This manual will explain the installation and operation of your Zeron 770. A thorough understanding of how the 770 functions will help you adapt these instructions to local customs. It is recommended that you read this manual completely and then practice before competing in an event. An old rally, rerun without the pressure of staying on time, can check your mastery of the instrument.


The 770 utilizes an integrated circuit technology that is susceptible to damage from static electricity, e.g., the kind that shocks you after walking across a rug on a dry day. Therefore, several precautionary measures should be taken when you handle this unit. There is no danger of harming the 770 when the unit is in its case. If you must remove it, hold it by the front panel and avoid touching the printed circuit boards and their components. Even if you do accidentally contact a part, don't worry. It's very unlikely that any harm was done.

Mounting studs are located on the top and back of the case. Refer to the illustrations at the end of the manual should you find it necessary to drill more holes. If you intend to use the 770 on performance rallies, the case must be supported along the bottom so that the mounting screws do not take all the shocks and jolts (the case may crack). The 770 will slide out of the case after the screws in the corners of the front panel are removed.

The heatsink which protrudes from the back of the case is connected to the +12 volts side of the car's electrical system at all times. Be positive that it cannot touch the car's chassis or other metal parts connected to the chassis. This won't hurt the 770, but it will blow the fuse and shut you down! The heatsink, as you might imagine, gets hot. Not hot enough to harm your car, but it will surprise you if you reach behind the unit and touch it.

The 770 is built with components which operate at temperatures between -40 and +85 degrees Celsius. Operation outside this range may cause incorrect calculations. While you won't be rallying with the inside of your car at -40C, temperatures at the high end are possible under certain conditions. On hot days when the car is closed and in direct sunlight, temperatures on the dash may exceed the upper limit. Watch out for this at lunch breaks and rest stops. Turning off the displays will help keep the 770 from overheating if you must leave the car in a poor location. When the displays are off, the 770 draws a current of about 0.06 amp (instead of 1.5 amp). A weak battery will not be run down.

Sending Unit:

Zeron sending units are designed for permanent mounting between the car's speedometer and the cable driving it. Disconnect the cable from behind the speedometer, mount the sending unit to the back of the speedometer, then reconnect the cable. The short square pin included with the unit goes between the speedometer and the sending unit. When the car is moving, the cable should turn evenly without binding or making strange noises. A small amount of speedometer needle bounce at low speeds is normal; large fluctuations indicate a binding cable. Check for any sharp bends you may have caused in the cable. It might also be helpful to change the position of the cable or to rotate the sending unit on the back of the speedometer.

On a small number of cars, it is very difficult to mount the sending unit directly behind the speedometer. In some cases, a short extension cable can be placed between the speedometer and the sending unit to allow easy access for making the connection. If your car is one of these, we'll let you know when shipping your 770. We can supply the cable at a small cost.

The sending unit can also be run from an un-driven wheel cable or from an auxiliary cable connected thru a 1:1 tee gear to the odometer cable. A few cars require this setup as it is impossible to mount the sending unit behind the speedometer. In either of these two cases, static electricity may develop in the cable and cause false inputs to the 770. GROUND THE SENDING UNIT BY RUNNING A WIRE BETWEEN THE CAR'S CHASSIS AND THE UNIT. Doing this is very important! Nearly every report of trouble made to Zeronics has been caused by failing to perform this one simple step.

When driving the sending unit from a separate cable, do not over grease the cable. The inner cable core needs only a light coating for proper operation. If the cable is packed with grease, the excess may work its way into the sending unit and block the sensor. The same holds true for a new odometer cable should you replace the one in your car. Don't grease it! The factory sends it ready for use.

The sending unit is not weather proof and my be damaged if left exposed. Always mount it in the car's passenger compartment or trunk. When the sending unit is not mounted underneath dash, protect it from direct sunlight. High intensity light may leak in through the mating surfaces and cause the unit to operate incorrectly.

Your sending unit comes equipped with the proper cable fittings for your car. If you change cars, they can be replaced with a different type. With replacement, it my be necessary to adjust the washers located on the sending unit's rotor. The bent washers apply a slight drag to the rotor and prevent false outputs from extraneous vibrations. Increase or decrease the drag by bending the washer(s) until the rotor offers a small resistance when turned. Check the drag every 20,000 miles. A diagram of the sending unit's parts and assembly is found at the end of this manual.

Most cars use odometer cables which terminate with a 0.106" square pin. Some British and French cars use a 0.118" pin. Your sending unit comes equipped with the proper rotor to accept the pin size of your car. Should you switch cars, the other size rotor can be ordered from the price list at the end of the manual.

Electrical Connections:

A prewired power cord is included with your 770. Connect the red wire to +12 volts (uninterrupted by the ignition switch) and the black wire to ground. Attach the leads as securely as you can make them. Only a momentary power loss, either +12 volts or ground, will erase the data in the 770's displays. Do not select a power lead from under the dash. It may contain numerous slip-on connections as it is routed from point to point. Your best tap point is at the fuse holder where the main power lead enters. If there are no screw-on terminals in the holder, strip back the power lead's insulation and attach the red lead at that point. Solder and then wrap with electrical tape for a secure joint. Attach the ground lead under a screw found in a metal part of the car's chassis. Scrape away any paint which might cause a poor electrical path. Using a cigarette lighter plug for your power connections is inviting disaster during a rally.

You will have noted that the power connector has 4 pins instead of only two. The pins are wired in pairs, two for +12 volts and two for ground. If your installation requires additional connectors, follow this system of doubling up on the pins. This is another precaution against a momentary power loss.

A 3 or 4 amp. fuse is located in the +12 volt lead of the power cord. Never run your 770 without this fuse in place. It is your protection against shorting the heatsink. A spare fuse is taped to the back of the case. In the unlikely event more fuses are needed, they can be found at most service stations, electronic supply stores and TV repair shops. In desperate circumstances, a fuse from your car's electrical system may work. Take one from the fuse holder that runs an un-needed circuit (radio, fan, etc.).

