Re: stereo amp cleaning


"Mark D. Zacharias" <spammenot@xxxxxxxxxxxx> wrote in message
news:X3U6i.5973$C96.2887@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Dave wrote:
"Mark D. Zacharias" <spammenot@xxxxxxxxxxxx> wrote in message
news:FMf6i.7530$4Y.6327@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

P.S.

At this point I'd say it's a virtual certainty that the outputs ARE
bad on that channel. Probably the only reason it's not just blowing
fuses is that one or both of the emitter resistors, R471/473 are bad
also.

Checked the outputs... they test OK on my transistor tester! No
leakage, both PNP and NPN had gain, although not anywhere near the
same amounts... I assumed they'd be "about the same"... anyway, the
outputs ARE NOT shorted and R471 and R473 DO NOT read open circuit
although I haven't pulled the resistors yet to test.

Started pulling transistors from the beginning of the signal path,
tested Q401 through Q413 so far, 5 more to go and they all test okay,
no shorted junctions and all show gain.

If it's a heat-related issue with a failing transistor, I'll never
find it. I don't think it is, because the DC bias on the outputs
shows up right away when the amp is powered on.

I'm out of ideas if the transistors all check out okay. I'm inclined
to blow $8 and replace all but the outputs, replace the half-dozen
smoked resistors, put it all back together and see what happens.

Dave

OK, well you're lucky then, the outputs are OK. Replace the bad resistors,
test the drivers and predrivers carefully, and test the amp while
monitoring the bias as measured across the emitter resistors on a
millivolts DC range of your DMM. A variac would be most useful. Lacking
that, one can pull the main fuse and substitute a 100 watt light bulb
which will limit current in the event of a hard overload. If the lamp
glows bright, all is not well. A light glow after initial charging of the
filter caps would be normal.

If the amp appears to be running OK, with no more than about 25 millivolts
across the emitter resistor pair with no load, I would recommend turning
your attention to the bias diode D405 which looks like it ought to have
about 1.2 volts across it in normal operation. If the diode opens up, the
voltage across it will increase, maybe slowly, maybe suddenly. If this
happens, that would be proof the diode is bad. You can compare the voltage
to the one on the other channel. They should be within a few millivolts of
each other.

That was my next question to you... the varistor. How does one test it? My
understanding of varistors is that they are a high-value resistor at low
voltages, and a low-value resistor at higher voltages. Kinda' like a zener.
It's listed under diodes on the parts list and its' schematic icon looks
like a diode, which would indicate polarity, no? Failing to find a
data*** or specs on this particular part, I'll just check it against the
one in the other channel for resistance with no power. Maybe hook it up to
15V with a series resistor and measure the voltage drop across it vs. the
drop with a 3V source? The "12" in the part number along with the expected
voltages across it (<2v) indicate a 12V transition point. Note to self:
keep an eye out for a variable DC power supply. If this "diode" is bad,
what would I replace it with?

I've got a device I made for TV troubleshooting, an isolation transformer /
series light bulb / GFI box, I'll use the series light bulb to check things
out.

The voltage across the emitter resistors is adjustable with V403 and is
speced at 33mV. It's nearly impossible to achieve with a single-turn 1K
pot, but the working channel is close.

Dave


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