S.V.N-MCA(cbcs)


	S.V.N-MCA(cbcs) - BigTrak

Reverse-Engineering the PCB for the Big Trak Toy

2004 was the 25th Anniversary of the Big Trak!

The Big Trak was introduced in 1979 for the suggested retail price of $43 USD. That's equivalent to around $115 in 2004, accounting for inflation.

The Big Trak includes:

A cool-looking plastic exterior with multicolor stickers.
Six wheels with the center pair driven by two motors. This allows the Big Trak to drive forward, backward, stop, spin left, or spin right.
Photon canon (incandescent bulb with blue plastic filter).
Speaker to play various beeps and buzzes.
24-key membrane keypad.
Encoder and magnetic clutch so that driving distances and turn angles are fairly repeatable even as battery voltage drops.
Expansion plug port for Big Trak Transport cargo carrier accessory.
Beeps every thirty seconds of inactivity to remind you to turn it off to save battery life.
Can be user programmed with up to 16 steps.

The Big Trak notably lacks obstacle detection, line-following sensors, or any other kind of sensors to allow it to interact with its environment. Also, the lack of a display for showing command steps makes user programming more difficult. Of course, such items would have increased the price, which I imagine was a key constraint for the toy market.

After a complete examination of the Big Trak and the Big Trak Transport (trailer), I am in awe of the genius of its creators. From the mechanics, to electronics, to esthetics, to marketing, the Big Trak stands as testament to the talent of the team.

OBTAINING A BIG TRAK AND LOOKING INSIDE

I didn't receive a Big Trak as a child, nor did I suffer from Big Trak envy, so the purpose of this page is to satisfy intellectual curiosity rather than nostalgia.

There are a number of sites dedicated to the reverse-engineering, dissection, inspection, and hacking of various modern toys. The answer to what's inside a Big Trak is a little more elusive, since the product wasn't in the public spotlight during the age of the Internet. Not only is interest and availability in the Big Trak limited today, but datasheets are virtually non-existent for the obsolete parts contained within.

You might find a Big Trak in your parent's attic (if you were a lucky boy). Otherwise, you can obtain a Big Trak second-hand on an eBay auction. Expect to pay between $40 and $120, plus shipping, depending on quality and accessories.

Due to age or abuse, most Big Trak toys now have some form of damage: usually broken axles, front screw hole covers with broken pins, corroded battery contacts, battery leakage into the main compartment, body damage (especially to the corners), peeling stickers, and superficial mottling of the keypad.

I was not the first to open up some of the Big Traks that I got. (That's right, my accommodating wife let me buy more than one.) Missing screws, electrical tape, and yellowing super glue are telltale signs of Dad's or Junior's field operation. At one point, there were excellent disassembly instructions at www.robotprojects.com/bigtrak/bigtrak.htm, but that link is now gone.

The first Big Trak I received had moderate damage with tenuously effective repairs. Because the damage wasn't apparent on the outside or before disassembly, I doubt the seller knew anything about it. This means it is possible that you will encounter a similar fate, at no fault of the seller.

On the damaged Big Trak, the encoder board had been shattered and partially repaired years ago. This Big Trak also has a broken axle and broken corner (both glued back on) all physically proximate to the encoder board. Did this Big Trak get stepped on or dropped? The encoder repair was somewhat impressive since the encoder board is located at the maximum disassembly depth. Most consumers would have given up at that point, and discarded the twisted pile into the garbage.

Big Trak infrared emitter and detector board pulled up slightly from the gearbox to show encoder gear with holes.

The encoder is simple, consisting of an infrared emitter (LED) and an infrared detector (photodiode) aimed at each other with a gap in between. One of the gears in the gearbox has holes in it, so that the infrared light can pass through whenever a hole appears. By placing the gear in the gap between the emitter and detector, the Big Trak sees light on and off as the gear turns. By counting the number of blinks, the Big Trak can tell how far the gear (and therefore the wheel) has turned in order to measure the distance traveled or turned.

With a broken encoder, my vehicle never stopped driving or turning since it couldn't count distance. Yet, an unsuspecting seller would correctly observe that the toy could fire, make sounds, move or turn 'just fine'. The idea that the truck was supposed to stop on its own is not something that would occur to someone at a cursory glance. I ended up replacing the broken pieces of the encoder board with a fresh breadboard and a few wires connected to the original components. It works well now -- almost as good as new.

