Kustom K200B-4 hiss

[pontiacpete]

This amp's hiss is affected when turning the treble and the bass pot. The hiss is loud, much louder than I've ever heard out of one of these kustoms. Here is the schematic
https://docs.google.com/file/d/0B1Fx...?pli=1&rm=demo

Since it's the only treble and the bass pot that affects the hiss what would you look at next?

[Mark Black]

Last time I had a mystery hiss I was able to run it down fairly quickly by using some freeze spray - that one was a transistor as I recall. Best of luck!

[Enzo]

His is generally involved with a gain element, which here means a transistor. One of your transistors has gone noisy would be my wager.

[52 Bill]

Looking at the schematic, the entire top half is before the tone controls. If you turn off the clipper circuit, then that limits it to the first three transistors on the left and the last two on the right.

If the volume and bright switch have no effect on the hiss, then it would probably be the last two transistors on the right side of the schematic.

[pontiacpete]

I went around subbing 2n4401 for the PET8002s two at a time. There was no change in the amount of hiss. This faulty method I used of pulling two out at time and installing the new transistors might prove to be just that,, faulty. Perhaps it could be that each of these are creating their share to the hiss and I didn't notice when doing the subs. The only transistor not change was the 2n3638. I couldn't find a good equivalent here but found some online and put them on order.
All other effects are off and do not have an affect on the hiss, only the bass and treble seem to do something.
Could it be a carbon comp resistor? If so there's a lot of those.
Thanks for the help

[Enzo]

I would isolate the noise to a specific stage using a scope or a signal tracer..

[pontiacpete]

I've been tracing the hiss from the input with a signal tracer. Touching each transistor with the probe without a signal in, I found the hiss starting at the junction between Q407, Q408, and R426. There's no hiss on any of the transistors before this point and after there's hiss on every transistor to the power board. So I changed out the 10K resistor and the 2 PET8002's with 2n4401s and no change with the hiss.