Re: stereo amp cleaning


"Mark D. Zacharias" <spammenot@xxxxxxxxxxxx> wrote in message
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telus 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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Dave wrote:
"Mark D. Zacharias" <spammenot@xxxxxxxxxxxx> wrote in message
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Dave wrote:
You can swap Q405 and Q406 and see if the problem moves to the
other channel. Resistor R417 (560 ohm) can be checked in circuit.
R405, a 68K, would probably need to be checked out-of-circuit.
Experience tells me that high-value resistors can change value or
go open-circuit for no good reason. Worth keeping in the back of
your mind sometimes as you're troubleshooting problems such as
this.
R417 and R418 both test around 560 in circuit

R405 and R406 both test 66k in circuit

I'll try swapping Q405 and Q406 and see if the problem follows the
transistor. I found them relatively close (4 hour drive) in
stock, or I can order them in to my local supplier and wait a
couple of days/weeks to receive. How many do you think I'd have
to order to find a pair with DC gain matched to within, say, 15%?
The 2SC2603 crosses to NTE289, which is available as NTE289AMP, a
matched pair for use in amplifiers...
For sure Q405 is not turning on. I watched the output of Q406
and, about 5 seconds after turning power on, it goes from -0.4 to
-7.1V. Q405 just stays at -0.4V.

Dave

If they are using a 2SC2603 then it is not critical. The 2603 is a
pretty standard signal transistor - nothing special. A 2SC945 would
be fine, heck, even a ECG / NTE 123AP. Just make sure the basing is
correct. An American type transistor will usually go EBC rather
than ECB as viewed from the front.


OK. I have lots of 2SC945's, they're in everything. But... I
swapped Q405 and Q406 and the problem DID NOT follow the transistor.

I kept thinking about C405 and C407. If either one leaked, even a
little bit, that would throw off the base voltage of Q405, maybe
enough to keep it from turning on. So I threw in a new C405 and,
voila, Q405 now shows -5.9V at the collector. Great, I says to
myself, I am so smart. (Note to self, find new source for
electrolytic caps, two bad ones out of 30 so far). Set about
checking other voltages in the channel, but HEY! what the ????
Something else is not right.
The voltages that were out were:

Base Collector Emitter
Q409 +34.2 +34.2
Q411 +33.0
Q413 +33.9 +33.8
Q401 +0.2
Q403 +0.4

I didn't check the collector voltages of Q401/Q403, I'll do that
next but... I'd be surprised if they weren't +40.5 given that all
the other positive voltages check out.

The negative power rail is fine at -43.5V, and I am getting my -12.5
at the base of Q405 so it's not the negative supply.

If Q409 fails to turn on, then that would make Q413 not turn on
either. There would be then be no negative voltage applied to the
collector of Q411 to get down to the +1.1V we want to see. I'll
check out R439.
I wouldn't have expected to see a positive voltage at the collector
of either Q409 OR Q413 given that they're basically connected to the
negative power rail through a small value resistor. Almost makes
you think that if R439 is open, R445 would be too.

But then we're back to my basic lack of understanding of how the
transistor works...

I think I'm learning something here.

Dave

Q411 collector CANNOT be at +33 volts unless there is a major DC
offset at the output of the amp.
Same for the collector of Q409, and Q413.

Also, the negative rail from the main negative filter cap is shown
as -42.6
volts. Therefore, the voltage you gave (+33.8) at the emitter of
Q413 must be wrong, or else resistor R445 would have SMOKED with
better than 70 volts across the 100 ohm resistor. Also, if it was
just a typo, and it should read -33.8, then I'm concerned that the
main filter cap C9 might be open, and not filtering.You might mesure
the collector voltages at both channels' output transistors. Since
there are dual supplies, one open cap would show a significantly
lower voltage at the collector of the one channels' PNP outputs
versus those on the other channel, if the one cap were open-circuit,
that is.

The negative power rail is fine at -43.5V, and I am getting my -12.5
at the base of Q405 so it's not the negative supply.

I did see this, but wonder if you might have measured the -DC from
the other channel...

I also see the the voltages at the emitters of Q401 and 403, low
though they are, seem unbalanced - this supports my theory that you
have a major DC offset in the amp channel. Please check this and
repaort back to me. It's not uncommon the have a bad differential
pair at the amp input (Q401/403). This could have been your main
problem all along.


Mark Z.


Yes yes yes.

Yes, R445 _IS_ smoked. Well, it looks fine but it's open... +33V on
one side and -43V on the other.

Yes, there is +33VDC at the emitter of the output transistors on this
channel, zero on the other channel.

There is 0VDC at the output of the preamp and at the base of Q401.

I checked all my voltages again and yes, all the +33VDC ones are
correct. Confirmed the +0.2 and +0.4, steady as a rock, at the
emitters of Q401 and Q403, respectively.

How do I tell where the DC offset is being introduced? Do I pull
Q401/Q403 and test? If I replace them do I need to worry about
matching just the pair, or should I look at matching the gain with
the other channel pair?

Thanks.

Dave

I think it's worth just replacing the bad resistor. Sometimes that's all
there is. Do check the various drivers and outputs for any shorted or open
junctions first, though.

R465, R485 Q415, Q417, Q419, R459, R457 all should be checked, as well as
the output transistors and the .22 ohm emitter resistors R471 and R473.

Don't forget the small value resistors like R469 and R461.

All the resistors off the power rails are listed as "1/4W, Fuse". Does this
mean I've got to find 1/4W fusible resistors, or will a regular old 1% 1/4W
100-ohm metal film resistor do the trick?

Dave


.

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