Cook Race (BurgerTime Clone) PCB Fix

 Technical Info  Comments Off on Cook Race (BurgerTime Clone) PCB Fix
Feb 222015
 

I was generously given this PCB for nothing, and thought I’d try and get it working as I really like the game BurgerTime.

I started by building a Jamma adaptor for video, power and sound using the pinout below (Note the equivalent Jamma pins are in brackets):

On initial powerup I got the following:

https://flic.kr/p/rafQKE

I could not credit the game, nor could I hear any sound.

The PCB had quite a bit of corrosion on a lot of chips, so I cleaned up the legs or replaced the IC if it was really bad. I did end up replacing quite a number of chips. One chip, an LS138 @ 2L on the top board had also failed, so I replaced this with a working one. Also I could not find any schematics for this PCB, but it did seem similar to Burgertime in layout, so this would help.

After quite a bit of work there was much improvement

So the problems I still had were:

1) I couldn’t credit or start the game
2) The game has lines through it
3) There was no sound as far as I could tell without crediting the game
4) The game doesn’t consistently boot. When it doesn’t boot I just get a black screen with white lines:

https://flic.kr/p/rcydEK

So first thing I did was try and work out why the game wouldn’t credit. I had a look at the edge connector and found three broken tracks so fixed them up:

So I now got this and could credit a game. This confirmed I had no sound:

https://flic.kr/p/rafMfU

So I had a look at the sound section:

I swapped the 6502 for the 6502 on the main PCB – no difference. I also swapped the sound chips around and no difference.

Also I checked the larger capacitors in the sound section and they seemed ok. The amplifier also sounded like it is working. So I was stuck on problems 2, 3 and 4.

I did also try testing the larger caps on the main PCB, as I thought this might explain the inconsistent boot, but they also looked fine.

With the help of Philmurr from UKVac providing some schematics from BurgerTime we noted that the 6502 CPU and AY3-8910’s were doing very little in terms of activity on the sound address and data buses.

The next time I looked at the PCB I realised that the 6502 looked as if it had no power. Vss (ground) on pin 1 and 21 were low as I expected but Vcc (+5) on pin 8 was also low ?!?!!?!? Instead I measured pin 21 and pin 38, pin 1 and pin 38, both giving 4.79v, so it looked correct. I started to think that pin 8 was unused, but the datasheet said that it was a +5 pin. I was confused.

Phil noted some corrosion on the photo right next to pin 8, or thought there could be a “hidden” track underneath the socket of the 6502, so I removed it and took a picture of it so I could see where that corroded track went. Pin 1 of the LS74 and pin 2 of the 6502:

Phil pointed out that this was the LS74 which is 3F on the BurgerTime schematics…..which makes that heavy rail +5V! Clearly the track has been damaged to pin 8! So with this fixed I got the following:

https://flic.kr/p/rgmwsA

I spent another hour on the PCB trying to fix the white line issue. Having done some reading around white lines/jail bars, I came to the conclusion it was probably a ROM issue. Several articles on different PCB repairs mentioned mask rom problems in relation to jail bars, so I thought I would start re-seating all the roms, despite not having any mask roms on the board. The PCB also had them one-sided pin sockets which are fairly poor, so I thought it wasn’t a bad place to start.

As I knew the sound data was held on the eprom labelled 6 I left that alone, and instead started by lifting out eproms numbered 5, 4, 3, 2, and 1 from the top board separately. Removing number 5 and powering up the board resulted in colour/sprites missing, but the lines were still visible. Damn! Lifting out number 4, and the same thing occured but with different colours/sprites as expected. However when I lifted out the number 3 eprom the lines disappeared!

So reseating eprom number 3 and booting up the PCB generated a game with no lines! However now there was no sound!!!!

So I thought I would lift eproms 4 and 5 out again, and see if that would make a difference as that was the only thing that I did, and I got this:

https://flic.kr/p/rj179C

So it looks like the sockets are causing the issues. I’m also wondering if this is causing the game to fail to boot sporadically? I probably need to change them

Anyway I would say the game is fixed now, so thanks for those that provided help and guidance!

Logic Comparator

 Technical Info  Comments Off on Logic Comparator
Feb 142015
 

Today I’ve finally started some work on a self build logic comparator. This is almost identical to the HP 10529a comparator, however instead of only supporting 16 pin logic chips, which is the maximum for the HP, this self build unit will support upto 20 pins. Still got a lot of parts to buy and fit to finish it off, but soldered the first 14 sockets on today, so I’m on my way 🙂

It’s going to be a really useful tool with repairing PCB’s because essentially what it does is compares the outputs of a chip in circuit to a known good chip placed in the tester. It’s perfect for someone who wants to learn more about what the chip functions are and for fault finding chips that can be hard to troubleshoot without a logic analyzer such as decoders (74LS138/139) and various flip flops (74LS74)

The other good thing is that this also supports the standard cards used in the HP comparator, so is backwards compatible 🙂

Logic Comparator