I'm pretty new to vintage equipment and very new to repairing it but I am eager to learn so I bought myself a dead Drumtraks recently in the hopes of learning a few things about diagnostics and repair. Luckily I have a neighbor who is a retired electrical engineer who is helping me but he has no experience with instruments and little experience with chip-based electronics.
It came with some paperwork from a shop that tried to fix it but the guy I bought it from didn't want to pay the $325 they were asking to fix it. According to their paperwork, they replaced the OS and an LED driver and then suggested sending it to Wine Country to have the CPU looked at. I put my Prophet 600's CPU in there (they both use a Zilog Z80) but that didn't solve anything.
We replaced some caps and a voltage regulator in the power supply section but it didn't power up until we swapped some of the RAM chips around. Just to be safe, I replaced all 4 RAM chips but didn't get any increased functionality from it. It currently powers up and makes sound (triggered via MIDI.) However, the only buttons on the control panel that work are some of the drum trigger pads. The sequencer-related buttons, pots, and number pad do not work. The LED works but displays r2 50. I checked all the ceramic caps since most people online recommend that and they all register a value. I'm not really sure what to look for other than 0.00V. They all have a +/- 5V or +/- 15V value. My neighbor thinks the problem lies in the chips which of course are hardest to diagnose. Can anyone help me with some voltage readings that each chip/pin is supposed to have so that I can narrow down the bad one or have any other suggestions?
Lastly, I only have a multimeter, no scope or freq counter, so that will probably make things harder to diagnose... Thanks for any help!