Fender Hot Rod Deluxe Problem

[Radmaximus]

Hey guys, I have a problem with my Drive channel that I am having trouble finding. When in the drive channel now if you turn any of the tone controls past about 5 it starts making the most ungodly rumbling/squealing noise I have ever heard. I also notice if you hit the bright button while in drive channel it goes into a high frequency squeal. I happened to have my multimeter hooked up to the bias point and ground and noticed the bias voltage goes from normal to the sky as it begins the rumble/squeal when you hit the mid point on the tone controls. The tubes are brand new and check out fine on my tube tester, any ideas?

Thanks,
Rad

[52 Bill]

When in the drive channel now if you turn any of the tone controls past about 5 it starts making the most ungodly rumbling/squealing noise I have ever heard. I also notice if you hit the bright button while in drive channel it goes into a high frequency squeal.

Might be a bad filter cap in the preamp supply. If you have a new one, you can temporarily clip it in across the existing one to see if the noise stops. This is a fairly common problem.

[Jazz P Bass]

Yup.
It sounds like a decoupling cap took a dump.
The first cap all the way on the left is a good place to start.
That is the one that affects the drive circuit when it is bad.

Hook up your meter to read Volts ac.
Clip the leads across that cap & turn on the amp.
If you are seeing a pretty high Vac reading, that cap is bad.
As 52 Bill stated, hooking up another cap in parallel will be a good test.
If the measured ripple drops, the original cap is bad.

[Radmaximus]

Ok great guys thanks for the info, I will give that a check. I appreciate the help...

Rad

[Radmaximus]

Yep guys it was indeed the C36 cap gone bad, thanks for the heads up. Replaced it and all is fine!

Radmaximus