On Tuesday, February 23, 2016 at 6:26:58 PM UTC-5, Cursitor Doom wrote:
On Wed, 24 Feb 2016 00:04:13 +0100, Dimitrij Klingbeil wrote:

On 23.02.2016 19:43, Cursitor Doom wrote:
On Mon, 22 Feb 2016 08:49:11 -0500, legg wrote:

If it's slow, it looks like a short when the power transistor is
trying to turn on, stressing the current snubber around L1804.

I don't think this diode is the culprit, TBH. Just out of curiosity I
hooked it up and tested it this afternoon. The faster diodes turned up
so I thought it might be instructive to compare them. The main flaw in
my test is that I'm unable to replicate actual working conditions. I
just hooked up each diode in series with a 1k resistor and fed the
arrangement from my 600ohm sig gen using 10VAC p-p. Slow recovery was
certainly visible on the scope with the BY134, but it wasn't *that*
bad. In fact it was still able to function as a viable rectifier right
up to nearly 600kHz. There were no signs of slow recovery with the
UF4007 of course, but the difference at 20kHz, whilst still noticeable,
is unlikely to be causing the issues I've experienced. But as I say, it
was in no way a scientific test and only when the new diode is in
circuit will we know for sure. I won't be holding my breath!

Well, I don't think that it's the main culprit either. But it may impair
the working of the energy recovery circuit far enough to make it
inefficient, forcing it to dump too much power into the resistor. If
everything else was well, that might still have worked to some extent.

But you're trying to troubleshoot it, and something is obviously wrong
that causes the resonant circuit to appear as too low impedance. Either
the transformer is broken or the output circuits (rectifiers) or the
whole thing is operated on wrong frequency too far out of resonance.

If the energy recovery circuit was working well, it should be able to
protect the resonant circuit, even at some overload, by diverting the
energy back into the main capacitor. That would allow you more time to
"probe around", checking what is the cause of the overload.

Slow diodes usually become worse with rising currents, so one that is
able to drive an 1k resistor from a signal generator may just as well
behave like an RF short circuit if one tries to push significant amps
through it. So it's really difficult to compare.

Anyway, while I don't thing that it's enough, I was hoping that making
that part work efficiently again would at least lower the load on the
resistor to some extent, and give you more time for such more complex
things like resonance frequency measurements or even adjustments.

Also, as for testing the transformer (out-of-circuit, with a poor man's
IWT equivalent), see my other post.

Regards Dimitrij

Many thanks as ever for your thoughts, Dimitrij. One question on your
other post before I forget: your schematic shows pulsing the transformer
input at 100Hz, so we're just testing the primary winding in this
instance, right? We're not concerned in this test about what's coming out
of the secondaries? I assume so because 100Hz is so far off its intended
frequency range but would be grateful if you'd confirm if I have this
right.

I fully agree with your observations on my diode test's shortcomings.

The only other thing I'm waiting for is some replacement caps for the
original tropical fish types that don't look very healthy. They test okay
at low voltage but may be misbehaving badly at closer to their working
conditions. They're in really poor shape visually and I could certainly
believe THEY might be responsible for the issues I've had. They should be
here tomorrow or Thursday so by the end of this week, I should have some
firm results one way or the other.


I haven't read all the posts, but way back when I suggested pulling every cap and checking for value and ESR *out* of circuit. Have you done that?