Toshiba TV29C90 problem; Image fades to black...

"deepsadvertising@hotmail.com" wrote:

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
Dave wrote:
"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
Metal film would be fine. I'd usually avoid carbon types for this
application.

Mark Z.
 
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:cBd5i.2697$u56.42@newssvr22.news.prodigy.net...
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.

I checked ALL of the resisitors in circuit and when I found one that didn't
jive with the expected value, I tested the same component in the other
channel. The resisitors used must be all 1% or better, they are BANG ON
their reported values... everything under 100 ohms was within 0.3 ohms and
this is a 20-plus-year-old amp.

R419, R421, R439, R445 and, strangely, R453 and R461 all tested open whereas
their duplicate in the other channel tested OK. R462 in the opposite
channel also tests open... It looks like about half the transistors should
be testable in-circuit, if one tests bad I'll pull and test
it(oscilloscope/curve tracer would save some time here).

Might take awhile.

Dave
 
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:cBd5i.2697$u56.42@newssvr22.news.prodigy.net...
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.

I checked ALL of the resisitors in circuit and when I found one that
didn't jive with the expected value, I tested the same component in
the other channel. The resisitors used must be all 1% or better,
they are BANG ON their reported values... everything under 100 ohms
was within 0.3 ohms and this is a 20-plus-year-old amp.

R419, R421, R439, R445 and, strangely, R453 and R461 all tested open
whereas their duplicate in the other channel tested OK. R462 in the
opposite channel also tests open... It looks like about half the
transistors should be testable in-circuit, if one tests bad I'll pull
and test it(oscilloscope/curve tracer would save some time here).

Might take awhile.

Dave
Dude... (Hurley voice here...)

You got a blown amp channel.


Mark Z.
 
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:8Pp5i.8813$H_.1042@newssvr21.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:cBd5i.2697$u56.42@newssvr22.news.prodigy.net...
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.

I checked ALL of the resisitors in circuit and when I found one that
didn't jive with the expected value, I tested the same component in
the other channel. The resisitors used must be all 1% or better,
they are BANG ON their reported values... everything under 100 ohms
was within 0.3 ohms and this is a 20-plus-year-old amp.

R419, R421, R439, R445 and, strangely, R453 and R461 all tested open
whereas their duplicate in the other channel tested OK. R462 in the
opposite channel also tests open... It looks like about half the
transistors should be testable in-circuit, if one tests bad I'll pull
and test it(oscilloscope/curve tracer would save some time here).

Might take awhile.

Dave

Dude... (Hurley voice here...)

You got a blown amp channel.


You know, for the amount of time I've got into this, I could have
realistically spent about $30 on parts and replaced every transistor and
resistor in the channel... hopefully the outputs are ok, they're pretty
robust compared to the resistors that blew. Theyr'e also fairly expensive.

What causes an amp channel to blow other than shorting the outputs?
 
"Dave" <dspear99ca@yahoo.delete.com> wrote in
news:8gC5i.59374$V75.23360@edtnps89:

"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:8Pp5i.8813$H_.1042@newssvr21.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:cBd5i.2697$u56.42@newssvr22.news.prodigy.net...
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.

I checked ALL of the resisitors in circuit and when I found one that
didn't jive with the expected value, I tested the same component in
the other channel. The resisitors used must be all 1% or better,
they are BANG ON their reported values... everything under 100 ohms
was within 0.3 ohms and this is a 20-plus-year-old amp.

R419, R421, R439, R445 and, strangely, R453 and R461 all tested open
whereas their duplicate in the other channel tested OK. R462 in the
opposite channel also tests open... It looks like about half the
transistors should be testable in-circuit, if one tests bad I'll
pull and test it(oscilloscope/curve tracer would save some time
here).

Might take awhile.

Dave

Dude... (Hurley voice here...)

You got a blown amp channel.


You know, for the amount of time I've got into this, I could have
realistically spent about $30 on parts and replaced every transistor
and resistor in the channel... hopefully the outputs are ok, they're
pretty robust compared to the resistors that blew. Theyr'e also fairly
expensive.

That is what is commonly done in service shops, even when the real
problem is located. Since the remaining parts have been badly stressed.
It is also a good idea to repalce the good channel components so the unit
does not have the other channel die a month later...

What causes an amp channel to blow other than shorting the outputs?
failed output transistors.

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Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:8Pp5i.8813$H_.1042@newssvr21.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:cBd5i.2697$u56.42@newssvr22.news.prodigy.net...
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.

I checked ALL of the resisitors in circuit and when I found one that
didn't jive with the expected value, I tested the same component in
the other channel. The resisitors used must be all 1% or better,
they are BANG ON their reported values... everything under 100 ohms
was within 0.3 ohms and this is a 20-plus-year-old amp.