WHAT'S INSIDE

All of the Big Trak electronic components are located on a single motherboard PCB, with the exception of the separate encoder board which is mounted on the gearbox. Various output electronics (expansion plug, speaker, motors, and light bulb) are located at the extremities connected via wires.

The light bulb (sorry, photon cannon) and motors were the only connections fortunate enough to be on removable connectors as opposed to soldering the wires directly to the board. It seems financially prudent that connectors would only be included where necessary to make initial assembly easier. Since I plan to assemble and disassemble my Big Trak over and over again, I found it easier to snip all of the wires and add Molex connectors.

Main PCB (motherboard) with part number and wiring information from the Big Trak.

The motherboard is single layer/single sided. There are between 8 and 10 jumper wires acting as second-side traces on the component side. The PCB does not include a silkscreen (printed layer), so the component numbers are not listed anywhere. As such, the part numbering referred to on the page is mine, arbitrarily based on which IC pin the parts are connected to.

Over the years, a number of people have claimed to have reverse engineered the Big Trak schematic. Although slightly time consuming, it isn't a complex feat. Unfortunately, I haven't found anyone who's shared this information publicly, so I am pleased to do so here.

Schematic / wiring diagram for the Big Trak.

"Newer" (higher serial number) Big Traks use a 32-ohm speaker instead of an 8-ohm speaker. This was an intelligent design change.

The speaker is in series with an 18-ohm resistor to reduce power usage. The power source is nominally 6 V.

In such a setup, an 8-ohm speaker receives peak current of 0.23 amps, with peak power of 0.43 watts. A 32-ohm speaker receives peak current of 0.12 amps, with peak power of 0.46 watts. The higher resistance of the 32-ohm speaker reduces the total current, but because it receives a greater share of the voltage relative to the fixed 18-ohm resistor, the total speaker power is nearly unchanged.

So, this part change produces approximately the same volume of sound for half the current, thus increasing battery life with no functional loss.

THE BRAIN

The Big Trak's brain is a 4-bit TMS1000 microcontroller running at approximately 0.2 MHz (at 6 clock cycles per instruction that's 0.033 MIPS). Members of that chip family were used a lot in calculators and consumer appliances. It cost less than $3 at the time, in quantity.

The program is burned into the 1024 byte ROM and is not modifiable. Because the program is in the microcontroller itself, not a separate ROM chip, it is not possible to replace the microcontroller with just any another TMS 1000 chip. You'd need to pull one from another Big Trak.

Texas Instruments TMS1000NLL 4-bit microcontroller from 36th week of 1979 (7935) and 5th week of 1980 (8005) with MP3301A (Big Trak) program. Note in the example of the right, the part labeling drops TMS1000 and adds 4955, which was the model part number for the Big Trak.

Texas Instruments TMS1000NLL 4-bit microcontroller from 36th week of 1979 (7936) and 5th week of 1980 (8005) with MP3301A (Big Trak) program. Note in the example of the right, the part labeling drops TMS1000 and adds 4955, which was the model part number for the Big Trak.

I don't know if there were different versions of the Big Trak program. The "MP" portion of the label on the chip refers to the particular program. So far, I've only seen MP3301A as the program label for the Big Trak.

The bottom number on the chip label is the two digit year and two digit week of manufacture of the chip. For example, 7936 is the 36th week on 1979. The label establishes only that this Big Trak can be no older than this date. Since custom chips are produced in large order batches, the toy manufacturer would have several months supply all with a similar date code (depending on how many chips could be made in a week). Thus, there could be a significant gap between the date on the chip and the actual assembly date of the Big Trak.

THE MEMORY

The TMS1000 chip contains a whopping 64 bytes of 4-bit RAM (which equals 64 nibbles or 256 bits or 32 bytes of modern 8-bit RAM). Not 64 gigabytes, not 64 megabytes, not even 64 kilobytes -- just plain 64 bytes!

Some websites mistakenly claim the TMS1000 had 32 bytes of 4-bit memory or even as little as 64 bits (not bytes) of memory. But, that can't be the case. There are at least eight commands (FIRE, FORWARD, SPIN LEFT, HOLD, SPIN RIGHT, BACKWARD, REPEAT, OUT) and all but one require a value between 1 and 99. Storing eight commands requires at least 3 bits and a number between 0 and 99 needs almost 7 bits. Therefore, it would take at least (3 command bits + 7 number bits) * up to 16 steps = 160 bits of memory for the longest user program. I tried a bunch of random combinations of commands and numbers to make sure pattern compression wasn't a factor. So, the chip does indeed have at least 160 bits of RAM.

