Krank Krankenstein volume drop issue

[Petey Wheatstraw]

Hey everyone, great forum. I'm hoping one of you may have come across this issue before. I'm working on a Krankenstein Plus amp and on the "dime: channel, whenever I increase the master volume past 25% the audio drops out. The clean channel works fine. I tested the pot out of circuit and it reads fine, I even went as far as to swap the pot with another 1 meg pot and it didnt help. I checked the pre amp tubes by swapping with a known good tube one at a time, and I'm assuming the power tubes are ok because the clean channel is working fine. I checked for loose cables and components and took out the main board to check for solder defects, everything looks good. It seems to be a pretty well made amp, but I'm having the toughest time figuring out this problem.

Any idea's would be much appreciated.

I have a full bench set up, been working on amps over 10 years and recently started doing repairs at home on the side.

[52 Bill]

Sounds like a parasitic oscillation is happening when you turn up the gain. If you have a scope look for a high frequency signal that comes and goes as you turn up the control.

[Petey Wheatstraw]

Thanks for responding Bill, I do have a scope. Should I be looking at the output for the high frequency signal, or on the pot?

[Billy R aka DynaFreak]

I've worked on some Kranks. Nice amps, build quality is very good. I would not hesitate to call Krank. they are very helpful, small company, chances are you might talk to the guy who built your amp.

[Jazz P Bass]

Have you checked the coupling caps on the "dimed" stage?
You may have a leaker.

[rf7]

There's a button on the back and a pot that increases the gain but will cause an oscillation. Turn that button off and your output will come back. There needs to pedal effects hooked up to the effects loop for the amp not to oscillate with that button engaged.

[Petey Wheatstraw]

Thanks for the reply's everyone, I have a call in to the tech department. I will check the coupling caps, I have already tried the effects loop pot it makes no difference.