Fender Hot Rod Deville

[Rob Stmple]

Hi guys.

I am new to your group, but, I have a question that I hope someone can help me with.

I work for a rental house (Bauder Audio Systems) here in Philly. This Hot Rod Deville that I am working on has me stumped.

When the amp is on the drive channel, the amp starts to flutter on the output. If you have the driver pot turned all the way up, the flutter decreases in frequency. If you turn it down, it increases.

When the amp is in the clean channel, it is fine, until you turn the treble up to around 2 or 3 O'clock. (8 on it's scale) At that point, the amp goes into high oscillations.

I have gone though the schematic, and have replaced two DC blocking caps. C-2, and C-7. Thinking that they may have been leaking. Not so. I also lifted all of the high resistance resistors and measured them. All tested in spec. I removed and tested pot R-7 (Drive), and R-13 (Treble). Both are fine. I have re soldered a few traces that looked a little suspicious as well. The tubes have been tested and also replaced. To no avail.

I thought that it could be the power supply. I removed the tubes, and tested the output of the power supply. It was stable. At this point, I am at a loss.

Has anyone encountered this problem before? Any help, or direction at this point, would be most welcome.

Thanks.

[52 Bill]

Did you check the filter caps? I've seen a bad filter cap in the pre-amp supply cause that type of oscillation.

[Rob Stmple]

Yeah, I have checked them. But, it does look like it should be in that area. I think I am just going to order the diodes and caps for the power supply, and just rebuild the whole section.

Other wise, I don't think I will have any sanity left.

Thanks for getting back to me on this.

[52 Bill]

I don't know how you checked them, but if you haven't tried this, try carefully clipping in a matching cap in parallel across the one on the board and see if it clears up the problem. I find that's the fastest way to check them.

[Rob Stmple]

Thanks 52 Bill. You were spot on. Turns out C-36 was bad. The way I was checking it was, by putting my meter on it, and checking the DC voltage. Then putting the scope on there to see the AC ripple.

The Voltage was correct at 386. And the AC ripple was under a half of a volt. From that information, I assumed the cap was good.

After removing the cap off of the board. I found that it measured .053 nf. Instead of the 22 uf that it's suppose to be.

So, thank you again.
Rob

[52 Bill]

Glad to hear it's working again.