After you have completed your power connections, plug in the 770 and see if it will turn on. If the leads have been reversed, the unit won't operate. No harm will occur if they have been accidentally switched. This test should be performed without the sending unit connected. Plugging the sending unit in when the leads are reversed may blow out an internal fuse and cause the sending unit to become inoperable.

Keep the sending unit and power connector pins free of dirt and grim. Never wrap masking tape around the pins. A light brushing with steel wool once a year will keep the contact in good condition. You may use tape to hold the connectors together if you feel they might separate because of their location in the car.

Your 770 will operate on voltages from 7 to 20. Fall below 7 volts and data will start dropping out of the displays. Your car's output voltage normally varies between 10 when starting and 14 when charging. A good battery is your best protection against having the voltage go below 7 volts.

While the car's nominal output voltage is 12-14 volts, electrical noise and large voltage spikes can occur at the same time. The 770 has adequate safeguards to isolate it from this type of noise (from fans, lights, etc.). The one exception is solid metal ignition wires. While few cars come equipped with these wires as standard equipment (Alfa Romeos are one), if your car has them, they need replacement with standard carbon resistance wire. Don't forget the wire between the coil and the distributor.

Porsches have solid wires but are also equipped with supressors to eliminate the generated noise. No new wires are required. If you feel that solid wires are absolutely necessary, use a shielded type. This wire is available at some automotive parts outlets. It's relatively expensive, but it keeps the noise down.

Don't be tempted to "see what happens" before being convinced to change the wires. In some cases the effect is obvious and immediate... the counters start rapid and sporadic counting as soon as the car is started. In other cases, hours of close observation may indicate that all is well. Don't be misled by this apparent lack of trouble. This noise can be extremely elusive. Why risk getting bad checkpoint scores. Change the wires!

A small gooseneck lamp to light the 770 during night rallies can be ordered from the price list found at the end of the manual. Two tapped mounting holes are located at the top of the front panel and a power jack is provided on the rear of the case. The lamp comes with a prewired plug, spare bulb, and ON/OFF switch. Do not use the jack or front panel for any other type of electrical connection. Doing so my overload the 770's electronics and cause a component failure.


When the 770 is first turned on, the displays will show a meaningless pattern of digits and, in some cases, blank digits. This random pattern will also warn you that the computer has lost power. If it occurs during your shakedown drives, a faulty power connection must be found and eliminated.

The Odometer records official rally miles from 00.00 to 99.99 in increments of 0.01 mile. Once the computer is corrected at the end of the odometer leg, the Odometer will match the rally's mileage. The count direction, up or down, is controlled by switches you will read about later. To zero the Odometer, press the small button to the left of the display while also pressing the button on the lower right side of the front panel. The second button, referred to as the "Safety" from now on, has several purposes. One of them is to prevent you from accidentally zeroing the Odometer. Because it must be pressed along with the other button, an unintentional bump on the zeroing button won't cause the loss of your mileage.

The Auxiliary counter has three separate modes controlled by the toggle switch to the left of the display. Mileage (Dist.) or time are recorded in hundredths of a mile (minute) when the switch is in the appropriate position. Whenever the 770 is put into an off-course mode, the Auxiliary automatically begins to count backwards. Any distance or time measurement will then read correctly when you regain the rally course.

The Auxiliary should be used for actions keyed to short intervals. For example, "Left 2.67 miles after county line" or "Increase your speed 5 mph 2.70 minutes after a bridge". When "time" Is recorded, it's computed time, not real (clock) time. If the car is stopped, time does not register.

When switched to P/G, the Auxiliary registers the amount of pauses or gains entered into the computer. The pause or gain procedure will be discussed later. For now, it's only important to learn that they register here in the middle counter. The toggle switch normally remains in the P/G position. After a pause or gain is recorded, zero the counter so it's ready for the next action. You don't need the safety to zero. Because the Auxiliary spends most of its time at 000, there is little chance of losing its data accidentally.

In place of a fourth digit, the Auxiliary has a warning flasher to alert you whenever a rotary switch is not in a normal operating position ... ON COURSE and +T. The counter, therefore, only counts to 9.99 where it starts over at 0.00. Most time and distance related actions are keyed to short increments. Time or distance can be added directly to the readings in the CTC or Odometer for long increments.

The Clock shows real time of day from 00.00 to 59.99 minutes. It restarts each hour at 00.00.

The time calculated by the 770 is recorded in the Computed Time Counter (CTC). It counts in decimal minutes from 00.00 to 59.99. Its count direction is also controlled by switches you will read about later. The safety must be pressed to zero the CTC.

The display's intensity is determined by the switch located at the bottom of the front panel. Use the BRT (bright) position for daytime rallies, the DIM for night ones. The OFF position turns the displays completely off.

Correction Factor:

The computer correction factor adjusts the sending unit pulses to allow the 770 to run at official rally miles. The four digit factor is set in the thumbwheel switches found on the lower right of the front panel. The 770 has the capability to calculate the correction factor for you. The procedure will be given in a later section. The following paragraphs will acquaint you with the basic correction factor information.

The formula for adjusting your factor at the end of the odometer leg is:

                   OFFICIAL MILES 
      NEW FACTOR = -------------- X OLD FACTOR
                   COMPUTER MILES

      Example: Your factor at the start of the odometer leg is 4800.
               The odometer leg is 14.92 miles long and the Odometer
               reads 15.07.

                            ----- X 4800 = 4752

               4752 is your factor for the reminder of the event.
               Of course, you can change it should conditions
               later require you to.
Before running your first rally, you may want to establish a "base correction factor" (BCF). This is the factor which will run the 770 at statute miles (km) in your car. Once obtained, make a habit of starting each rally with this factor. It will normally keep you within a few percent of the rally's mileage and thus make the odometer leg easier to run.

To find your BCF, it will be necessary to run your car against a known statute distance. Many highways have mileage markers, or you could use an old rally measured in statute miles. When a suitable course is not available, lay one out using the car's odometer. A more precise BCF can be developed when the opportunity arises.

The course you select can have any length although it's a good idea to make it at least 5 miles. Set in a factor of 5000 and run the course. Use the formula to calculate your BCF. A rerun of the course with the new factor will find your Odometer mileage equal to the course's "official" length.