R419, R421, R439, R445 and, strangely, R453 and R461 all tested open
whereas their duplicate in the other channel tested OK. R462 in the
opposite channel also tests open... It looks like about half the
transistors should be testable in-circuit, if one tests bad I'll
pull and test it(oscilloscope/curve tracer would save some time
here). Might take awhile.

Dave

Dude... (Hurley voice here...)

You got a blown amp channel.


You know, for the amount of time I've got into this, I could have
realistically spent about $30 on parts and replaced every transistor
and resistor in the channel... hopefully the outputs are ok, they're
pretty robust compared to the resistors that blew. Theyr'e also
fairly expensive.
What causes an amp channel to blow other than shorting the outputs?
You do need the check the outputs. They can fail for a variety of reasons. A
bias diode can go open-circuit, even momentarily. Bad solder at a bias
transistor. Shorted speaker wires or defective speaker(s). Abuse by the
user. Other failed components or solder connections.

Mark Z.
 
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:8Pp5i.8813$H_.1042@newssvr21.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:cBd5i.2697$u56.42@newssvr22.news.prodigy.net...
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.

I checked ALL of the resisitors in circuit and when I found one that
didn't jive with the expected value, I tested the same component in
the other channel. The resisitors used must be all 1% or better,
they are BANG ON their reported values... everything under 100 ohms
was within 0.3 ohms and this is a 20-plus-year-old amp.

R419, R421, R439, R445 and, strangely, R453 and R461 all tested open
whereas their duplicate in the other channel tested OK. R462 in the
opposite channel also tests open... It looks like about half the
transistors should be testable in-circuit, if one tests bad I'll
pull and test it(oscilloscope/curve tracer would save some time
here). Might take awhile.

Dave

Dude... (Hurley voice here...)

You got a blown amp channel.


You know, for the amount of time I've got into this, I could have
realistically spent about $30 on parts and replaced every transistor
and resistor in the channel... hopefully the outputs are ok, they're
pretty robust compared to the resistors that blew. Theyr'e also
fairly expensive.
What causes an amp channel to blow other than shorting the outputs?
P.S.

At this point I'd say it's a virtual certainty that the outputs ARE bad on
that channel. Probably the only reason it's not just blowing fuses is that
one or both of the emitter resistors, R471/473 are bad also.

Mark Z.
 
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:FMf6i.7530$4Y.6327@newssvr19.news.prodigy.net...
P.S.

At this point I'd say it's a virtual certainty that the outputs ARE bad on
that channel. Probably the only reason it's not just blowing fuses is that
one or both of the emitter resistors, R471/473 are bad also.
Checked the outputs... they test OK on my transistor tester! No leakage,
both PNP and NPN had gain, although not anywhere near the same amounts... I
assumed they'd be "about the same"... anyway, the outputs ARE NOT shorted
and R471 and R473 DO NOT read open circuit although I haven't pulled the
resistors yet to test.

Started pulling transistors from the beginning of the signal path, tested
Q401 through Q413 so far, 5 more to go and they all test okay, no shorted
junctions and all show gain.

If it's a heat-related issue with a failing transistor, I'll never find it.
I don't think it is, because the DC bias on the outputs shows up right away
when the amp is powered on.

I'm out of ideas if the transistors all check out okay. I'm inclined to
blow $8 and replace all but the outputs, replace the half-dozen smoked
resistors, put it all back together and see what happens.

Dave
 
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:cRM5i.23287$JZ3.2783@newssvr13.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:8Pp5i.8813$H_.1042@newssvr21.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:cBd5i.2697$u56.42@newssvr22.news.prodigy.net...
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.

I checked ALL of the resisitors in circuit and when I found one that
didn't jive with the expected value, I tested the same component in
the other channel. The resisitors used must be all 1% or better,
they are BANG ON their reported values... everything under 100 ohms
was within 0.3 ohms and this is a 20-plus-year-old amp.

R419, R421, R439, R445 and, strangely, R453 and R461 all tested open
whereas their duplicate in the other channel tested OK. R462 in the
opposite channel also tests open... It looks like about half the
transistors should be testable in-circuit, if one tests bad I'll
pull and test it(oscilloscope/curve tracer would save some time
here). Might take awhile.

Dave

Dude... (Hurley voice here...)

You got a blown amp channel.


You know, for the amount of time I've got into this, I could have
realistically spent about $30 on parts and replaced every transistor
and resistor in the channel... hopefully the outputs are ok, they're
pretty robust compared to the resistors that blew. Theyr'e also
fairly expensive.
What causes an amp channel to blow other than shorting the outputs?

You do need the check the outputs. They can fail for a variety of reasons.
A bias diode can go open-circuit, even momentarily. Bad solder at a bias
transistor. Shorted speaker wires or defective speaker(s). Abuse by the
user. Other failed components or solder connections.

Mark Z.

