Re: stereo amp cleaning

Dave wrote:
"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

The bias diode is essentially two diodes in series. (Sometimes three,
actually). In this case two, looks like. A diode check function of a DMM
would be OK for a static check, and if it became necessary you could use the
DMM to help match a replacement pair of diodes to replace it with. However,
using regular diodes might not work as well in terms of correctly responding
to heat changes.
The proof is in the operation, and in this case that means the actual
voltage drop across the device in-circuit, while the amp is running. Bias
diodes opening up is uncommon but it does happen, and when it does,
catastrophe is the result.

In your case the resistors may have burned because of a momentary short at
the speaker wires, etc although this would ordinarily have destroyed the
output transistors as well.


Mark Z.


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