CAPACITORS

I was struck by the low capacitance values of the capacitors. If you add up all the capacitors in the Big Trak, there's less than 1 microfarad of total capacitance. Compare that with all of today's robots running around with 2200 µF capacitors across their battery inputs. Also, the Big Trak doesn't include any fancy low-ESR tantalum, metalized polyester, or aluminum organic capacitors either!

Part of the reason the Big Trak can get away with this is the separate power supplies. While four 'D' cells power the majority of the robot, an independent 9 V battery powers the TMS1000 brain. This electrical isolation helps prevent electrical noise from the motor, speaker, and lamp from resetting or interfering with the microcontroller.

An old axial capacitor that looks like a resistor.

The lowest value capacitors on the Big Trak (C18, C20) are in axial packages with color bands that look like resistors. That's something you don't see anymore.

THE BRAWN

The Big Trak's brawn primarily consists of a 75494 Hex Digit Common Cathode LED driver. This chip is also known as SN27423, SN27914, or National Semiconductor equivalent DS75494. Usually, this chip would drive the individual LEDs that form numbers on a numeric LED display, like on a pre-LCD calculator.

The term "Hex" means this chip has six inputs and outputs. The term "Common-Cathode" means its outputs were designed to sink (0 V) the cathode end of the LED. This chip is not capable of outputting a high (greater than ground) voltage.

Think of the 75494 chip as a six-pack of transistors -- although it actually is slightly fancier as the inputs can be controlled by the weak output pins of early microcontrollers and logic devices. And that's the reason why it exists inside the Big Trak. It takes a weak output signal from the TMS1000 and delivers heavier current (several hundred mA) to thirsty electronic consumers such as the lamp, speaker, and Big Trak Transport trailer motor.

For ease of upgrading, I decided to desolder the TMS1000NLL and SN75494N chips and to reinstall them in DIP sockets instead. Desoldering isn't easy, especially for components with more than a couple of pins. Destructive desoldering (cutting off the component pins with a Dremel and then desoldering pins individually) is easy, but here we have irreplaceable components that we don't want to destroy in the process.

Removing DIP chips soldered directly to the board by heating the board with a heat gun while pulling the chip away with a spring clamp.

Use a soldering iron and a conventional solder sucker (bulb or braid) to remove as much solder around each pin as you can.
Mount the circuit board in a bench vise.
Remove all flammables from the area and have a fire extinguisher nearby just in case.
Open a window or supply some other adequate source of ventilation.
Attach a spring clamp to the ends of the chip you want to pull out.
Heat the targeted area on the soldered side of the board with a heat gun (like the kind used to strip paint) on the low setting. Please be careful! I used a Milwaukee 1220HS which generates approximately 750 degrees Fahrenheit on low. Distribute heat evenly. Keep the nozzle moving up and down the rows of pins on the chip you want to remove. Keep the heat gun away from delicate parts, such as the plastic keypad and your hands. Gloves may be a good choice.
While heating, tug on the spring clamp so that the chip pulls out when the remaining solder has melted on all the pins. This shouldn't take more than ten seconds if done correctly.

My TMS1000 chip pulled out cleanly and neatly. I was so proud!

However, the SN75494 chip had a slight accident, as I must have lightly squeezed and released the spring clamp as I was pulling the chip out. The edge of the chip broke off as the clamp tightened again on the corner, sending the chip flying to the back of the work bench. But, that wasn't the worst insult that the SN75494 would have to endure.

I brought all of the parts upstairs after extraction. Along the way, the SN75494 must have fallen to the tile floor. Back at my desk, I couldn't find the chip and so I figured I had left it downstairs by mistake. As I entered the tile hallway, I stepped on the chip!! That's right. 25 year old chip. Irreplaceable. I stepped on it.

And, you know what?
It made a crunching sound when I stepped on it.

16-pin DIP with bent pins damaged from being stepped on and a broken edge (see arrow) from a spring clamp during desoldering. The pins have not yet been cleaned of residual solder and the exposed portions of each pin show dark oxidation from age. On the right, the pins have been carefully straightened and the old solder has been smoothed or removed.