Your exact statute mile factor does not need to be your BCF. Your calculation will be easier if you round off the last two digits. 5529 would go to 5500, 3579 to 3600, and 4841 would round to 4800.

There my be times (it may start raining) when you will want to adjust your factor during the rally. Small changes can be mentally calculated quite easily. For each 0.01 mile you wish to adjust your mileage in 10 miles, change the factor by the value of its first digit. Increase the factor to increase your mileage. For less mileage, decrease the factor. Suppose your factor was 6012 and you wanted to increase your mileage 0.01 mile for every 10 you travel. Change the factor to 6018 (6012 + 6). As another example, your factor is 4850 and you need a decrease of 0.02 mile per 10 miles. Change the factor to 4840 (4850 -4.8 -4.8).

If you know the number of revolutions per mile at the input of the sending unit, you can calculate your BCF from the formula below. A little math will tell you that you need a minimum of 470 revolutions per mile to run the computer. Most cars have between 800 and 1600. None are lower. Direct drives from an undriven wheel vary between 800 and 1000, depending on wheel size.

      CORRECTION FACTOR = --------------------
                          REVOLUTIONS PER MILE
Most American cars and many foreign models have odometer cables which turn at 1000 revolutions per mile. The BCF in these cases would all be around 4700. The 770 will also run in kilometers. The correction factor has been designed so that it will not normally go above 6000. This gives you sufficient range to increase the factor to let the Odometer read 1.00 at the end of 0.62 mile (1 kilometer = 0.62 mile...old factor X 1.61). The 770 doesn't know or care what the unit of measurement is. That's for you to decide. Of course, speeds are now entered in KPH, not MPH.

Speed Banks:

Rally speeds are set in miles per hour with the switches located at the top of the computer. The correction factor corrects the entire computer, therefore the speeds you enter are the ones given in the route instructions. There's no need for corrected tables, conversion factors, etc. Any speed from 00.3 to 99.9 mph in increments of 0.1 mph can be used. Speeds of 00.0, 00.l. and 00.2 should not be entered. These 3 speeds will not yield correct computed time.

The toggle switch between the two banks selects the active bank. The unused bank can be preset to the next rally speed. You may change the active bank's speed while the car is moving. This technique is helpful with small repetitive changes, say from 40 to 41 to 42. For unexpected or forgotten speeds, use the inactive bank and then throw the switch.

Avoid extremely slow speeds whenever possible. This is one time when the 770's accuracy may hurt your score. Consider the following instructions taken from an actual rally:

           34.45       33. "Stop". Change average speed to 1 mph.

           34.92       34. "Go". Change average speed to 35 mph.
The time for this distance is 28.20 minutes. The rallymaster was giving the crews a break at the gas station which occurred between the two instructions. When he measured the course, he rounded off the mileages to the nearest hundredth. Since he gave you two hard mileages, the time of 29.20 was correct. However, the 770 calculates on pulses received every 3-4 feet. If the actual distance were 0.474 miles, the 770 would have calculated a time of 28.44 minutes. A 0.24 minute error at the next checkpoint! You should treat this type of problem as a transit zone of 28.20 minutes. No reasonable rallymaster would expect you to maintain speeds this low. He would expect you to be able to calculate the elapsed time given the speed and distance.

Checkpoint Switch:

This control locks the data displayed in the Odometer, Clock, Auxiliary and CTC whenever it is thrown to "checkpoint". The counters continue to function, but their additional data is not shown until you return the switch to the OFF position.

Throw the switch as you cross checkpoint timing lines. The mileage to the line, your actual arrival time, and your computed arrival time will then lock in the displays. If you are mistimed, the Clock will verify the time you want. After you have recorded whatever information you desire, turn off the switch.


The driver readout can display three sets of information. The normal readout function is to display the computed time error (differential). The others allow either the Odometer or CTC readings to be sent to the readout. The driver will find running the odometer correction leg easier when mileage is displayed in front of him. If you lay out rallies, both mileage and computed time will be useful during course design and measurement. The toggle switch located below the Odometer selects the readout's mode.

Your computed time error is the difference between the Clock and the CTC. As will be illustrated later, you will rally on a time-of-day basis. That is, when you are "on time", the time in the CTC will be the same as the time in the Clock. When the times diverge, the readout shows the error. Only the three least significant digits are displayed when viewing computed time error. Drop- ping the ten minute position from the differential mode makes for quicker checks during the event.

Differential mode examples:

                 012 -- .12 minute ahead ("up")
                 001 -- .01 minute ahead
                 000 --  on time
                 998 -- .02 minute late ("down")
                 972 -- .28 minute late
As you can see, the readout drops below 000 when you are late. It may occur to you that a + or - indicator would make the driver's job easier. However, as you gain experience, you will find that driving a little up is the most effective tactic for getting good scores. Since you will then spend most of your time up a hundredth or two, a reading such as 002 will automatically indicate that you are ahead. It won't be necessary to associate an extra piece of data (the + or -) each time the readout is checked.

Mental arithmetic when you are extremely late is not required in most cases. Suppose you found yourself delayed by traffic and when you checked the readout, it read 861. In this situation, it's more important to get back on time; not worry about being precisely 1.39 minutes late.

The relationship among the Clock, CTC, and readout are illustrated below:

                    CLOCK    CTC     READOUT
                    0000     0000    000 (on time)
                    0122     0000    878 (1.22 late)
                    0122     0200    078 (0.78 early)
                    1234     1240    006 (0.06 early)
                    1234     2240    006 (10.06 early)
Notice how the readout in the last example appears to indicate that you are only up 0.06 minutes when you are really up 10.06 minutes. You must always insure that the Clock and CTC are in the same 10 minute period. Though there is little chance for this to happen while you rally on time, you should compare the counters at the end of the odometer leg, the end of transit zones, and whenever you recover from being off course.