For what it's worth, the speakers and wires are fine, they're being driven
by another amp while this one is being repaired. The amp was not
abused/overdriven prior to failure. I think I'd better get out the
magnifying glass and start looking at solder connections...
 
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:FMf6i.7530$4Y.6327@newssvr19.news.prodigy.net...

P.S.

At this point I'd say it's a virtual certainty that the outputs ARE
bad on that channel. Probably the only reason it's not just blowing
fuses is that one or both of the emitter resistors, R471/473 are bad
also.

Checked the outputs... they test OK on my transistor tester! No
leakage, both PNP and NPN had gain, although not anywhere near the
same amounts... I assumed they'd be "about the same"... anyway, the
outputs ARE NOT shorted and R471 and R473 DO NOT read open circuit
although I haven't pulled the resistors yet to test.

Started pulling transistors from the beginning of the signal path,
tested Q401 through Q413 so far, 5 more to go and they all test okay,
no shorted junctions and all show gain.

If it's a heat-related issue with a failing transistor, I'll never
find it. I don't think it is, because the DC bias on the outputs
shows up right away when the amp is powered on.

I'm out of ideas if the transistors all check out okay. I'm inclined
to blow $8 and replace all but the outputs, replace the half-dozen
smoked resistors, put it all back together and see what happens.

Dave
OK, well you're lucky then, the outputs are OK. Replace the bad resistors,
test the drivers and predrivers carefully, and test the amp while monitoring
the bias as measured across the emitter resistors on a millivolts DC range
of your DMM. A variac would be most useful. Lacking that, one can pull the
main fuse and substitute a 100 watt light bulb which will limit current in
the event of a hard overload. If the lamp glows bright, all is not well. A
light glow after initial charging of the filter caps would be normal.

If the amp appears to be running OK, with no more than about 25 millivolts
across the emitter resistor pair with no load, I would recommend turning
your attention to the bias diode D405 which looks like it ought to have
about 1.2 volts across it in normal operation. If the diode opens up, the
voltage across it will increase, maybe slowly, maybe suddenly. If this
happens, that would be proof the diode is bad. You can compare the voltage
to the one on the other channel. They should be within a few millivolts of
each other.

Mark Z.
 
Dear All,

I have a malfunction Technics RS-TR555 Double Cassette Deck. It has soft
touch control buttons for controlling, auto reverse and independent motors.
Recently when I tried to check out my tapes before disposing them. It seems
that the logic goes wrong. For example, press "play" will forward your tape.
And after repeatedly pressing the buttons and you finally got it in the play
mode, the wrong wheel spin and the tape kept squeezing out.

Anyone got a hint on what to look for or where to find a schematic diagram?
Thanks.
 
On Tue, 29 May 2007 21:40:57 +0800, Zebra wrote:

Dear All,

I have a malfunction Technics RS-TR555 Double Cassette Deck. It has soft
touch control buttons for controlling, auto reverse and independent motors.
Recently when I tried to check out my tapes before disposing them. It seems
that the logic goes wrong. For example, press "play" will forward your tape.
And after repeatedly pressing the buttons and you finally got it in the play
mode, the wrong wheel spin and the tape kept squeezing out.

Anyone got a hint on what to look for or where to find a schematic diagram?
Thanks.
Try unplugging the mains cord for a few minutes.
--
#1 Offishul Ruiner of Usenet, March 2007
#1 Usenet Asshole, March 2007
#1 Bartlo Pset, March 13-24 2007
#10 Most hated Usenetizen of all time
#8 AUK Hate Machine Cog
Pierre Salinger Memorial Hook, Line & Sinker, June 2004
COOSN-266-06-25794
 
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:X3U6i.5973$C96.2887@newssvr23.news.prodigy.net...
Dave wrote:
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:FMf6i.7530$4Y.6327@newssvr19.news.prodigy.net...

P.S.

At this point I'd say it's a virtual certainty that the outputs ARE
bad on that channel. Probably the only reason it's not just blowing
fuses is that one or both of the emitter resistors, R471/473 are bad
also.

Checked the outputs... they test OK on my transistor tester! No
leakage, both PNP and NPN had gain, although not anywhere near the
same amounts... I assumed they'd be "about the same"... anyway, the
outputs ARE NOT shorted and R471 and R473 DO NOT read open circuit
although I haven't pulled the resistors yet to test.

Started pulling transistors from the beginning of the signal path,
tested Q401 through Q413 so far, 5 more to go and they all test okay,
no shorted junctions and all show gain.

If it's a heat-related issue with a failing transistor, I'll never
find it. I don't think it is, because the DC bias on the outputs
shows up right away when the amp is powered on.

I'm out of ideas if the transistors all check out okay. I'm inclined
to blow $8 and replace all but the outputs, replace the half-dozen
smoked resistors, put it all back together and see what happens.