After I got done with the obscenities, I removed the chip from the sole of my shoe (yes, it was embedded). I then began the long process of cleaning; starting with gently bending each pin back into position. Then, I cleaned the chip with an ultrasonic cleaner with detergent, smoothed/removed the remaining solder with a soldering iron and no-clean flux, cleaned it again with an ultrasonic cleaner and isopropyl alcohol, and then finally nudged it into a DIP socket (which also helped align the pins). Believe it or not, the chip works beautifully.

MOTOR DRIVERS

The 75494 isn't powerful enough to drive the Big Trak's main motors. For that job, discrete (stand alone) transistors are necessary.

Each motor can go forward, reverse, or coast, allowing the Big Trak to go forward, backward, spin left, spin right, and stop. Brake mode is not actually supported, electrically and mechanically speaking. However, due to the friction of the gearbox, the coast mode actually acts like a brake.

The motor drivers are ingenious and little perplexing. Before opening up the hood, I was expecting to see a motor driver chip or four transistors and four diodes for each motor. Boy was I surprised!

Schematic of the Big Trak motor driver circuit with bipolar transistors. Only a half bridge is needed per motor due to intelligent use of a split power supply.

The first sneaky thing is that the Big Trak engineers managed to use only two output transistors per motor. Thus, the Big Trak has a half bridge, instead of a full bridge per motor, yet can still drive each motor forward, coast, and reverse.

The key to this accomplishment was center-tapping or splitting the battery pack into 6 V and 3 V. One motor wire is always connected to 3 V. The other motor wire is either electrically disconnected (no transistors on) for coast mode, electrically switched to 0 V (9113 NPN transistor on) for one direction, or electrically switched to 6 V (9112 PNP transistor on) for the other direction. That's a great trick!!

One potential drawback to this approach is that the motors can only receive half the battery pack voltage at one time (6 V to 3V or 0 V to 3 V; which is 3 V either way). But, that's actually fine if you want to use a motor rated at half the voltage.

Another potential drawback is that, if the designers aren't careful, half the batteries will drain faster than the other half. For example, if the forward command engages both PNP transistors, then both motors will draw only from the second pair of D cells (6 V to 3 V portion of the battery pack). If the forward command is used more than the backward command (which it usually is), then one pair of cells will be exhausted first.

Like in most robots, since one motor faces the opposite direction than the other, it would be perfectly natural to engage one PNP transistor for one motor and one NPN transistor for the other motor. In that case, each motor would draw from a different pair of cells, balancing battery usage for forward motion. Spinning left or right would consume power from only one pair of cells at a time, but as long as you turn right as often as you turn left, it would balance out.

Unfortunately, instead of flipping transistors to compensate for the motors facing opposite directions, the Big Trak designers flipped motor wire connections. As such, the Big Trak sucks power (300 mA to 500 mA under load) from only one pair of D cells when driving forward. If your Big Trak drives forward more than backward (which it usually does), then one pair of D cells will be exhausted first.

OTHER MOTOR DRIVER COMPONENTS

There's a non-polarized ceramic capacitor (C6, C12) across each motor to consume spikes and reduce electrical noise. R6, R12, R13, R15 are current-limiting resistors on the base of each transistor. These parts are ordinary and appropriate.

Now here's where I start to get confused. R5 and R11 are presumably pull-up resistors to make sure the PNP transistors are off by default. R14 and R16 are pull-down resistors to make sure the NPN transistors are off by default. The higher pull-down value (15 kilohm versus 10 kilohm) is probably to avoid overwhelming the weak output of the TMS1000 microcontroller.

I don't understand why pull-up and pull-down resistors are even necessary on current-driven devices (bipolar transistors). Do they provide value as spike reduction paths?

C13 and C15 are equally mysterious. I can't decide if they exist for spike reduction on the lines that connect to the sensitive DMOS microcontroller (as opposed to the more rugged bipolar driver chip) or if they are there to absorb the output of the microcontroller on power up.

Based on oscilloscope tests, The TMS1000 outputs seem random at power up. Therefore, perhaps sometimes the outputs will randomly specify that both transistors turn on for a single motor. Such a condition would result in a brief short circuit (bad). Perhaps the capacitor delays the turn on time for at least one transistor long enough for the microcontroller to switch to the correct output state?

