Coleco Telstar Arcade Repair

The Telstar Arcade was a cartridge based video game console released by Coleco in 1977. Unlike later cartridge based systems the cartridges for this system didn't just contain program code, but instead used the MOS Technology MPS-7600-00x chip series which contained a simple CPU, code ROM and some custom hardware for generating video that may have been different in each cartridge. You can find a good article here about this chip:

http://oldvcr.blogspot.com/2022/09/confirmed-mos-76007601-pong-chip-is.html

I acquired one of these systems a long time ago but never bothered to try it out. I recently dug it out and tried powering it up. I could not find the power supply for it which is 9VDC 200ma, so instead I used an Atari 2600 power supply which if 9VDC 400ma and has the same connector and polarity. 

I connected it to a TV and when I powered it up I got no change in the display. Even if a video game system has major problems you will usually see some change in the screen. I disassembled the unit and checked the connections between the boards and everything looked ok. The next step was to check the power. 

The power from AC adapters enters the main board on pins 10 and 11 of connect J3. That is the left side of the connector in this picture.

With the power switch off I can see the 9VDC at this connector and it also makes it to the switch. When I turned the switch on the input power dropped so something was pulling it down. Before I continued troubleshooting I traced out the circuit for the power regulator on the main board:

This is a simple voltage regulator based on a Zener diode, CR3. Here is a picture of that part of the board. The leg on the transistor is lifted because I took this picture when I was testing something. 

With the circuit mapped out I check the components with an ohm meter but nothing looked obviously bad. I tried lifting the emitter leg of the transistor to see if that helped but I was still not getting anything out of it. I ordered some replacement transistors and replaced the one on the board with one of those. When I powered it up I still wasn't getting any output from the transistor, and then I started seeing smoke from the component side of the board. Took me a bit to find the burning component, but it turned out to be the CR3 Zener diode and this also burned out the new transistor. 

Now I had a problem. The diode was burned so bad that I could not read the whole part number, all I could see was 1N75 which told me it was a Zener diode but the last two digits tell you it's operating voltage. I also could not find schematics for the system, so I have no way of knowing what voltage the Zener is. 

I happened to have some surplus 1N75 series diodes so I put in a 1N7532 which is a 5.6v Zener and replaced the transistor one more time. With those changes I finally got a 4.4V output from the transistor. This is as far as I have taken the troubleshooting for now. I would like to figure out what the proper diode is. If you are reading this and have access to one of these I need to either know the full part numbers on CR3, or need a measurement of the voltage between the two points shown below.