Re: stereo amp cleaning


"Mark D. Zacharias" <spammenot@xxxxxxxxxxxx> wrote in message
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Dave wrote:
"Mark D. Zacharias" <spammenot@xxxxxxxxxxxx> wrote in message
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Dave wrote:
"Mark D. Zacharias" <spammenot@xxxxxxxxxxxx> wrote in message
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Man.

This might be really easy. Q 405 conrols the turn-on delay. HK's
don't use relays, so they mute the signal til the amp stabilizes.
On this model it is Q405 and Q406 respectively. The transistor
could be bad, but I would be especially concerned with D401, R405,
C405, C407, and D403, which is a 15 volt zener.

Could just be solder connections relating to the above, but in any
case you need to see that Q405 turns off hard a few seconds after
turn-on. The -12.5 or so volts at the base is critical.

Mark Z.
Something is still failing in my amp. As I noted, I put the
original caps (C401, 403, 405, 407) back in and the problem went
away. For awhile. It's back... I measured voltages after the
right channel had cut out.
The collector of Q407 was 0V. So was the collector and base of
Q409, and the base of Q413. Makes sense as they're all tied
together. The collector of Q405 was also 0V. the collectors of
Q411 and Q413 were ALSO 0V. I noted that although R437 tested
fine, it looks like it's been run hot. As in it's dark brown and
you can't really read the color bands any more.
I don't understand how Q405 works as far as the turn-on delay, but
if it were bad could it be causing no voltage where I've indicated?

I tested the other, working channel and all voltages were correct.

Thanks

Dave

I need the base and emitter voltages on Q405 versus Q406. I wasn't
thinking clearly before, but the basic principle is the same. As the
power supply comes up, a positive voltage is passed through D401 and
D402 to turn ON Q405 and Q406. In the case of Q405, you can see that
there should be a negative voltage at the emitter, and a somewhat
LESS negative voltage at the base. This turns ON the transistor and
should pass a negative voltage through the transistor, so that the
negative 7 volts or so appears at the collector. If there's no
voltage at the collector of Q405, it's likely there is no negative
voltage at the emitter either. Possibly R429 or R429 are opening up,
or there could be bad solder connections on one or both of them.

Let me know what you find.

Oops I guess I omitted information... had typed it then deleted it as
insignificant. I did test the negative power rail at R417 where the
schemo shows -14.2V. It was, I think around -14.9V. In the zone
anyways which would rule out R427 or R429 as culprits. If one of
these had opened up would I not likely see a positive voltage at the
base of Q405?
Base Collector Emitter
Q405 -14.2 -0.4 -14.7
Q406 -12.7 -6.9 -13.5

As you can see the base vs. emitter of the two transistors has a
comparable delta... say a half-volt lower at the base which is what
you'd expect I think.

So, if I have the correct voltages at the base and emitter of Q405,
but the wrong collector voltage, I should be looking hard at Q405? I
would think that the -0.4V seen at the collector would be derived
from the 2 x -0.6V coming out of Q401 and Q403 attenuated by
R413/R415/R483 with no contribution from Q405.

Dave


I do think I'd replace Q405 at this point. You say the B-E voltage is
comparable but I don't think so. There's a big difference between 0.5
volts and 0.8 volts when it comes to turning on a transistor. If the
transistor isn't the problem I'd still say it's not turning on
sufficiently. Could be a resistor is not open but the value has changed,
for example.


Well, I'll lift the legs of R427 and R429... and, I suppose, R407, R417 and
R405 just to rule out the resistors as I likely have replacements kicking
around for them whereas the transistor will need to be procured. Something
has got to be causing the ~1.5V differential between the R and L amp
channels in the first place.

I could add a small ~40-ohm resistor to the base, too and see if Q405 turns
on with a 0.8V B-E drop... FYI the spec drop is only 0.6V and I doubt the
designer would cut it so close that a 0.1V difference causes failure... but
them I'm often surprised.

Will post results, might be awhile before I get my hands on the transistor.

Out of curiousity, what does the Q5/Q6 circuit which biases Q405 do? As
near as I can tell, it takes 30VAC prior to rectification, runs it through a
diode to separate out the negative half of the wave, then past a 12V zener
to drop the voltage. Not sure what the function of D9 is, it is oriented
the reverse of D8. you end up with -1.1V at the base of Q5. The net result
being -0.56V out of the collector of Q6 to bias Q405/6. Seems like a lot of
work to derive a half-volt of regulated power but I guess they wanted it
fully independent of the amp channel power rails.

Thanks again.

Dave


.

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