On Tue, 2008-04-08 at 10:54 -0400, Gene Heskett wrote:
> In this case, I believe I would insert a transformer between the signal source
> and your input transformer. I'd suspect something in the generator was
> responding to the grounding of its output, particularly if the generators
> output is an electronically balanced output without a transformer to isolate
> it. Grounding one side will often cause a rise in the opposite sides output
> because of the extreme levels of feedback in such circuitry.
>
> If you still see that effect then, there is something bogus about that input
> tranny. And since they are often wound with 40+ gauge wire, I would not be
> willing to bet they are really good as little as 5 years later, due as much
> to the solders amalgamation with the copper of so fine a wire, the failure
> rate under those conditions is legendary.
Output transformer... But nevertheless, they (*) all exhibited the same
peculiar behaviour.
* The internal tone generator has a transformer coupled output
* The master channels have transformer coupled outputs
* Likewise for the reverb send and PFL.
* All of them are unbalanced driver outputs to a transformer, and
they're all floating.
The tone generator was the first one that put me onto it. It's been
flakey, and I'd suspected the circuit. I'd unplugged it years ago, when
it stopped working, and never got around to repairing it, and put it
back in on a whim recently. It started working again, which surprised
me. Then went up and down in level. Traced that back to bad soldering
on its output transformer (buried far away somewhere else in the
chassis), resoldered the terminals several times (it kept failing), then
it died altogether.
It turns out the oscillator output is DC coupled, at half voltage
(around 20 Volts), so they put 20 Volts DC through the transformer.
Eventually that open-circuited the primary. Must have been a design
stupidity (duh, I don't know, lets put 20 Volts DC into an audio
transformer that carries just a few volts of audio...), there's a cap in
the output circuitry, they've just tapped of before it. The other
driver circuits are AC coupled at that point.
I put a cap in, put a resistor over the output equivalent to the old
transformer primary as the driver circuitry wanted loading down,
replaced the transformer with a cheapy ripped out of some other junk
(it's only a 400 Hertz tone generator, it doesn't need good frequency
response). That got that working, but still left me puzzling over the
voltage changes dependent on grounding the other output transformers. I
can't recall if the same issue cropped up on my replacement, I'll have
to check later.
--
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important to the thread.)
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