"David Farber" wrote in message
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"Arfa Daily" wrote in message
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"Mark D. Zacharias" wrote in message
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"Arfa Daily" wrote in message
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"Eeyore" wrote in message
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Arfa Daily wrote:

If all else fails, the temperature that the output
transistors run at on the rebuilt channel, compared to those on the
other
channel, is usually a pretty good guide as to how evenly the two
channels
are running with respect to one another. This should be checked with
the amp
just idling.

And the load not connected.

Graham


I've never really thought about whether or not a load should be
connected
when making that assessment. I suppose it might depend on how the bias
arrangement works, or if there are any other adjustments such as
offset,
but in theory, surely if it's a standard complimentary output stage,
with
the output DC coupled to the midpoint, and with the amp just idling
with
no input, there should be no current in the load ? If there's any kind
of
asymmetry, leading to a DC offset at the midpoint, then yes, I can see
some value in not having a load connected, but also, I would suggest
that
it would indicate a further problem that needed resolving ?

Interested in your thinking here, Graham.

Arfa


Even a slight DC offset would affect the voltage measured across an
emitter resistor, if a load is connected. I put a pencil to it and
found
that with a 50 millivolt offset (a bit high but certainly not uncommon)
and a 6 ohm DC resistance representing the speaker, this would mean
about
18 millivolts additional across a .22 ohm emitter resistor. Reason
enough
to adjust the offset first, I would say. Unfortunately, many amps have
no
offset adjustment as such. I will try to look up the 2240PE bias
adjustment at work and e-mail Dave directly. I've seen some NAD's which
require measurement across a 1 ohm resistor which is made available by
desoldering two adjacent foil pads. Once the adjustment is done, the
pads
are resoldered.

Mark Z.

That's a good point, and I expect that was what Graham was thinking

Arfa



Thanks for all the tips. The dc offset was about 1 mv. Not too bad. Now
that
Mark has e-mailed me the details about the desoldered joints, it will be
easy. I should note that I did notice the solder bridge across the 1 ohm
resistor and I thought that I had caused the solder bridge when replacing
the transistors. Long story short, you don't want to run the amp under
stress with the foil unbridged or else you will be fixing more problems.

By the way, is anyone familiar with the component on the power amp board
that looks similar to a microswitch? It measures a direct short. Is it a
resettable fuse? The location numbers are BK-1 and BK-2.



Yes. I've seen these in NADs before. They should self reset, but I have had
them go totally open.

Arfa