PCB Repair Logs Truxton
Got a few of Bwodie's boards here at the moment, 1st one off the pile is Tatsujin aka Truxton. Fired it up on my Commodore monitor on the test bench and couldn't see anything wrong with it. The JAMMA loom here is partially knackered and is missing all the control lines, so I plugged it in to my cab so I could coin it up.
It gave a load BLARK noise and booted up normally, but on a larger screen it was obvious that the whole video output was shimmery, the game coined up and played but after about 20 seconds the sound got really loud, then very quiet and then turned into a metallic screaming noise.
A hot amp and interference with the video signal sounded like a problem with the amp circuit, the amp chip itself, or both.
Went round the electrolytic caps in the amp section and soon found the culprit, a single 470uF 16volt cap showing an ESR of 67 ohms, it should have an ESR of 0.2ohms.
The scruffy looking yellow cap is actually fine, but its been bodged onto the legs of its predecessor, as the plate through holes look like they have taken a pounding and I could see that one of the tracks it connects to is one of the inner layers of the board I decided to leave it well alone, its firmly fixed and doing its job, if I remove it the last remnants of the plate through hole may come with it and then it would be a nightmare to make the internal connection again.
After being desoldered the capacitor was registering as open circuit, which means it either just died, or always was dead and the 67ohm reading was from a proximal pathway in the circuit and wasn't the cap at all.
Slapping in a good cap fixed all problems...
...no more squeal on power up, no interference in the video, no crazy volume changes and the amp chip is now a healthy warm when running. I would assume that the amp chip started oscillating, causing high levels of interference that showed up in the video output. It would also cause the amp to get very hot which would trigger its thermal shut-down (if it has one), am surprised the amp chip was not totally fried actually, it wouldn't take too long before it roasted itself out of existence.
Thought I had one more fault left, there is an odd feature with this game, when you start and the music plays as you take off, your shots are silent, as are the first few enemy kills. Once the tune finished you get all the sound effects back, MAME does the exact same thing so its clearly a "feature" not a bug.