Re: Bias current in Power Amps


"Arny Krueger" <arnyk@xxxxxxxxxx> wrote in message
news:4M2dnajVJ_gKgqnVnZ2dnUVZ_uqdnZ2d@xxxxxxxxxxxxxx
"Gareth Magennis" <sound.service@xxxxxxxxxxxxx> wrote in
message news:8O2dnR3MJNZVganVRVnyiQA@xxxxxx

The protect happened once yesterday for about a second,
after 4 hours on the bench, reassembled with no load,
and the fans weren't on so it wasn't hot.

That clarifies a lot. Obviously, there should be no protected condition.

OTOH, what about the power? I recall that most protect circuits come on
during the power-up cycle. A momentary loss of power could cause the
symptom you observe.
The next question would be about the logic that goes into detecting a
protected state.

I've seen some moderately complex circuits for turning on the "protect"
light. The fault could be in a part in the protection circuit, not a fault
of the circuit(s) being protected.


I spoke to a tech who fixes a lot of these, and he suggested an output
device as the most likely culprit, so I am following up this line of enquiry
first, as well as learning a few things here along the way. The next step
will be to remove one power amp from circuit and wait (another whole day for
God's sake) for it to go into protect or not, then try the other. If I
could force this fault to happen it would make things a lot easier.


As far as your power sugestion goes, I might look a lot more closely at the
low voltage rails. A blip in one of those could well be the culprit.



Cheers,



Gareth.




.

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