The TMS1000 had an odd way of specifying outputs, so I would think the output matrix could have been set for the Big Trak to prevent it from allowing both transistors to be enabled at the same time. If so, then maybe the capacitor delays the turn on time for the transistor so that the motors won't jerk at power up?

Another mystery of this motor driver is the lack of diodes across the transistors to protect them from motor spikes. (The lamp also lacks a diode across it.) I mean, every classic schematic with a bipolar transistor and an inductance-based device shows a diode across either the device (single transistor designs) or the transistors (more than one transistor). Are the capacitors across the motor leads enough? Were discrete diodes expensive enough to be worth leaving them out? An oversight?

I was unable to locate datasheets or specifications for the 9112 and 9113 transistors. But, if you need to, it seems like you could replace them with 2907A and 2222A transistors respectively. Don Kerste would probably recommend power bipolar transistors for better performance, like the Zetex 718 and Zetex 618 respectively.

Left: Standard 9112 and 9113 transistors in TO-92 packages. Middle: 2N6715 in a TO-237 package. Right: 9112 and 9113 in TO-237 packages.

Left: Standard 9112 and 9113 transistors in TO-92 packages. Middle: 2N6715 in a TO-237 package. Right: 9112 and 9113 in TO-237 packages.

Left: 9112 and 9113 transistors in TO-92 packages. Middle: 2N6715 in a TO-237 package. Right: 9112 and 9113 in TO-237 packages.

Half or more of the Big Traks had 9112 and 9113 transistors in standard TO-92 packages. I was surprised to find a 2N6715 transistor in a TO-237 package (metal heat sink fin at top) substituted for the 9113 in a "newer" (higher serial number) Big Trak. Then, I was surprised again by finding both the 9112 and 9113 in a TO-237 package in an even "newer" Big Trak. And finally, I was surprised yet again to find the 9112 and 9113 back to standard TO-92 packages in the "newest" (highest serial number) Big Trak.

My first hypothesis is that the transistors were overheating and so the designers switched to a package with better heat dissipation, the TO-237. However, that doesn't seem to fit with the return to a standard TO-92 package in the later Big Traks.

My second hypothesis is that they simply ran out of stock of the 9112 and 9113 transistors in TO-92 packages. So they substituted the 2N6715 transistor for some 9113 transistors, and then eventually had to purchase 9112 and 9113 transistors in a different package. I don't know if this is what happened.

THE OUT COMMAND

The Big Trak has an optional trailer accessory (purchased separately for $13 USD 1979) to cart around small objects behind the Big Trak. When the Big Trak reaches the destination appointed by the user, the OUT command can dump the cargo.

Big Trak Transport trailer.

Just like the Big Trak, the Transport trailer is simple, yet ingenious. The Transport consists of:

a 1.5 V 'D' cell (yes, yet another battery)
an electric motor
a gearbox and crank to lift the trailer bed
a 0.15 µF capacitor across the motor for electrical noise reduction
an electrical plug to begin dumping
and a mechanical switch to continue driving the motor until the bed returns to a level position

The dump mechanism can be demonstrated by placing an electrically conductive material (paper clip) across the two plug contacts furthest from the plug tip. This completes the motor connection to the battery, causing the motor to turn the worm-driven gearbox. A crank mechanism connected to the final gear dumps and levels the trailer bed over and over again.

When the plug is inserted into the Big Trak expansion plug port and the OUT command is given, one output from the Big Trak's SN75494 connects the trailer motor wire to ground, just like the paperclip did. (The ground wire from the 1.5 V battery joins the ground wire from the Big Trak's 9 V battery and 'D' cells). Thus, the Big Trak can control the dumping of the Big Trak Trailer.

The OUT command only powers the trailer motor for a preset amount of time (3 to 4 seconds). The Big Trak Transport trailer plug doesn't purposely provide electrical feedback as to the angle of the trailer bed, which would have allowed the TMS 1000 microcontroller to detect when to shut off the trailer bed motor.

Rich Harman wrote to me and explained how the trailer is able to return to a level position by itself. A mechanical switch is built into the side of the trailer gearbox. When the trailer is not at a level position, a spring attached to the side of the battery cover pushes the switch contacts together, which completes the motor circuit, causing the trailer bed to go through its motions. The motor continues to turn until the crankpin (a raised nub on the final gear) pushes apart the switch contacts, thus disconnecting the motor.