You will soon notice that the differential made does not display all the Clock and CTC pulses. If the car is stopped, the readout counts steadily downward as the Clock counts up. Once you are up to rally speed, the readout will hold a constant value even though the Clock and CTC are trying to pulse it up 1 count, then down 1 count, then up 1... The driver can hold the readout at a constant number without the distraction of digits flipping up and down. This fixed number system is achieved by updating the readout once every 0.01 minute. Because of this, you may occasionally see the readout skip some numbers. This is normal. It only means that the readout received several pulses during the 0.01 update interval.

The driver can also use the readout for pauses and gains. Suppose the route instructions called for a pause while the navigator was busy. You can run the pause on the readout until the navigator is free to enter it normally. If the pause were 0.25 minutes, slow the car until the readout drops to 975 (0.25 minute late). Then continue with 975 as your "on-time" reference instead of 000.

Mileage Alarm:

A preset mileage can be dialed into the thumbwheel switches located below the CTC. When the Odometer reaches this mileage, a buzzer will sound to alert you. To turn off the buzzer, press the pushbutton next to the switches. The alarm system is always on. When another mileage is entered, the buzzer will sound when the new distance is attained. The buzzer will also go off if the mileage in the switches comes up again. This might occur after an odometer zero point or when you are returning from off course.

In most cases, you do not want the buzzer to sound at the exact mileage you are concerned with. Using one slightly prior to this mileage will give you time to prepare for the required action.

When you don't need the alarm, dial in a large mileage, e.g., 99.00. This will, in effect, turn off the system.

Mode Control:

This rotary switch selects the operating mode for all of the 770's functions. The 770 is only off when the switch is in the PWR OFF position.

DIST. ONLY (distance only) and DIST. ONLY REV. (distance only reverse) - These modes are used when computed time is not required, e.g., during an odometer leg or transit zone. The Odometer and Auxiliary will both register distance. No computed time is registered in the CTC. As will be explained shortly, you are not prevented from adjusting the CTC with the add time pushbuttons. This allows you to set the CTC to your time out from the end of the odometer leg as you run the leg. You can also add in a transit zone time as you complete the distance.

ON COURSE - This is the normal operating mode for the 770. Both the Odometer and CTC count forward. The warning flasher indicates when the switch is not in this position.

OFF COURSE - Both Odometer and CTC are reversed. Since the readout follows the CTC, it also reverses. Use this mode to retrace the rally course after a course following error or to back up to check a clue.

PARK - This position turns off the 770's speed and distance functions. No mileage or computed time are allowed into the counters although the add pulse circuits still operate. Use it whenever you wish to leave the rally course during a break. The blank positions on the switch are also PARK. If you accidentally overshoot one of the operating positions, you won't turn the 770 off or put it in TEST.

The PARK mode also comes in handy when you are off course. Once you've decided to turn around, pick out a landmark (tree, pole, etc.) and switch to PARK as you pass it. U-turn at the first opportunity and switch to OFF COURSE as you pass the landmark from the other direction. This procedure eliminates guessing at when to reverse the 770 as you turn around. Switch directly to ON COURSE when you regain the rally route.

TEST - This mode activates an internal pulse generator to run the 770 as if it were in a moving car. You can test or demonstrate the computer with this mode. Using a correction factor of 5860, the 770 behaves as if it were in a car going 60 mph.

When TEST is in use, remember that it's the "car" traveling at 60 mph, not the 770's electronics. With a speed of 60 mph dialed in the speed bank, the 770 will yield a computed time of 1 minute computed time over a distance of 1 mile while taking exactly 1 minute of real time to run this amount. The readout would hold steady at a constant value showing that you are neither losing nor gaining time. If the speed bank had 30 mph in it instead of 60 mph, after a similar 1 mile in 1 minute of real time, the computed time would read 2 minutes. Since the "car" is traveling at 60 mph and the "rally speed" is 30, the readout will show that you are getting farther ahead as the 1 mile is run. Had the readout been at 000 when the 1 mile started, you would be exactly 1 minute "up" at the finish.

Pulse Control:
The +T, -T, +D, and -D positions, in conjunction with the pushbuttons, are used to pause or gain, add or subtract computed time, and add or subtract mileage. The other three positions will be covered later. The switch is inactive when it is in one of the blank positions.

The rotary switch selects the type of pulse, the pushbuttons the amount. For example:

      Pause 2 minutes 30 seconds --- Set the selector to +T, press the
                                     1.00 (100 pulses) button 2 times and
                                     the .10 (10 pulses) button 5 times.
                                     Remember that the seconds must be
                                     converted to hundredths.
Pause (+T) and gain (-T) pulses go into the CTC and also register in the Auxiliary when it's in the P/G mode. The Auxiliary will double check the amount of pause or gain you enter. If the Auxiliary were in use with a distance turn, pauses and gains are still fed into the CTC but you cannot check yourself.

Since pauses and gains are simply the addition and subtraction of computed time, You can use +T and -T whenever you need to set the CTC to a specific time. As an example, you will need to set the CTC to your time out from the end of the odometer calibration leg. Suppose this was 48.00 minutes. Assuming the CTC is at 00.00, switch to -T and push the 1.00 button 12 times to Subtract 12 minutes. This method will also be used at checkpoints to set the CTC to your assigned time out, at time of day restarts and at the end of transit zones.

The +0 and -0 positions adjust mileage. Use them to correct your Odometer to official mileage at the end of the odometer calibration leg, to adjust for wheel slippage, and to correct the Odometer when it is in error at checkpoints.

Of course, the above procedures apply whether or not the car is moving. It makes no difference to the computer.

The Auxiliary registers extra added or subtracted pulses when it is in the Time or Distance mode. For example, a pause entered while the Auxiliary was in use for a time turn would go into both the CTC and the time being recorded by the Auxiliary. The rallymaster takes into account any pauses or gains which occur during the time interval. Enter them as usual and turn when the proper amount is displayed.

Time and mileage actions can also be run by using the CTC and Odometer directly. An action required 2.50 miles after a certain point can be accomplished by noting the mileage in the Odometer as you pass the clue and when performing the action 2.50 miles later.

Keep the rotary switch in the +T position except when specifically needed for other actions. Pauses occur most often during normal rallying between checkpoints. The flasher will remind you when you are switched to another position. Hopefully, this will prevent your next "pause" from going into the odometer.

