On Sun, 29 Oct 2017 05:28:38 -0600, wrote:

On Fri, 27 Oct 2017 20:25:42 -0500, Ken wrote:

wrote:
Actually lytics are worse when cold, the opposite of your situation.

The caps I was thinking of were disc caps across the diodes, not
electrolytics elsewhere in the circuit. What puzzled me about the 30
minute delay was that electrolytic DO behave that way. If the problem
was the caps across the diodes, that would make sense since they seem to
be the source of the noise.

One of my latest additions to the shop tools is a digital infared
thermometer. They are made to measure the temperature of anything, but
are sold for automotive use, such as finding a hot brake drum, or hot
spot on an engine. But I have found this thing is pretty accurate on
electronic components too. It has a built in laser pointer to highlight
the thing it's pointed at. So it will tell the temp of a heat sink, or
CPU, or even an opamp or other chip or for that matter, any component.

If you have one of them, or get one, try to get the temperature of
different parts on the chassis or PCB when you first turn it on, then do
the same after a half hour. Certain things will get warmer like
heatsinks, power resistors, etc but if there are passive components or
semiconductors getting quite warm or hot, you may want to check them
further. They used to sell some sort of stuff in a can to cool parts. It
might have been freon??? I used it many years ago, I have not even
looked to see if they still sell it, but if it's still sold, that could
help see if the sound changes when a suspected part is cooled.

I did not read this whole thread, but I think you said those disc caps
across the diodes were open. I assume you mean the power rectifier
diodes. I cant understand why those would open, unless you had a
lightning surge in your power lines. That makes me wonder if those
diodes could be a little flaky. Usually diodes are either good or bad,
but lightning surges can do strange things and nothing in electronics
has a 100% definite solution. Regardless what the books say, sometimes
strange things happen that have no real explanation.
IR thermometers are pretty handy but can give wildly false readings.
This is because of the emissivity of the surface being measured. For
example, bare aluminum will measure muich cooler than it really is
when the temp is measured with an IR thermometer while black anodized
aluminum will measure pretty accurately when the IR thermometer is
used.
Eric