SVT4PRO won't run cold

[booj]

It runs dependably when warmed up a few minutes, but when cold it will oscillate between the fan running fast and the speakers switching off and the fan running slowly and the speakers switched on, about 3 or 4 seconds on each cycle. After about a half dozen cycles it will operate normally.
Also, with the top off I can take advantage of my un-even work bench surface and rock the chassis level to about 5 degrees and it will obediently turn of and on.
Only clue I have to offer is the amp must have stood in water for some time. The innards are clean, but the tips of the board holding screws are rusted as are the bolts that hold the power tranny and the fan.
I've had the thimg completely apart and have done a solder re-flow on all the power transistors, major filter caps, controls, and all the jacks with only very slight "improvement" in symptoms.
Once it's warmed up I cannot get it to act up at all.
I haven't removed and re-installed the power tranny yet.
Thanks for your time!

[Enzo]

Hey, that is what freeze spray is for. Get it warmed up and working, then methodically freeze small areas one at a time.

Isolate the problem.

See if chilling any area causes the "cold" symptom to return. If you can find what is warming up to make it stop doing this, then you are on your way to repairing it.


Failing that, while it is doing this, monitor the various power rails. See if any are cycling off and on. If the AC to the supplies is steady, then the PT is likely fine.

But since your fan cycles and the speaker relats as well, that poiints to the thermal protection circuit and IC2 at its heart. When the amp is cold, watch TP13 there at the output of IC2. What does it do? How about the +22 it runs on at pin 1?

IC2 controls IC3, which is also common to both fan and relay circuits. And that IC requires both polarities of 15v. Are they up and stable when the amp is cold?

And your relays cut the speakers off, but not the preamps. APply a signal to the amp input and run lines from the preamp out jacks to some other amp for a listen. Are they working, or are they cycling off and on as well?

Every time you rock your amp over the hump, you are causing flexation inside the thing. All the more reason to look at IC2 - tiny little thing on the heatsink between two large xstrs. Always worth a re-solder. If that doesn't help. also look to see if a leg is broke off at the body.

[booj]

Thanks, Enzo for your kindly response! I shall incorporate your thinking into the saving of the beast. If you wrote a troubleshooting book I would buy it! I'll get back with what happened.

[booj]

U2 had three little halo's around their solder connections, apparently from stress related to the big heatsinks vibrating? It did take a while to find that puppy on the drawing. Thanks again!