Pulses add or subtract as directed by the rotary switch. The Mode Control does not affect them. When switched to OFF COURSE, pulses add or subtract as usual. If you enter a pause while rallying off course, you would put in a gain on the way back to the proper route.

The CALC and CALC/M positions tie the pushbuttons into the 770's time and distance circuits. Don't use these two functions when the car is moving. Your calculations will be incorrect.

The easiest way to explain CALC and CALC/M is through a sample exercise. Let's assume that you switch to CALC/M, that the active speed bank has 40 mph, you are ON COURSE, and that the 770's counters contain the information as shown below. Now push the .10 button twice.

                                    BEFORE     AFTER
                    Odometer         34.00     34.20
                    Aux. (in P/G)     0.00      0.20
                    CTC              15.00     15.30
As you can see, the pushbuttons are tied to distance while the speed circuits generate the corresponding computed time for the mileage entered. (0.20 mile at 40 mph yields 0.30 minute.) Yet another way to visualize what happened is to imagine the rally car traveling 0.20 miles almost instantaneously. The information accumulated in the counters would be the same had you actually driven the 0.20 mile at a normal speed.

The .01 and .10 pushbuttons add in the amounts indicated. The 1.00 pushbutton does not put in 1 mile increments. Instead, mileage begins running in at about 1 mile per second for as long as the button is pressed. The .10 mile increment takes about 1/4 second to enter. If you watch closely, you will see the 10 counts (and computed time) enter the 770. It is possible (but not probable) to push the .10 button a second time while the first .10 mile is still registering. In this case, the second push will have no effect. However, since the Auxiliary is recording the actual amount entered, it will indicate that only .10 mile was received. You would also note this in the Odometer reading. The .01 button enters data so rapidly that it is impossible to get in a second push before the 770 finishes the calculation.

As mentioned above, these modes are similar to moving your car very rapidly over the distance entered. The 770's other functions, therefore, respond normally. If the Auxiliary were switched to TIME or DIST., the proper amounts would be recorded. Switching the Mode Control to OFF COURSE would cause the distance and time to subtract. There is no reason to use CALC or CALC/M when in one of the distance only positions.

CALC is the same as CALC/M, but without mileage entering the Odometer. The distance punched in still records in the Auxiliary (P/G Mode) so you can check the amount entered. CALC is most useful at checkpoints where timing is from timing line to timing line and when an out-marker is also provided a short distance after the line. The sample rally covers the procedure in full.

The safety and elapsed time sections will give further insight on the CALC and CALC/M Modes.

The final position on the pulse control switch activates the remaining rotary switch. When the knob points straight down, the functions on the third rotary are available for use.

Setting the Clock:

The Clock is set by using the three Clock positions on the third rotary switch. This switch must be turned on by pulse control rotary switch before it will work. Even so, it is best to leave the knob pointing to a blank position until you want to adjust the Clock or calculate a correction factor.

To zero the Clock, switch to ZERO and press the .01 button. To hold the Clock at a constant value, use STOP and the .01 button. To add time, turn to SET and use the pushbuttons to add in the amount needed.

The quickest method for setting the Clock to time of day is as follows. Just prior to an even minute on your time reference, zero the Clock. While still pressing the .01 button, switch to STOP. The Clock will now be held at 00.00. When the even minute arrives, release the button. The first time count will occur exactly 0.01 minute later. This has set the hundredths part of the Clock. Next, switch to SET and use the 1.00 button to run the minutes up to the correct time. That's all there is to it.

Factor Calculation:

Your 770 uses some of the functions already described to calculate a new correction factor at the end of the odometer correction leg. The calculation is not done instantaneously and may take the 770 ten to twenty seconds to complete the operation. The actual time depends on the length of the odometer leg. However, all the data is visible throughout the calculation. You won't need to do it twice to double check yourself. Also, the calculation circuit does not round up the last digit when it is closer to the next higher number. Even so, with a typical factor of 5000, the error will be less than 0.01 mile in 60. This is far greater accuracy than you could ever hope to achieve while trying to duplicate the rallymaster's measurements.

The 770 is set up at the end of the odometer leg as described below. In a real rally, you would not sit at the exact ending point to perform the steps given. As always, you should pull ahead to clear the ending point for other cars. The sample rally covers the procedure in detail. It shouldn't take more than a minute or two to prepare the 770 for the rally's first timed section.

          OFFICIAL MILEAGE:  Set in alarm switches. (Can be dialed in during the
                             odometer leg.)
        UNOFFICIAL MILEAGE:  Set in the four right most speed bank switches. The
                             selector most be thrown to the right bank.
                OLD FACTOR:  Already in the factor switches.
              MODE CONTROL:  Set to ON COURSE.
          ODOMETER and CTC:  Cleared to zero.
             ROTARY SWITCH:  Set to FACTOR CALC. Remember to turn on the switch
                             with the other rotary switch.
When you have completed the preliminary steps, press the .01 button. The 770 will begin the calculation. If the switches are not in the correct positions, the calculating process will not start. This feature will help keep you from coming up with an incorrect answer. When the calculation is completed, the alarm buzzer will go off and the new factor will be displayed in the CTC. Simply dial it into the factor switches.

One thing to remember is that the CTC zeros when it reaches 6000. If your new factor is larger than this value, you'll need to add 6000 to the amount shown in display. It should be obvious when you are required to do so.

The following illustrations use the same data as the example given in the correction factor section. Note that the Odometer contains official mileage at the calculation's conclusion. Unless the rally mileage is zeroed, you will want to start the first leg with the Odometer set to this value.

You are already acquainted with using the safety to zero the Odometer and CTC. A second feature clears all the 770's internal counting functions each time the button is pushed. As an example, imagine that you are at a checkpoint out-marker and are preparing for the next leg. You set the CTC, check the mileage, then begin to pull ahead to clear the out-marker for other cars. As you do this, you notice that the CTC and Odometer count almost immediately. When you first reached the out-marker, your Odometer may have pulsed a few feet before you arrived there, or 50 feet prior to the marker. The same is true of the CTC. Of course, it doesn't make any difference where they pulse before the marker, but it would be beneficial if the first mileage pulse occurs 52 feet into the next leg and the first time pulse occurs at the correct mileage for the speed in the computer. A tap on the safety will insure that the next counts occur at the proper point.