Dave

OK, well you're lucky then, the outputs are OK. Replace the bad resistors,
test the drivers and predrivers carefully, and test the amp while
monitoring the bias as measured across the emitter resistors on a
millivolts DC range of your DMM. A variac would be most useful. Lacking
that, one can pull the main fuse and substitute a 100 watt light bulb
which will limit current in the event of a hard overload. If the lamp
glows bright, all is not well. A light glow after initial charging of the
filter caps would be normal.

If the amp appears to be running OK, with no more than about 25 millivolts
across the emitter resistor pair with no load, I would recommend turning
your attention to the bias diode D405 which looks like it ought to have
about 1.2 volts across it in normal operation. If the diode opens up, the
voltage across it will increase, maybe slowly, maybe suddenly. If this
happens, that would be proof the diode is bad. You can compare the voltage
to the one on the other channel. They should be within a few millivolts of
each other.

That was my next question to you... the varistor. How does one test it? My
understanding of varistors is that they are a high-value resistor at low
voltages, and a low-value resistor at higher voltages. Kinda' like a zener.
It's listed under diodes on the parts list and its' schematic icon looks
like a diode, which would indicate polarity, no? Failing to find a
datasheet or specs on this particular part, I'll just check it against the
one in the other channel for resistance with no power. Maybe hook it up to
15V with a series resistor and measure the voltage drop across it vs. the
drop with a 3V source? The "12" in the part number along with the expected
voltages across it (<2v) indicate a 12V transition point. Note to self:
keep an eye out for a variable DC power supply. If this "diode" is bad,
what would I replace it with?

I've got a device I made for TV troubleshooting, an isolation transformer /
series light bulb / GFI box, I'll use the series light bulb to check things
out.

The voltage across the emitter resistors is adjustable with V403 and is
speced at 33mV. It's nearly impossible to achieve with a single-turn 1K
pot, but the working channel is close.

Dave
 
"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:X3U6i.5973$C96.2887@newssvr23.news.prodigy.net...
If the amp appears to be running OK, with no more than about 25 millivolts
across the emitter resistor pair with no load, I would recommend turning
your attention to the bias diode D405 which looks like it ought to have
about 1.2 volts across it in normal operation. If the diode opens up, the
voltage across it will increase, maybe slowly, maybe suddenly. If this
happens, that would be proof the diode is bad. You can compare the voltage
to the one on the other channel. They should be within a few millivolts of
each other.
Next question. If the diode/MOV D405 is shorted, am I ever going to see the
correct voltage across R471/R473?

Dave
 
On May 29, 6:40 am, "Zebra" <t_perpet...@hotmail.com> wrote:
Dear All,

I have a malfunction Technics RS-TR555 Double Cassette Deck. It has soft
touch control buttons for controlling, auto reverse and independent motors.
Recently when I tried to check out my tapes before disposing them. It seems
that the logic goes wrong. For example, press "play" will forward your tape.
And after repeatedly pressing the buttons and you finally got it in the play
mode, the wrong wheel spin and the tape kept squeezing out.

Anyone got a hint on what to look for or where to find a schematic diagram?
Thanks.
Yes, you can find the schematic on Google, as always.
 
<Dicky.Links4@gmail.com> Lied in message
news:1180416052.098349.10780@r19g2000prf.googlegroups.com...

http://bulletdownloads.info/ - this free download site is way faster
than rapid share and no limits!
Except that there are no downloads, just spam links AND the site is for sale


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On May 29, 12:18 pm, "Homer J Simpson" <nob...@nowhere.com> wrote:
Dicky.Lin...@gmail.com> Lied in messagenews:1180416052.098349.10780@r19g2000prf.googlegroups.com...

http://bulletdownloads.info/- this free download site is way faster
than rapid share and no limits!

Except that there are no downloads, just spam links AND the site is for sale

--
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.
.
.
.
.
.
.
.

--
You have been reported for sending spam to sci.electronics.repair and
for impersonating a cartoon character.
 
On May 29, 1:38 pm, Malissa Baldwin <sitremaganais...@yahoo.com>
wrote:
On May 29, 6:40 am, "Zebra" <t_perpet...@hotmail.com> wrote:

Dear All,

I have a malfunction Technics RS-TR555 Double Cassette Deck. It has soft
touch control buttons for controlling, auto reverse and independent motors.
Recently when I tried to check out my tapes before disposing them. It seems
that the logic goes wrong. For example, press "play" will forward your tape.
And after repeatedly pressing the buttons and you finally got it in the play
mode, the wrong wheel spin and the tape kept squeezing out.

Anyone got a hint on what to look for or where to find a schematic diagram?
Thanks.

Yes, you can find the schematic on Google, as always.
Now, the challenge to you, Ms. Malissa... would be to show us exactly
where this schematic resides (on google, of course)... can you?

Peter Wieck
Wyncote, PA
 

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