Carlsbro Cobra 90 Lead - Reverb pot acting like a volume pot

[Ukdigger]

Just bought this amp for £10. Reverb pot acts like a volume pot on both high and low inputs and treble pot has high pitch whine when turned up above half way, but only on low input. Any help would welcomed.

[redwire]

I would clean the jack switches with DeOxit. You can pull a small piece of paper through the contacts to clean them as well.
See what signal you get at the effects-loop jacks, the Send and Return.
It might have no dry signal, only reverb signal and that is what you are hearing.
The schematic is half a page, where is the rest?

[Ukdigger]

Tested all the weird looking grey caps and didn't get any readings from these ones, in either direction c1 c11 c12 c13 c14 c17 c21. All the others gave a reading or started charging up. I cleaned all the pots and put it back together and all my problems went away. Don't know if I disturbed something or just got lucky. Thanks for your help, I will update if anything changes.

[sleizen]

Crazily I happen to be working on the same amplifier; although mine is the mk1 (no green on the control panel ).
You can do me a favour, I need images of how the power amp links to the control panel pcb, and the power amp wiring generally, colours, etc.
. Regarding your amp, I notice the reverb output cable to pin 8 is missing (it's the reverb ground wire- there are 4 connections to the pcb from reverb tank, in out shield shield). That would account for the noise floor you're getting when reverb turned up. Those accutronics reverb tanks , they corrode bad, so flaky sig/ground connections can fuck up the reverb or make it sound ghostly quiet, etc, etc

[Ukdigger]

Thanks for the info, hope these pics can help you.

[Ukdigger]

I resoled the reverb output cable ground wire to pin 8 and now I've got the reverb pot acting like a volume pot again on both low and high gain inputs. Also the treble pot has a high pitch whistle on the low gain input at any setting, but is ok on the high gain input.

[sleizen]

Yeah, mine does that too... ..and mine cost £20!! In the case of my cobra 90, this is probably due to the lack of proper power amp section. When the signal is sent to the power amp, there are smoothing caps and god knows what else, preparing the signal before it goes to the output. I don't have any of that, so it sounds like dirt.

[sleizen]

also I found the suzz/gain and reverb controls all affect each other- even with a previous carlsbro amp which was fine. A bit unpredictable, you could say

[Ukdigger]

Just took the reverb tank out and checked everything was ok, when I put it back in I reversed the input and out put connectors and everything works as it should now. Think someone must have put them the wrong way round