Hi there!
I've been searching / reading other users CP-70 issues here, thank you all for helping others out! I've got an issue with mine that is puzzling me a lot:
I bought this guy from a friend. Had no idea CP-70s existed until I saw it, and it's perfectly weird and great for me. He said the output was not working, but that it did work right before it was moved from his grandfather's house to his. So I think, "hmm, probably just a jack solder knocked loose in moving or like such as."
The output jacks are working, but the signal is very very low and noise is super high, on both XLR and 1/4". I did a visual check of everything and poked around in the pre and power amp with a chopstick with the piano on, trying to find a loose connection, but everything seems good. Capacitors seem to all have been replaced with modern ones, which is great.
I've been using the effects loop to try and diagnose, and this is what puzzles me:
1. Play piano with amp plugged into EFFECTS OUT ->- Pickups sound great, normal signal, no noise/hiss at all
- Okay, so the pickups / 1st half of the preamp circuit and power section are good
2. Play signal (iPhone) into EFFECTS IN with amp plugged into 1/4" OUTPUT jack ->- iPhone signal comes out fine, some noise but not overwhelming
- Volume knob working correctly, though dirty
- Tremolo works fine
- But it's hard to tell the iPhone gain vs. CP-70 pickups gain, maybe not a fair test of 2nd half of circuit?
I used one of these signal trace tools like you would with a guitar amp
http://musicfromouterspace.com/analogsynth_new/HOT_TIPS/ampsignaltracer.html...in the preamp and I've found some quiet places on the board, but nothing that sounds super broken or one component that's really losing the signal. So I'm at a loss of what to do next!
Looking at the schematic, all the outputs have the same quiet levels, so maybe it's something that effects all outputs, like Mosfet 4 or 5? But using the signal trace on those, seemed like they are good (see attached)
I know this is almost more of an amp/board question, but any suggestions are appreciated!
Thank you!