The OUT command on the Big Trak need only power the trailer motor long enough for the crankpin to move away from the switch contacts, allowing the spring to push the switch contacts together, thus completing the motor circuit until the crankpin rotates around again. With a fresh battery and a light load, it is possible for the 3-4 seconds of the Big Trak OUT command to rotate the trailer bed beyond a single dumping cycle, such that the mechanical switch carries the trailer bed through the end of a second dumping cycle. However, this isn't a big deal.

The spring on the side of the battery cover has rusted away on one of my Big Trak Transport trailers. The other trailer is missing the spring altogether. So, neither of my trailers are able to continue their dump cycles and return to a level position after the timer on the Big Trak OUT command ceases to power the
motor.

THE IN COMMAND

There's a mysterious command button on the American-model keypad labeled The IN command key on the Big Trak.

. According to the manual, the IN key will be used with an accessory that is not yet available. However, upon internal inspection of my Big Traks, it does not appear that this was ever possible.

The accessory expansion plug port has three possible connections. One is always connected to ground and the second is always connected to an output on the SN75494. The third is not connected to anything on both of the Big Trak toys that I have. That third connection should have been wired to support the IN command.

Three holes for the missing transistor (labeled U1 here) and one hole for the missing wire on the expansion plug port on the Big Trak.

Examining the circuit board in the Big Trak, there's a cluster of three holes that look suspiciously like a place for a transistor. Depending on your Big Trak, there are two unused resistors properly located for pull-up and current-limiting. Near the other two plug wires, there's an empty hole which connects to the current-limiting resistor which connects to the base lead of the missing transistor. Assuming a PNP transistor (U1) was installed, this would be a perfect way for an accessory to send an input signal by simply grounding the third connector on the accessory plug.

Since at least some of the Big Traks shipped without this transistor and with the third wire on the accessory plug cut off, then those Big Traks were simply not capable of supporting the IN command with an accessory, contrary to the instruction manual.

IN EXPERIMENT

I added a wire to the third connection on the accessory plug and connected it to the empty hole on the motherboard. I then installed a 2907A PNP bipolar transistor to the group of three holes on the motherboard. This particular Big Trak already had the resistors and jumper traces.

According to the manual, the IN command does nothing other than skip the next step. I had hoped that with this extra hardware installed, that the IN command would act as a conditional statement (like an IF) by not skipping the next step when the third connection on the plug is grounded.

The Big Trak performs normally until the IN command is reached. At that point, it either skips the command (third connection not grounded) as usual or pauses indefinitely (third connection is grounded) until the third connection is not grounded. This demonstrates that this portion of the circuit definitely affects the behavior of the IN command.

Regardless of what command followed the IN command (OUT, FIRE, HOLD, move), the next step was skipped. I'm perplexed as to what value the next step provides.

At this point, it looks like the only action performed by IN command is to pause and wait for some sort of input to continue.

IN QUESTIONS

Was the IN function supposed to work this way?
Or had the programmers run out of time / program ROM to complete this feature?
Or was there a bug in a batch of these unmodifiable chips causing the manufacturer to abandon their original plans?
Or did an extra transistor affect their profit or key price point such that they figured they should just drop support of the IN command? (How much did a PNP transistor cost in 1979?)
Or did someone in marketing or a major retailer decide that one accessory was enough?
Or did they actually intend to release the accessory later and then realized they trapped themselves on the day they first shipped an "incompatible" Big Trak without the transistor?
Or was the IN command just so lame that they couldn't come up with a useful accessory?

It seems odd that the resistors, jumper traces, holes, and the third plug wire (which was then cut off) remained in production for some Big Traks when the lack of the transistor made those components completely worthless. At some point the manufacturer apparently realized this and completely eliminated all of the unused parts.

The support resistors and jumper wires (left) disappeared (right) from later Big Trak boards. The color change in the capacitor is unimportant.

Shortly after writing this web page article, I acquired my third Big Trak, serial number 1007748. Given the early date on the chip (7923) and the pre-printed "1", I've come to the opinion that this is actually the 7748th Big Trak manufactured.

This early Big Trak includes the "missing" transistor, a 3906! The 3906 is a PNP bipolar transistor, which matched my educated guess. (However, I had figured it would be a 9112 to keep the parts consistent and save money due to volume.)

Now comes the surprise. Despite the inclusion of the resistors, jumper traces, and transistor, the third wire to the accessory port was cut off! Why? Why do that? Why incur the cost of including all of the parts, only to remove the required wire? This pretty much rules out my hypotheses on the manufacturer originally eliminating the IN function to save manufacturing costs.

