Fender Twin Po Tube = 100 watt boat whistle

[booj]

Uncontrollable. Starts whistling as it warms up. Take out V2 or V3 and it stops. Re-soldered the board in the preamp section. Nothing looks bad or out of place. Swapping tubes doesn't help. V3 shows the strongest signal strength.
If you have experienced this problem and have a clue, your help appreciated. Awesome amp I'm sure when it works!
I can't paste a PDF file to look at, but the schematic is avialable at

http://www.fender.com/support/amp_sc...Amp_SchE45.pdf

[mikeboone]

When is the last time the amp worked properly? What changed? When you say "swap" tubes, did you swap them with new tubes or swap V2 & V3 around?

You might also try wiggling the tubes in the sockets to see if one has any affect. It could be a fractured solder joint on the PCB around one of your tube sockets.

I have no experience working on this particular amp, but I have re-flowed solder on the tube PCB's on several Deluxe and DeVille series amps.

Food for thought

[booj]

Thanks for the replly.
I swapped V2 out with aq known good tube, then did the same with V3 with no change. The tubes all had to be removed to lift the board up for inspection. All the sockets look good as well as their connections. I also checked the relay contacts in the off mode, and the contacts read normal.

[Steve Conner]

Could be a dried-up electrolytic. This tends to lead to motorboating, which is more of a put-put noise, but maybe you just have really fast motorboating. You can test by temporarily sticking a known-good electrolytic across the suspect caps.

[booj]

I gave the electro's a quick check by putting a diode meter across them to supply a small voltage and then read that voltage to watch how fast the voltage dropped off. That statement contains a lot of voltage!

[booj]

Amp troubleshot

You were right on Steve. My little diode check system doesn't work. It was an open cap. Reads 160pf on a cap checker. I found it by jumping a good cap across the bad cap and the oscillation stopped.
Of course after that epiphany, I tried it and the drive channel was dead! I found that problem was an open winding in the switching relay, K1.