rocknroller
Active Member
Picked up a CR400 on CL. Has random noise on both channels, different levels and at different times. Sounds like heavy paper rustling with loud pops. Can be perfectly clean then start minor and get really bad, then clean again. Can go for minutes in any given stage,on either channel. I do measure some peaking DC voltage on the speaker terminal when it's really bad (meter might shows 1.5 or even 2-3 volts for a fraction of a second, probably when it's making the loud pops). I believe I have ruled out every but the power amp board itself. When it occurs, it is irrespective of any front panel selections and the preamp volume can be completely off and it will still happen. With audio tester, I confirm the input from the preamp to the amp is clean and unaffected. The problem is occurring in a later stage of the amp. At least as earlier as the collector of TR609 and that path (see schematic). Definitely heavily audible along there (when it is occurring). Thinking it might be a cap and given several measured out of tolerance (uf and in some cases esr too) I have changed out all electrolytics that were 10uf and higher, except the 3300uf power caps - they do measure way out of spec about 5500uf, but esr meter shows good - also did a temp replace with a new 4700uf on one of them (closest I had) and no change to the issue on that channel at all. The amp is also passing the preamp signal all the way (when preamp turned up of course) and the signal from the preamp sounds good, but there is this underlying random noise that is clearly occurring and at times dangerous to the speaker when it gets loud and really starts popping.
Thoughts on where to measure/check in the circuit?
Thoughts on where to measure/check in the circuit?