Mar 282016
 

I’ve spent what has seemed like months trying to repair a Sega/Gremlin Frogger PCB. I found the schematics fairly difficult to read, as they use codes to show connections between chips, which is not what I am used too

The board itself had a constant resetting CPU, and I was not able to get it out of this state. Even when I disabled the watchdog the board booted to the same crashed state each time.

I got to a point where my patience had run out, and I asked a fellow collector Sarj if he had a spare working PCB I could compare against. Luckily he had about five sets, and was able to kindly pop one in the post for me to examine.

When I received his board, I was able to swap his sub and logic boards for mine, and use a different interconnect cable between both boards.

To my surprise my main logic board worked fine with his sub-board. Then I tried my sub-board with his main logic board, and found that I was unable to credit a game, and could not test sound, so there was at least one fault here. This made me think that there was issues with my interconnect cable or cable headers. So I tried my own logic board and sub-board with his interconnect cable, and to my surprise the PCB was working!! No CPU resetting!!

So I then used my own interconnect cable, logic, and sub board, back as they were originally. I expected the game to not boot, but it was still working!! Very bizzare!!

The only explanation I can think of is that there was a dirty pin on one of the headers and using Sarj’s interconnect cable had cleaned that somehow. I unplugged and re-plugged my own interconnecting cables many times before this, so it is very odd that the board should now boot as normal.

I put Sarj’s board to one side, now that my own PCB was booting, and found I was still unable to credit a game, or hear any sound. So I looked at the schematics and could see that pin 37 on the 8255 @ IC7 was toggling when I was trying to credit the game, which meant that the resistor array, and capacitor the input went through was working. I could then see that the LS00 @ IC41 was showing no signs of the game being credited at pin 8, and the second 8255 @ IC40 was not being selected. I pulsed some of the input pins on the LS00 to check if I could see any logic changes on the outputs, and any changes at the 8255, and it looked like they were both working correctly, so I suspected that the 8255 @ IC7 was not working as expected. I desoldered this, and checked it with my Chipmaster and it had failed. I replaced it with another 8255, and the game could now credit

I was then able to test the sound and I could hear nothing at all which made me suspect the M51516L amp

It must be noted out at this point that if you don’t have a volume pot connected to the 6 pin header on the sub board you will get no sound. Thanks to Sarj for providing me with one

Even with a pot connected I could still not hear anything. So I swapped the sound chip, an AY-3-8910, for a working one and this made no difference, so I started prodding the pins at the LM741 op-amp. If the main amp was working I expected to hear some noise when prodding the op-amp pins, and I heard nothing. So I desoldered the M51516L amp and fitted a new one. To my relief the PCB was now booting, crediting, and I could hear sound 🙂