Sega/Gremlin Frogger Repair

 My Projects  Comments Off on Sega/Gremlin Frogger Repair
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 🙂

Vectrex Repair

 My Retro  Comments Off on Vectrex Repair
Mar 062016
 

I bought a Vectrex system made in 1982 on an Facebook retro group a couple of weeks ago fairly cheap, that had a strange fault where the game would loop on the internal minestorm game. Here is a video of the fault.

My guess was that it was an issue with the rams (which are 2114 rams) or an issue with the rom.

Yesterday I took it apart, which is not an easy task, and removed the rams and fitted sockets. Both rams tested ok. I then removed the rom which was a D2763-4. I managed to read it as a 2764 eprom on my programmer, and checked it against the bios “exec rom intl 284001-1” from Mame, which showed it had one “bit” of bad data! I went a step further and used the rom data on Mame to see what it did. Unsurprisly it crashed at the same point as the Vectrex, which confirmed the rom was at fault. I re-burnt a new bios file to a 2764 eprom, put the Vectrex back together, and I now have a working system 🙂

Nintendo Balloon Fight PPU rom mod

 Nintendo Vs  Comments Off on Nintendo Balloon Fight PPU rom mod
Mar 052016
 

Today I’ve been testing out a rom mod for Balloon Fight on the Nintendo Vs. system to see if it is possible to run the game on different PPU’s on each side of the board, and to check if the rom mod works on original hardware:

Ballon Fight Rom Mod