Use this feature at the beginning of the odometer leg, at the end of the odometer leg, and wherever the Odometer is zeroed. Of course, when you zero the Odometer the safety is also pushed. You automatically make the next pulse occur at the proper mileage. The safety should also be used to clear the internal circuits prior to using CALC, CALC/M or FACTOR CALC. Doing so will keep your calculations accurate. In the case of FACTOR CALC, the safety is pushed when you clear the Odometer.

Elapsed Time Calculations:

When you put on a rally of your own, you will find the CALC/M mode very convenient for running the official elapsed time (OET) for each leg of the event. With a few exceptions noted later, these OET's will be as accurate as those obtained using conventional methods. Examine the following exercise.

                                        ODOMETER        CTC
       Start (Speed bank at 36 mph)     00.00(0)     00.00(000)
       After one push on .01 button     00.01(0)     00.01(666)
       After another .01                00.02(0)     00.03(333)
       After 8 more (or a single push
              on the .10 at the start)  00.10(0)     00.16(666)
       After 9 more .10 entries         01.00(0)     01.66(666)
       CHECKPOINT                       01.54(0)     02.56(666)  OET: 2.57 min.
Notice that the CTC time is shown as only 0.01 minute after the first 0.01 mile. The approximate time for 0.01 mile at 36 mph is 0.0167 minute. The 770 calculates and accumulates these "unseen" partial hundredths. Under actual rally conditions, the CTC would have flipped over to 0.01 about half way thru the first 52 feet traveled. The 0.02 minute and 0.03 minute counts would occur between the 0.01 and 0.02 mile points. This gives the CTC a smooth count rate although it might appear otherwise as you run OET's.

Theoretically, the 770's computed time has an infinite number of places to the right of the decimal point. For purposes of this section, let's assume that there are only 5.

Once the checkpoint mileage is finally punched in, you must determine whether the internal digits might cause you to round the OET up to the next 0.01 minute. To help you understand the rounding procedure, consider what would happen if you added 0.01500 minute to the CTC values (and internal digits) in the two problems below.

                      CTC  2.56(412)          CTC  2.56(666)
                         + 0.01(500)             + 0.01(500)
                           2.57(912)               2.58(166)
As you can see, when the internal CTC digits are less than half a hundredth, the visible sum is only one more than the original CTC value. Where the unseen digits are more than half a hundredth as in the second problem, the sum increases by 2.

How can you duplicate the process of adding in 0.01500 minute? Switch to CALC, put 40 mph in the active speed bank, then press the .01 button. (0.01 mile at 40 mph equals 0.01500 minute). Simply by noting whether the CTC value increases by one or two, you will be able to determine whether the original CTC value was correct or whether it should be increased by one to yield a proper OET.

In most cases, a new OET calculation will start with the previous timing line mileage in the Odometer and the CTC cleared to 00.00. Pauses and gains should be entered at the appropriate place using +T and -T. Speed changes are made at the mileages you obtained during course measurement. After a little thought, you will easily see how other rally timing procedures (transit zones, etc.) can be entered into your OET calculations.

Using your 770 to calculate OET's containing extremely low speeds is not recommended. Your computer runs on approximate 0.001 mile increments. When you switch from one speed to another, you may travel as much as 0.001 mile extra at the old speed before it is cleared out and the new speed data is entered. At normal rally speeds this will not be evident in your OET's (either in ones you calculate or the ones you obtain as you actually run a rally). Suddenly entering a speed of 1 mph from a speed of around 40 mph, might or might not cause your final OET to be slightly in error. In situations where very low speeds are required, calculate them separately and enter the time as a transit zone. Refer to the example given in the speed bank section.

If you overshoot a mileage during your calculations, don't switch to OFF COURSE and back it out. Not all of the 770's internal circuits run in reverse. This might cause your time calculation to be slightly in error. Recalculate the OET from the beginning. Using OFF COURSE and the calculating modes during an actual rally won't give you a discernible timing error.

When running OET's, you will often use the 1.00 button to rapidly advance your mileage. When the button is released, the mileage my stop at a point between two hundredths, e.g., 42.61(3) miles. You will only see 42.61 in the Odometer, but the CTC may have some additional calculated time for the extra 0.003 mile. It might then appear that the CTC was not correct for the mileage shown. As soon as you push either the .01 or .10 button, the Odometer and CTC get back in sync with one another. In the above case, after one .01 entry, your mileage would be exactly 42.62(0).


Install the computer and check that it is properly connected and in good working order before you leave home. Let's assume the following:

     1. The first car leaves the start at 8:01.00 AM.
     2. Your car number is 10.
     3. Your BCF is 4800.
     4. The general instructions require a speed change to 35 mph
         whenever your mileage is an even multiple of 22.00 miles
         (22, 44, etc.).
Turn the computer on about 30 minutes before your time out (8:10.00). Zero the counters and set the Clock. Enter your BCF. The remaining time can be spent double checking things in the car and going over the general instructions one final time.

     0.00    1. Begin odometer correction leg at "Start". Zero your
                odometer. Leave this point at 8 AM plus your car
                number in minutes. Take 30 minutes to reach instruction 8
check your Odometer to see that it is zeroed, be sure you are in DIST. ONLY, get your route instructions, then leave. Send mileage out to the driver readout so he can follow the odometer leg while the navigator checks the other route instructions.

     11.52    8. End odometer correction leg at "Stop". Begin 40 mph.
Write down the mileage in the Odometer (let's call it 11.41), push the safety to clear the 770's internal circuits, then pull ahead to allow following cars access to the end point. Try to pull up to a place where the Odometer just turns over to a new hundredth. Let's say you pull ahead exactly 0.04 mile. You can measure this distance simply by going to 11.45 an the Odometer. The new correction factor is not in yet, but a short distance at the old one won't give you a timing error. Once you have stopped, zero the Odometer (the CTC should still be at zero), enter the mileage data, and calculate the new factor (4846). Dial the factor into the switches.

