Mark Zacharias wrote in message
...
"N_Cook" wrote in message
...
Mark Zacharias wrote in message
...
"N_Cook" wrote in message
...
rodifications wrote in message
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Yamaha model htr 5790 7.1 pro logic surround receiver regardless
of
input function or out put channels , produces audio for two to four
seconds then powers off. Press power on with vol. all the way down
then
slowly bring up to -30.0 db and it stays on , any more , and it
powers
down.




--
rodifications


protection circuit genuinly or more likely falsely operating ?, check
main
power rails while it is operating and look for a most likely SIL IC




Like ALL modern receivers this model's protection function is
microprocessor
based.
The TA7317P and uPC1237HA protection IC's have not been used for over a
decade by any major manufacturer. Dedicated protection IC's ARE NOT
used
except in units which do not employ microprocessors. Certainly not in
Yamaha
models. Nor Sony, Pioneer, Marantz, Kenwood, Denon, HK, etc etc.

The fact that the protection only activates when you raise the volume
indicated either a load problem - the speaker or wire has a problem,
there
could be a bias problem, or there could be an open base-emitter
junction
on
one of the affected channel's output transistors.

BTW the OP has given two different protection scenarios. One is a 3
second
timeout, the other volume based. The shutdown after 3 seconds is
another
common problem, caused by bad low-frequency rolloff capacitors in the
amplifier feedback circuit.

The HTR-5790 is a difficult unit to service. Any doit-yourself type
repair
attempt has a very low probability of success. Service should probably
be
referred to a professional.

Mark Z.
Authorized Yamaha service
Wichita, KS. USA



Do uC based amps use the same protection criterea of power up /no AC =
mute,
and unbalanced/ out of range power rails? or do they add heaps more
criteria?





Same criteria, sometimes different implementation. For example, a
TA-7317P
will have one pin dedicated to monitoring a lightly filtered DC off the
power supply for a fast cutoff at power-down. Many uProc versions will
take
a direct AC sample, perhaps squared off, for an "AC power good" signal.
AS I recall, the 7317 does not monitor more than the one power supply
line.
A micro is not limited here. Pioneer has two PS monitor lines on recent
models, and each of those may be tied to summing networks monitoring most
or
all DC power supplies and amp channels.

There are variances from one manufacturer to another, that's for sure.
Most
use a conventional "pull-up" resistor from a micro pin to the power
supply.
Any trigger will pull that line down.
Some older Yamaha models may have a micro pin set at about 1.2 volts and
positive or a negative bias from either the power supply summing network
or
the amp sensing network will trigger either up or down.

DC sensing resistors to the amp channels are generally around 22K for most
manufacturers, but Yamaha will use different values on each channel so
that
in the unlikely event of equal but opposite DC offsets on different
channels, the protection circuit will not be fooled.

Mark Z.



So as long as you can identify the sensing routes into the micro then much
the same divining process, assuming the uC or its firmware is not corrupt.
Monitoring and fudging the monitored DCs until an erroneous external
component is found in the amp or monitoring chain.