On Tue, 25 May 2010 07:33:01 -0700 (PDT), "William R. Walsh"
wrote:

.......[snip!}.........

So that's got me thinking...

Perhaps this serves to protect the hybrid module from such things?
It's my belief that the hybrid power amplifier is already something of
a delicate part.

I've noticed already that this receiver also contains additional relay/
outboard circuit protection against failure of the hybrid possibly
putting a power supply rail on a speaker. Earlier and other models did
not do that, instead relying on ineffective protection measures built
into the hybrid module.

Maybe the Technics/Panasonic engineers were trying to improve the
reliability of their design? I've seen a lot of these receivers that
were dead, usually as a result of hybrid module failure.


Although I repaired a number of amps with discrete and module
outputs, I don't have enough experience to say which are worse. The
early discrete circuits were pretty vulnerable.... I don't remember
any customers that that suggested electrostatic discharge as a cause
for the amp's failure. Usually it was power on/off, or load abuse
(shorting speakers, disconnecting while at full volume, etc.).
I believe that ESD (electrostatic discharge) into the speaker line
is not likely to bother the output circuit. The output devices are
designed to withstand short bursts of current, usually in excess of
10-20 amps. The half microsecond or so 2-3 amp transient generated by
the human ESD model is not likely to bother the output devices, since
there is sufficient capacitance in the devices themselves and the
associated circuitry to absorb significant charge. The output circuit
is not likely to be in a high impedance state. The tiny DC current
generated by scuffing across a rug, even though driven by a very high
voltage would not affect the module, unless the module circuitry was
in excess of hundreds or thousands of megohms. The components that are
likely to suffer from static voltages are the power supply where it
isolates the input AC line. The thousands of megohms insulation will
build up a high voltage as the static current tries to get to ground.
That's where the weird little resistor allows a path for the
current to seek ground. As has been stated earlier, if your amp has a
receiver in it, you can also get some very impressive static voltages
(driven by very tiny currents) from external antennae .

As you say, the newer amps have protection circuitry that senses
overcurrent, etc, and shuts off the output terminals. I remember
looking at the schematic of a cheapy 5.1 ch. Yamaha amplifier - it
monitored about 10-15 voltages and currents, and the microcontroller
would only energize the output if all the parameters were in the
correct range. There were hybrid modules throughout this amp. I
suspect the paranoia was not so much for the amp, but to protect the
customers speakers. (There is a very negative reputation for an amp
that blows a customers speaker!) I was quite impressed with the degree
protection and control of this amp... you could even run diagnostic
routines on it to check its performance, yet its price was around
$200. The fault I found was someone hadn't tightened the screw that
held one of the output modules to the heatsink, and its overheating
would shutdown the amp at high output levels.

......[snip !].........

Fortunately, I do not think that this will be a problem. The unit
seems to operate safely in every regard. If anything, I will probably
be replacing the main filter capacitors as a precautionary measure.
The power supply section of this receiver runs hot and the original
filters are ~23 years old. They work fine now, but I do not want a
problem.

William

The little electrolytics throughout the amp are just as likely to
fail as the main ones (maybe even more so), so if the main caps are
cool, and your amp doesn't have 120 Hz hum in it, why bother? I would
only do a total recap if the amp was something very exceptional. Your
chances of screwing something up doing a complete recap are quite
high.
If the thing dies badly, you can always go scrounging on "big
garbage pickup day" and get another Japanese amp that has been tossed
out and will probably remain useful for many more years.

Paul G.