Toshiba TV29C90 problem; Image fades to black...

[b]

On 11 mayo, 17:19, "Interesting Ian" <spam.me2DELETET...@ntlworld.com>
wrote:

Hi,

My present TV is no longer working (picture keeps fading away into
nothing) so I need to buy a new TV.

That is a well known fault and almost always due to a dry solder joint
on the tube filament heater lines. A tech should be able to solve this
in very little time and for little more than labour cost- if the set
is otherwise ok, I'd go this route. Don't be quick to dump it in the
landfill, this is just waste . As others have said it will probably
outperform a new lcd set! Let us know if you need help solving the
problem. You may be surprised.
B.

 

[Mark D. Zacharias]

Dave wrote:

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:8K82i.3600$mR2.2417@newssvr22.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
newsm__h.4408$H_.3864@newssvr21.news.prodigy.net...

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.


Mark Z.

Mark Z.

 

[Dave]

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
newsZt2i.132$u56.98@newssvr22.news.prodigy.net...

Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:8K82i.3600$mR2.2417@newssvr22.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
newsm__h.4408$H_.3864@newssvr21.news.prodigy.net...

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

 

[Mark D. Zacharias]

Dave wrote:

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
newsZt2i.132$u56.98@newssvr22.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:8K82i.3600$mR2.2417@newssvr22.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
newsm__h.4408$H_.3864@newssvr21.news.prodigy.net...

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

The function of Q5 and 6 is just to provide a more positive bias to Q405
after a delay (mainly determined by R24 and C22), enabling the amp channel
to "un-mute". It neen't be actually "positive" just more positive than the
negative rail at the emitter 0f Q401. Diode D9 is a question mark to me
also, but I suspect that since the capacitor C21 is only 33uF then some
ripple would still be present, and the diode might then act similar to a
zener and clamp the voltage drop across R24. I have to admit that since I'm
no engineer I can lose it when trying to figure the theory aspects. I just
fix them when they break.

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.

Not sure how you would use a 40 ohm added to the base of Q405.... in series
with R405 (68K) would do nothing, and in parallel with the BE junction of
Q405 would just keep the transistor from ever turning on in this
application.

At this point I think R427 and 429 are probably OK, or you wouldn't see the
voltage at the emitter of Q405. Our problem seems to involve getting 405 to
turn on, which is to say, getting the base of Q405 a bit more positve (less
negative) than it is right now.


Mark Z.

 

[Dave]

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...

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

 

[Dave]

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...

Not sure how you would use a 40 ohm added to the base of Q405.... in
series with R405 (68K) would do nothing, and in parallel with the BE
junction of Q405 would just keep the transistor from ever turning on in
this application.

I meant in series with R407 to drop another 0.2V at the base of Q405 to try

to get it to turn on.

 

[Mark D. Zacharias]

Dave wrote:

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...
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.

Mark Z.

 

[Mark D. Zacharias]

Dave wrote:

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...
Not sure how you would use a 40 ohm added to the base of Q405.... in
series with R405 (68K) would do nothing, and in parallel with the BE
junction of Q405 would just keep the transistor from ever turning on
in this application.

I meant in series with R407 to drop another 0.2V at the base of Q405
to try to get it to turn on.

Ah. I get it.

40 ohms wouldn't get it. I'd try maybe about 1 kOhm to about 2.2K.

Mark Z.

 

[Dave]

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:Yyg3i.796$C96.162@newssvr23.news.prodigy.net...

Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...
Not sure how you would use a 40 ohm added to the base of Q405.... in
series with R405 (68K) would do nothing, and in parallel with the BE
junction of Q405 would just keep the transistor from ever turning on
in this application.

I meant in series with R407 to drop another 0.2V at the base of Q405
to try to get it to turn on.

Ah. I get it.

40 ohms wouldn't get it. I'd try maybe about 1 kOhm to about 2.2K.

Mark Z.

I figured that if 3.6K dropped the negative rail voltage from -42.6 to -14.2

(through R427/R429) then that's about 125 ohms/V. 40 ohms ~= 1/3V.

 

[Dave]

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:Vvg3i.795$C96.649@newssvr23.news.prodigy.net...

Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...
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

 

[Dave]

"Dave" <dspear99ca@yahoo.delete.com> wrote in message
news:Vsj3i.36782$Xh3.2600@edtnps90...

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:Vvg3i.795$C96.649@newssvr23.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...
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


My kids have a game which is a 4 x 4 grid of holes. At any given time,

there is a mouse poking out of one hole. One pounds the mouse into the hole
with a hammer and this forces another mouse out of another hole. Sort of
how I feel here... fix one thing and it appears to break another.

 

[Dave]

"Dave" <dspear99ca@yahoo.delete.com> wrote in message
news:Vsj3i.36782$Xh3.2600@edtnps90...

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:Vvg3i.795$C96.649@newssvr23.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...
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


What would happen if C415 were open?

 

[Marra]

I think you need to replace any components that wear with time.
Switches, potentiometers and electrolytic caps usually go first.

www.ckp-railways.talktalk.net/pcbcad21.htm

 

[Mark D. Zacharias]

Dave wrote:

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:Vvg3i.795$C96.649@newssvr23.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...
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.

 

[Charles]

Are they going to fix our broken electronics?

 

[Michael Kennedy]

"Charles" <charlesschuler@comcast.net> wrote in message
news:JNKdnZlh4Im_Lc3bnZ2dnUVZ_vmqnZ2d@comcast.com...

Are they going to fix our broken electronics?

Illegal aliens? I doubt they will.

 

[telus]

"Marra" <cresswellavenue@talktalk.net> wrote in message
news:1179529881.656101.174240@h2g2000hsg.googlegroups.com...

I think you need to replace any components that wear with time.
Switches, potentiometers and electrolytic caps usually go first.

Um, if you read back in this post you'll see that replacing time-worn
components is what started this mess in the first place... I replaced all of
the electrolytic caps in the amp and pre-amp and have and have had nothing
but trouble ever since. I also cleaned up all of the pots and switches...

www.ckp-railways.talktalk.net/pcbcad21.htm

 

[telus]

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:O0B3i.9316$rO7.3940@newssvr25.news.prodigy.net...

Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:Vvg3i.795$C96.649@newssvr23.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...
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

 

[Mark D. Zacharias]

telus wrote:

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:O0B3i.9316$rO7.3940@newssvr25.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:Vvg3i.795$C96.649@newssvr23.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...
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.

Mark Z.

Mark Z.

 

[Dave]

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:jIL4i.9697$rO7.7174@newssvr25.news.prodigy.net...

telus wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:O0B3i.9316$rO7.3940@newssvr25.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:Vvg3i.795$C96.649@newssvr23.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:0sX2i.6296$RX.639@newssvr11.news.prodigy.net...
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