Next, set the CTC to your time out from the end of the odometer leg (40.00). The Odometer (11.52) and CTC (40.00) are now both set as if you were sitting right at the end of the odometer leg and were awaiting your time to leave. In reality, you are 0.04 mile past this point. Put in the first rally speed (40). Let's "bring" the computer up to your present location. Switch to CALC/M and punch in 0.04 mile. The Odometer is now set to 11.56 and the CTC has 40.06(000). Assuming that the rallymaster gave you adequate time at the ending point, real time (Clock) should still be less than 40.06. Use the extra time to read through the route instructions. As real time catches up to your CTC, get the car up to rally speed. The driver readout will be showing how much time remains (it's counting down toward 000). Be sure that the Clock and CTC are in the same 10 minute period. You should be in ON COURSE. You switched there for the factor calculation.

If you decide to do the factor calculation on your own, follow these steps. During the odometer leg, set the CTC to 40.00. Enter the first speed (40). At the end of the odometer leg write down the mileage, adjust the Odometer to 11.52, switch to ON COURSE and pull ahead. The CTC will begin to count up based on the first rally speed. As before, your new factor is not in yet but a small distance at the old one won't matter. Calculate and enter your new factor. Your computer is now set. Simply wait for the Clock to catch up with the CTC.

CHECKPOINT! Throw the checkpoint switch as you cross the timing line. Compare the Clock with the time given you by the workers. If there's a discrepancy, straighter it out before you turn off the switch. When everything is taken care of, pull up to the out-marker. Correct your Odometer if necessary.

Most rallies use one of two common methods to start you on the next leg. When the checkpoint zone is dead mileage, i.e., timing is from out-marker to checkpoint line, the 770's procedure is effortless. At the out-marker, pulse the CTC up to your assigned time out. Assuming that you have been given adequate time by the workers, the CTC will then have more time in it than the Clock has. The readout will indicate the time left before you start the next leg. Pull ahead slightly to clear the out-marker for other rally cars. Begin the next leg as the readout nears 000.

When the rally is timed from checkpoint line to checkpoint line (no dead mileage), the calculated time between the checkpoint line and the out-marker must be added to your time out. As an example, you are given an assigned time out of 9:12.00 and you are now at the out-marker which is located 0.11 miles from the checkpoint line. Adjust your Odometer to the out-marker mileage (if necessary) and set the CTC to 12.00. Switch to CALC and add in the computed time for 0.11 mile. In this case, if the speed leaving the checkpoint line were 40 mph, the CTC would go to 12.16(500). Again, you would pull ahead slightly and wait for Clock time to catch up with the CTC. When you pull ahead, the CTC will count up. This is as it should be. The CTC is merely telling you what the computed time is to the point where the car is located.

     21.90 miles ---the buzzer will sound (you did set it, didn't you?).
                    Enter 35 mph in the inactive speed bank in preparation
                    for the speed change at 22.00 miles. Dial in 43.90
                    (or the next important mileage).

     28.36    23.   Begin transit zone of 20 minutes to next instruction.

     28.90    24.   "Max". End transit zone. Zero your Odometer. Begin 40 mph.
At 28.36 miles, switch to DIST. ONLY and note the time in the CTC. During the zone add 20 minutes to the CTC. Plan to arrive at instruction 24 before your time is up. At "Max", set in the new rally speed, zero the odometer, switch to ON COURSE and pull ahead to wait out the balance of your time. Be sure that the CTC and Clock are in the same 10 minutes when you leave.


The accuracy of your 770 is much greater than you will need on a rally. When your correction factor falls in the normal 3000 to 6000 range, you can correct your mileage to about plus or minus 0.01 mile in 60 miles. If you and the rallymaster can agree to within 0.01 mile after 20 miles or so, that would be about normal. All things considered, you can expect to average about 0.01 minute error per checkpoint.

The Clock will stay within 0.01 minute over the period of an all day event. Extreme temperature fluctuations may slightly affect its accuracy. This may happen should you leave the 770 turned on overnight in cold weather. Even in these cases, the Clock will not vary by more than 0.02 minute per day.

The 770 has been constructed to withstand all the normal jolts and bounces it might receive during a rally. When using your 770 on stage events, don't mount the unit rigidly to the car's frame. This will only transfer every shock taken by the car directly to the 770. Application of a little care in your mounting arrangement will insure many years of trouble-free rallying.

Although your 770 is built only with quality components, there is always the possibility it my stop functioning correctly. As a first step in correcting the problem, notify Zeronics and describe the malfunction. It may not be necessary to return the unit. Most major parts are mounted in sockets for easy removal and replacement. This feature is found on all Zeron models. It's your assurance of quick and inexpensive repair of any problem you might have.


The math problems you find most often on rallies involve the elements of time, speed, and distance. It is usually easier to work in minutes per mile factors (MPM) than with miles per hour. A factor table has been printed at the end of the manual for your convenience.

For those who are not familiar with MPM factors, a quick explanation is in order. Basically, MPM factors tell how long (in minutes) it takes to run 1 mile at the assigned speed. The factor for 35 mph is 1.714. This means that it takes 1.714 minutes to run 1 mile at 35 mph. Moving the decimal point gives the time for 10 miles (17.14 minutes), 0.10 mile (.17 minute), and 0.01 mile (.017 minute-- about 2 hundredths).

Missed speed changes can easily be "fudged" by using factors. To illustrate, suppose you forgot the speed change that was part of the last route instruction. You've been driving at 30 mph (2.000) instead of 35 mph (1.714). Unless you can risk returning to the execution point, you'll have to make an educated guess at the gain time required to correct the CTC. With the MPM factors you can quickly see that it takes about 0.29 minute less to run 1 mile at 35 mph than it does at 30 mph. For every mile you've been at the wrong speed you will have to gain 0.29 minute. Let's say you estimate the distance back to the speed change as 1.5 miles. The amount of the gain would then be 0.44 minute (1.5 X 0.29).

