Hi all, first post on this forum. I've been doing a lot of searching for this problem I'm having and I've come across a post or two that were rather close but not quite exactly the same. I'm trying to repair my bass players Hartke 3500 mosfet amp. I'm by no means an expert at this (more of a beginner trying to cross into intermediate territory), and I'll do my best to describe the issue in as much detail as possible.
So this amp was being used by my bass player as well as the best player in another band. My friend went to use it one day and discovered that the other guy had the master volume cranked waaaay up, and the speaker cabinet was plugged into one of the effects slots (I assume the send) instead of the speaker out. So when my friend went to hook it up correctly, he longer was getting any signal and instead was just getting a loud white noise-like, WOOOOSH sound. I told him I'd take a look at it, and I'm at the point where I've pretty much narrowed it down to it being a problem at the preamp. First off, the master volume control is able to control the white noise level. Second, I input a signal generator with a 1 kHZ sinewave into the passive and active input slots and they are coming through the speaker loudly, but with the white noise sound also coming through louder and when I adjust the preamp tube and solid state knobs, they do adjust the volume of the signal generator. Also I read that you can determine whether it's a preamp problem by inputting the signal at the effects send input. I tried that with the bass guitar, and I am able to get a signal through there. It's weak, but it's there and it's louder than through the preamp. Things I've done so far: Replaced resistor R130 because it looked burnt, no change. Replaced the single tube, no change. Tested the 5 output transistors out of circuit, they checked out good. And that's about it so far. So at this point I'm thinking it's most likely in the preamp, but where should I start? Could it maybe be a bad op-amp? Filter capacitor? If anyone has any idea on which direction I should go at this point, it would be greatly appreciated, thanks!
