"Mark D. Zacharias" wrote in message
t...
Dave wrote:
"Mark D. Zacharias" wrote in message
news
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