Two helpful formulas are:
                                   60                           Elapsed minutes
      Speed (miles per hour) = ----------          MPM FACTOR = ---------------
                               MPM Factor                       Elapsed miles


Whenever you think that the rallymaster is beginning his elapsed time calculations
based on a perfect mileage, use the safety to clear out the 770's
internal circuits. In most cases he will do this at the end of the odometer leg
and at the start of each new leg. The safety will insure that no unseen data
from a previous rally section is included in the next one.

As stated earlier in the manual, when pauses and gains are given in seconds, they must be converted to hundredths. Some values do not convert into even hundredths. Take for example, 10 seconds. In hundredths, this is 16.7. A pause of 10 seconds would be entered as 17. If you had a series of 10 second pauses, putting them all in as 17 would give you an error of 1 hundredth per three pauses. You can eliminate the problem by making the first two 17 and the third 16.

While all rally teams have a favorite technique for crossing a checkpoint line on time, you may find the following method best suited for the 770.

As you will soon notice, when you are at rally speed and are on time, it's a simple matter to hold the readout at 000. The readout will occasionally count down to 999 or up to 001. You can easily correct this by speeding up or slowing down. But what if the readout changes just before you cross the timing line? You won't have time to correct for it. Even when you can see the checkpoint in the distance, it's impossible to drive the exact rally speed. The readout my still change right at the line.

As an alternative to driving at 000, try driving at 002. When you see the checkpoint, slow down to below rally speed. The readout can now only count down. With a little practice, you can make the readout drop to 000 just before the timing line. When a checkpoint occurs without warning, a tap on the brakes will lose the extra time.

PRICE LIST (Subject to change without notice):

The following items are available should you change rally cars or set up a
second car. We pay the shipping charges.

         Sending unit (specify car make and year or type of fittings)     $45.00
         Rotor (specify for 0.106 or 0.118 shaft)                           8.50
         Night light                                                       10.00
         Power cord (prewired with fuse and holder)                         5.00
         Halda T-Gear (used for new sending unit fittings)                 20.00

         For a fee of $7.00 you may return your sending unit and have the fittings
         exchanged for a different type. This offer is good only when the fittings
         to be switched are still being used on a currently produced car. Your old
         fittings will be kept by Zeronics and the new ones will be mounted on your
         sending unit. The rotor drag will be adjusted and the unit will be inspected.


Zeronics warrants this product to be free from defects in workmanship and material for a period of one year from original purchase. Our obligation under this warranty is limited solely to repairing or replacing, at our option, any part when the product is returned to us within the warranty period providing: (1) the defective unit is returned to us transportation prepaid by purchaser, (2) no modification or change has been made to the unit's circuitry or wiring, (3) the unit has not been damaged by misuse, neglect, improper operation, accident, or alteration as determined by Zeronics. No other obligation is implied or expressed.

Minutes per Mile Factors

      SPEED       SPEED       SPEED       SPEED
         1  60.00   16  3.750   31  1.935   46  1.304
         2  30.00   17  3.529   32  1.875   47  1.277
         3  20.00   18  3.333   33  1.818   48  1.250
         4  15.00   19  3.158   34  1.765   49  1.224
         5  12.00   20  3.000   35  1.714   50  1.200
         6  10.00   21  2-857   36  1.667   51  1.176
         7  8.571   22  2.727   37  1.622   52  1.154
         8  7.500   23  2.609   38  1.579   53  1.132
         9  6.667   24  2.500   39  1.538   54  1.111
        10  6.000   25  2.400   40  1.500   55  1.091
        11  5.455   26  2.308   41  1.463   56  1.071
        12  5.000   27  2.222   42  1.429   57  1.053
        13  4.615   28  2.143   43  1.395   58  1.034
        14  4.286   29  2.069   44  1.364   59  1.017
        15  4.000   30  2.000   45  1.333   60  1.000

                        OFFICIAL MILES                                           60
           NEW FACTOR = -------------- X OLD FACTOR             SPEED(mph) = ----------
                        COMPUTER MILES                                       MPM FACTOR


       MIN.  SEC.    MIN.  SEC.    MIN.  SEC.    MIN.  SEC.
       .01   0.6     .26   15.6    .51   30.6    .76   45.6
       .02   1.2     .27   16.2    .52   31.2    .77   46.2
       .03   1.8     .28   16.8    .53   31.8    .78   46.8
       .04   2.4     .29   17.4    .54   32.4    .79   47.4
       .05   3.0     .30   18.0    .55   33.0    .80   48.0
       .06   3.6     .31   18.6    .56   33.6    .81   48.6
       .07   4.2     .32   19.2    .57   34.2    .82   49.2
       .08   4.8     .33   19.8    .58   34.8    .83   49.8
       .083  5.0     .333  20.0    .583  35.0    .833  50.0
       .09   5.4     .34   20.4    .59   35.4    .84   50.4
       .10   6.0     .35   21.0    .60   36.0    .85   51.0
       .11   6.6     .36   21.6    .61   36.6    .86   51.6
       .12   7.2     .37   22.2    .62   37.2    .87   52.2
       .13   7.8     .38   22.8    .63   37.8    .88   52.8
       .14   8.4     .39   23.4    .64   38.4    .89   53.4
       .15   9.0     .40   24.0    .65   39.0    .90   54.0
       .16   9.6     .41   24.6    .66   39.6    .91   54.6
       .167 10.0     .417  25.0    .667  40.0    .917  55.0
       .17  10.2     .42   25.2    .67   40.2    .92   55.2
       .18  10.8     .43   25.8    .68   40.8    .93   55.8
       .19  11.4     .44   26.4    .69   41.4    .94   56.4
       .20  12.0     .45   27.0    .70   42.0    .95   57.0
       .21  12.6     .46   27.6    .71   42.6    .96   57.6
       .22  13.2     .47   28.2    .72   43.2    .97   58.2
       .23  13.8     .48   28.8    .73   43.8    .98   58.8
       .24  14.4     .49   29.4    .74   44.4    .99   59.4
       .25  15.0     .50   30.0    .75   45.0   l.00   60.0

Manuals Index
RRN Index