DIYbanter - View Single Post - Trying to fix a Marantz SR-2000 receiver

"jakdedert" wrote in message
...
Arfa Daily wrote:
"jakdedert" wrote in message
.. .
Arfa Daily wrote:
wrote in message
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Speaker connected (switch on or off for them), right channel power amp
still very hot. The sound is good on both speakers but I cant leave it
on for long, I am afraid to fry the power amp ($20.00)

On Mon, 06 Mar 2006 19:37:03 +0100, Damir wrote:

On Mon, 06 Mar 2006 13:12:54 -0500, wrote:

Thanks Charles,
I checked the voltages(DC) on all pins on both hybrid power amps and
they are the same. I swapped the input to the hybrid power amps
(left
and right channels) and I still have the same power amp getting very
hot. Not sure what to check next.

If the sound is good, but the hybrid is frying, then I would suggest
that it is hooting, maybe as high as RF. You need to get a 'scope on
the output to check for this. There may be a Zobel network ( R & C in
series ) on the output that was damaged by the previously faulty
hybrid. Alternatively, there may be a defective decoupling cap on one
of the bypass pins of the hybrid, or an open or high ESR supply
decoupling cap, and this is allowing it to hoot, which may be what
caused the demise of the original hybrid, and would ultimately do the
same to the replacement, if left.

Arfa
'Hooting', you say. I suppose by that--and your description of the
solution--you refer to high-frequency oscillation.

Hooting...that's a new one on me; but I like it.

jak

Oh, ok. Perhaps it's a funny British thing !! It's a term that was taught
to me when I was an apprentice nearly 40 years ago, by my apprentice
mentor, and I've always used it ...

Your assumption of its meaning Jak, is basically correct. It is a
parasitic oscillation of a stage which may be, but is often not,
detrimental to the basic operation of that stage. The OP's amp is a
classic example, if that is indeed what the problem was - pins 3,4 and 5
of that hybrid are all up the front end, and typically have caps
connected to them. The stage basically carried out its job of amplifying
' low freq ' music perfectly well, but was also outputting an additional
signal either at an ultrasonic, or low HF frequency, probably at close to
full power, depending on how good a bandwidth the output stage has -
hence it gets very hot. I don't know if you remember the old Texas
Instruments SN76003 / 013 / 023 series of power amp ICs. They had a sort
of " fan " shaped hetsink bonded to them. Well those used to suffer from
it badly, if the supply decoupling wasn't perfect.

I've also had genuine oscillators in mixers that have suffered from
parasitic oscillations. Their primary function has been basically fine,
except that every other radio in the vicinity is going bananas, due to
the high amplitude 'hoot' that is also occuring.

I used a Studiomaster desk that just sounded like utter sh*t. I couldn't
get a decent mix out of it to save my life, although the rest of the PA
system was really excellent. I couldn't put my finger on it, but
something was just not 'right'. I subbed in a cheap Sunn console, and the
whole system came to life!

Turned out the Studiodisaster had parasitics (up into AM radio
frequencies) in almost every stage.

Sorry if I've caused any confusion to anyone. I'll stick to '
contemporary ' terminology in the future ... !!

Hooting is fine. I like it, but it must be a British thing...like
'valve'.

jak
Arfa
Yes, that's the sort of thing. I've had similar where a high amplitude RF
signal is being created in an audio stage, and rectified in the following
stage due to its huge amplitude driving that stage to non linearity. The DC
then produced at the base of the transistor, upsets the bias, and bingo,
hefty distortion.

Arfa