Grounding the third wire on the early Big Trak causes the IN function to pause indefinitely, just like my other Big Traks. That would be a pretty lame feature and doesn't explain gobbling up the next command step.

So, here is my current favorite hypothesis:

The designers and manufacturer intended for the Big Trak to include a proper IN (input) command. The box, manuals, keypad, and circuit boards were all designed with this in mind.

A large batch of custom TMS1000 MP3301A microcontrollers was ordered.

Board assembly began, with passive components (jumper traces, resistors, etc) added first, and delicate semiconductors (transistors and chips) added last.

When the custom TMS1000 microcontrollers arrived and were installed, it was discovered there was a bug or defect that prevented the IN command from working properly. Since this couldn't be corrected (ROM isn't modifiable) and since the custom microcontroller was probably one the most expensive and longest-lead-time parts, they decided to go ahead as-is.

The third wire on the accessory plug might have been cut off to save the labor cost of soldering it on, assuming it was one of the last items added. Or, they might have been concerned that the IN command defect would cause interference with the trailer accessory. In any case, some boards still included the transistor because they had already been completely assembled. And, some boards included only resistors and jumper traces (but not the transistor) because they had already been partially assembled.

Keypad for later Big Trak missing hole for IN key.

The keypad on a later Big Trak is missing the hole for IN key.

In later Big Traks, the keypad plastic was altered to eliminate the hole necessary for the top pad and bottom pad to make contact for the IN key. In other words, pressing the IN key does nothing on these Big Traks, since the plastic insulator prevents the key from making an electrical connection.

The pictures I've seen of the European Bigtrak even eliminate the image of the IN key on the keypad.

BOARD REVISIONS

Revision information is on the back of the Big Trak printed circuit board. Thus far, I've seen REV C, REV D, and REV E. Apparently the European version of the Big Trak has a REV L board!

Two changes between the REV C and REV D boards are the patterns around the screw holes and the location of a jumper wire hole (see arrow).

The REV D board includes dot patterns around the screw holes. This may have helped hold the screws in place against vibrations. Or, the patterns may have reduced over-tightening or provided stress relief to prevent board cracking.

Additionally, one jumper wire hole changed from REV C to REV D. This may have been to reduce the length of the jumper wire or to make it consistent with the length of some other jumper wires on the board.

By moving a hole, physically smaller capacitors or shorter lead lengths could be used on the REV E board.

(REV E photos courtesy of Pablo Bleyer Kocik).
By moving a hole, physically smaller capacitors or shorter lead lengths could be used on the REV E board.

Another hole changed from REV D to REV E. This permitted the length of the leads on capacitor C9 to be greatly reduced, improving the effectiveness of the capacitor in decoupling/bypass. Also, it's a lot easier to insert the capacitor straight into the holes, rather than bending the leads to exactly the correct length.

The REV C and REV D boards I've seen have green solder masks. The REV E boards I've seen have a red solder mask.

REPLACEMENT PARTS

The photon cannon miniature incandescent light bulb (lamp) is a 2.25 V, 0.25 A, #222 GTL-3 screw base. It can be replaced with part number 1505K41 at McMaster-Carr or 606-CM222 at Mouser Electronics or 209533 at Jameco Electronics.

These bulbs measure around 1.2 ohms in resistance. Given that the Big Trak drives the bulb from 3 V (or up to 3.2 V with fresh batteries) and doesn't have a current-limiting resistor, at first I wondered how the current limit of 0.25 A was observed. It would seem that 3.2 V / 1.2 ohms = 2.66 A would be delivered. The 75494 is unable to deliver that much current and the internal resistance of the alkaline 'D' cells would also be a limiting factor. However, the fascinating truth is that the bulb filament increases in resistance (to around 9 ohms) when lit!

Left: Original zinc-plated Big Trak screw. Right: Acceptable stainless-steel replacement lacks cutting tip.

The Big Trak screws are zinc-plated steel, #4 (4-24) threaded, 3/8 inch length, type 25 (ASA-BT) point, pan head, Phillips, thread-cutting screws. I would prefer stainless steel, but I have not yet located a retail supplier of that exact type of screw in either type of metal. The screws make their own threads by cutting into the plastic when driven. This saves the cost of threading the screw holes by hand.

