Tetris (Bootleg) Repair Logs
Took pity on an old Tetris bootleg board that I acquired; think it was listed as having corrupt graphics.
Fired it up and that was pretty accurate!
Had already spotted the likely candidate though; a big scratch on the otherwise mint solder-side of the board.
As usual the tracks that looked the worst were actually OK and the ones that looked like they had survived were totally cut through, only 3 in fact. So I soldered in short lengths of hookup wire to bridge the cut tracks, also resoldered a very dry joint on one of the main smoothing capacitors, the solder had never wetted the leg of the cap and it was totally loose on that leg.
Plugged it all back in and fired it up...
Another nice easy one.
Symptom: No audio
I have received this board as a present from a friend, as a consequence of the video I showed him about the Tetris bootleg I own, with the UM3482 doorbell chip. He said: “...no, please, stop offending this classic! Leave that funny board alone and play this one which sounds as Nature intended.” LOL!
This bootleg emulates in standard TTL logic both the Slapstick and Atari's video custom IC, but has two proper Pokeys for the sound.
As I powered the board I couldn't hear any audio from the speaker, no matter the volume pot setting. I fired up the scope and analysed the cursor of the volume pot. The audio signal was present there. Moving downstream I reached up to pin 1 of the NEC µPC1181H and still could see the signal. The power amplifier was extremely hot, but its output was showing amplified audio. Going further downstream I found a 220µF 16V capacitor that measured as 18pF with an ESR of 14kohm...
Once replaced that capacitor, as well as another one that was in the same condition but was in charge of filtering the 12V power supply, I looked further downstream to check if there was any other mess in the way before reapplying power. I found out that the speaker pins at the Jamma connector were inverted: the top one was grounded while the bottom one was wired to the power amplifier. Some cabinets might not be happy with this, therefore I altered the PCB traces to comply to the standard setup: speaker + on top, speaker - on bottom (ground in this case).
At this point the game was happily playing music and sound effects. Happy ending, or so it seemed... I fired up the service mode to check the input ports and adjust the game difficulty and found out that the right coin slot was sensed by the CPU as permanently shorted to ground. I checked both the R-Pack that pulls the signal up as well as the smoothing 100nF cap without seeing anything strange. The coin signal goes straight to one of the Pokeys, therefore I tried swapping them. Now the game would move automatically from one test to the next in the service mode, because this time the player one rotate button was permanently sensed as pressed. No other chance but to replace the Pokey.
This is the time when I noticed that the service mode was complaining about bad CPU RAM. Although the game was seemingly playing fine I replaced the chip for good measure with a compatible part taken from a scrap board and the service mode stopped complaining about it.
Very quick and easy repair completed in two fun hours, although a bit unlucky all considered: new old stock Pokeys aren't cheap! In the picture below you can check out which parts I replaced.
Now I can enjoy Tetris music! I shall really take a recording of it, for the next time I'm loading my car with luggage for holidays... :D