The closest match that I've found for this screw is 92470A108 at McMaster-Carr. The McMaster-Carr screws are stainless steel (which is superior to zinc-plated) but lack the thread cutting point. This shouldn't be an issue since the threads have already been cut into the plastic by the original screw.

Left: Screw hole with threads. Right: Screw hole without threads.

Higher-serial-number Big Traks are missing some screws!

Some Big Traks only have 4 screws holding the main body to the base, instead of 6 screws. The middle 2 screws are missing.

Some Big Traks have only 2 screws holding down the circuit board, instead of 4 screws. The 2 screws on the opposite corners are missing.

You can tell if your Big Trak originally had all of the screws by looking in the screw holes. If any screw holes are missing threads, then they never had screws installed in the first place.

The full compliment of screws really isn't necessary. I suspect this was a money saving move by the manufacturer, not only saving the cost of the screws, but more significantly saving the labor cost and time.

Another item missing from higher-serial-number Big Traks is the cardboard insert underneath the keyboard. You cheap so-and-so!

Left: Rubber o-rings on the Big Trak provide traction. Right: The o-rings crack with age.

Each of the middle wheels (the other wheels are not connected to motors) has a #232, 1/8 inch width, 2 3/4 inch inner diameter, rubber o-ring for traction. The rubber tends to crack over time. I replaced mine with Buna-N (the least expensive material), part number 9452K161 at McMaster-Carr.

When you receive the replacement o-rings, don't worry if the diameter seems a little too small to put on at first. They stretch.

Lastly, if you lose the hex nut for the accessory plug, it is a 1/4-32 NEF (National Extra Fine). Part 91862A516 at McMaster-Carr is an adequate replacement, although the original nut has slightly larger outside diameter.

HOW MANY BIG TRAKS WERE THERE?

The Big Traks I know about have the following information:

Serial Number	PCB Rev	TMS Date	75494 Date	Speaker	Motor Transistors	Notes
1007748*	C	7923	7919	8 ohm	TO-92	Has U1, R1, R2, jumper traces, plug wire stub
1129128*	D	7932	7919	8 ohm	TO-92	Has R1, R2, jumper traces
1180691*	C	7936	7919	8 ohm	TO-92	Has R1, R2, jumper traces, plug wire stub
1238030	D	7936	7936			No IN circuitry.
1411838*	D	8001	7927	32 ohm	TO-92	No IN circuitry.
1438262	D	7936	7923	32 ohm	TO-92	No IN circuitry. Sticker with battery.
1706580	D	8005	8001			No IN circuitry.
1755076*	E	8014	7919	32 ohm	2N6715	No IN circuitry or key hole. Sticker underneath. Missing 4 screws.
1791200	D	8014	8018			No IN circuitry.
1806157*	E	8018	8018	32 ohm	TO-237	No IN circuitry. Missing cardboard and 4 screws.
2045315*	E	8014	8035	32 ohm	TO-92	No IN circuitry. Missing cardboard and 4 screws.

*Currently own.

If reading this article motivates you to open up your Big Trak, would you take a minute to jot down anything new you discover about your Big Trak and email it to me?

I'd be particularly interested in hearing from anyone with circuit board revisions other than C, D, or E or serial numbers less than 1 million or greater than 2 million. Also, I like to hear from anyone that has third wire from the accessory plug factory soldered to the board with the IN command transistor, or anyone who can figure out how to make the IN command useful.

Big Trak silver-colored serial number sticker. The first '1' digit (see arrow) appears to be pre-printed.

The serial number appears on a silver sticker located in the 9 V battery compartment (earlier models) or underneath on the gearbox (later models).

The first digit (1 million) appears to be preprinted on the sticker. Either they were trying to make it look like they were shipping a lot of Big Trak toys (likely), or they had already made a million in the middle of 1979 (unlikely). Sometime in 1980 or later, they either had made at least a million more Big Traks (likely), or they were incrementing the serial number in funny ways.

Recently, I was fortunate enough to obtain a Big Trak with a serial number in the 2 million range. I suspect these are much rarer, comprising as few as 5% of the total Big Traks produced. This Big Trak has four unthreaded screw holes and a few other distinguishing characteristics. This makes it harder to falsify than simply attaching a fake serial number sticker to an earlier Big Trak. (I have chosen not to post a picture of the 2 million serial number sticker to avoid fakes.)

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