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One fact can't be beaten - it is always the last thing you try."Readily Visible" <abc@xyz.com> wrote in message
news:BSBSl.23075$as4.9021@nlpi069.nbdc.sbc.com...
I understand. But doesn't the fact that the left channel continues to be
the one that drops out when I switch the right channel preamp output to
the left channel main amp input and same for the other two ins and outs
at the back of the unit... doesn't this isolate the problem to the main
amp section? The left channel is out at the moment and operating the
tape monitor switches does nothing to bring it back. But turning up the
volume to just past halfway *always* brings it back.
You are absolutely correct, I did not read your post clearly enough. If the
problem stays with the amp when the channels are switched, it's definitely
in the amp. Usually when transistors start to fail you get static, not the
fading in/out you describe. I'll keep my fingers crossed for you that
cleaning the relay works... it can be a real pain to track down this type of
fault. I have spent literally days working on a piece of equipment and it
ends up being something I never even thought of.
It's been 6 hours now and the left channel appears to be fine. I think"Readily Visible" <abc@xyz.com> wrote in message
news:BSBSl.23075$as4.9021@nlpi069.nbdc.sbc.com...
I understand. But doesn't the fact that the left channel continues to be
the one that drops out when I switch the right channel preamp output to
the left channel main amp input and same for the other two ins and outs
at the back of the unit... doesn't this isolate the problem to the main
amp section? The left channel is out at the moment and operating the
tape monitor switches does nothing to bring it back. But turning up the
volume to just past halfway *always* brings it back.
You are absolutely correct, I did not read your post clearly enough. If the
problem stays with the amp when the channels are switched, it's definitely
in the amp. Usually when transistors start to fail you get static, not the
fading in/out you describe. I'll keep my fingers crossed for you that
cleaning the relay works... it can be a real pain to track down this type of
fault. I have spent literally days working on a piece of equipment and it
ends up being something I never even thought of.
That was the easiest thing to try so I tried it first and that seems toI would not completely rule out the possibility of a bad speaker relay.
But next tiem you rip into the carb it will only take you 25% of the time!After all that aggravation with futilely tearing apart the carb in
frustration, I guess I was due for an easy layup with the Pioneer. I
guess I was tricked by the fading out and fading in into thinking it
couldn't be the speaker relay. I have never been so happy to be wrong!
Dave M wrote:
"Readily Visible" <abc@xyz.com> wrote in message
news:%YBSl.23076$as4.12524@nlpi069.nbdc.sbc.com...
PlainBill47@yahoo.com wrote:
On Sun, 24 May 2009 22:16:40 -0700, Readily Visible <abc@xyz.com
wrote:
I came into possession of this beautiful old Pioneer SX-838 receiver a
couple of years ago and I would like to fix this problem. After the unit
has been playing for a half hour or so, irrespective of source, the left
channel will drop out. When it drops out, it fades out over a period of
a second or two. It does not cut out instantaneously.
To isolate the problem I switched the preamp-to-main amp connections in
the back so that the left preamp channel feeds the right main amp
channel and vice versa. The left channel continued to drop out. This
tells me that the problem is in the main amp section (speakers are *not*
the problem). I was hoping that the problem was in the volume pot, this
behavior seems to rule that out and points to the left channel of the
main amp section.
If I turn the volume up high enough the dropped channel can be heard
faintly and if I turn it up even higher it will kick back in with a
crackle and play okay for a while before dropping out again. To avoid
blasting the volume out of the speakers when doing this I used the
speaker button on the front to disconnect the speakers. This technique
resurrects the channel as well as when I leave the speakers connected.
This seems significant as turning the volume up past a certain point
resurrects the channel whether or not current is flowing through the
circuit. Apparently, the higher voltage applied to the circuit is enough
to do the trick.
When the channel drops out, I can power down the unit for 10 or 15
seconds and when I turn it on again the channel is still out.
Can anyone suggest to me which type of component(s) might be causing
this behavior, resistor, capacitor or semiconductor? I have the
schematics and I don't see any coils. I don't suspect the power supply
because both channels seem to be powered by the same circuitry and if
one channel went out, both would go out.
I don't have a scope or a signal generator for probing the circuit, just
an analog and a digital meter.
Any help appreciated.
Thanks
I can think of a number of possible causes, but it should be possible
to run down the problem without any sophisticated test equipment.
Yes, a signal generator and scope would make it a trivial problem.
The schematic / service manual seems to be available free here:
http://www.hifiengine.com/manuals/pioneer/sx-838.shtml (registration
required).
I'd suggest setting the volume at a moderate level and measuring the
AC signal level at various points in the signal path, comparing the
levels at right and left channels. Use a mono source. You might get
exceptionally lucky and discover the signal pops back in when you
probe a particular point.
PlainBill
Excellent link, Bill, thank you!
I have the schematics but not the service manual.
I am preparing to bust open the case again.
You've shown a good thought process in diagnosing the problem area. I agree
with you that the most likely suspect is the power amp section. Now to get
into the circuit.
If you have the schematic and a couple of good multimeters, it's time to use
them. Open the unit to the point that you can easily probe the power amp
circuit board and the output transistors. And yes, they are plain old BJTs
(not FETs).
Play the unit until the left channel dies. While it's still powered up,
check the voltages around all the transistors on the power amp board (as
well as the power transistors for the left channel. Use the voltages around
the working right channel as a reference when the voltages aren't present on
the schematic. Since this is a DC coupled circuit, it might be a little
tricky to identify the bad guy immediately, but with a bit of logic and
transistor theory, it can be done. You will probably see a significant
discrepancy somewhere in the signal chain. If you have trouble sorting out
the measurements and what they mean, write them down and post them back
here. Some very good techs here can help.
I've seen transistors become intermittent. That is, they work for a while
and then for no apparent reason, they die. This could be due to one of the
bonding wires inside the transistor being broken away from the silicon die
or the transistor lead. Sometimes, a physical shock such as a tap with a
dowel or screwdriver handle will make it act up. Other times, when the
transistor heats up from use, the bonding wire will separate.
Since the audio dies slowly (as opposed to suddenly), the problem has the
bookmarks of a heat-related problem. As has been suggested, a very *quick*
blast of circuit cooler can be very helpful. Be aware of the *quick* blast
here. The most common mistake in using circuit cooler spray is the tendency
to freeze the components to the point that ice forms on them. A one-second
(or less) blast is all that's needed to cool a small component such as a
small transistor, diode, resistor, etc. Focus the spray on a single
component at a time, so you can easily identify it if the channel
immediately starts working again.
Hth,
Dave, I am thoroughly impressed with all the help I am receiving here.
Thank you all very much! Mark Z suggested checking the speaker relay for
bad contacts. This made sense at first as it is on the downstream side
of the preamp-amp connectors on the back and I have established that the
problem is downstream from this point and the speaker relays are about
as downstream os you can get.
But then I realized that I remember the left channel quickly fading out
in the past I jumped to the conclusion that a bad speaker contact would
tend to cut out instantaneously. But of course, I can't say why exactly
I arrived at that conclusion. At any rate, after cleaning the speaker
relay contacts, the left channel has been playing for about 25 minutes
without cutting out.
If it *does* cut out again, I will follow your advice as far as I can as
soon as I can get a can of freeze at the local electronics supply store.
I will keep the group posted.
Usually for me, intermittent goes away as soon as I pull the cover off and"Major Debacle" <Major_Debacle@pentagon.mil> wrote in message
news:FNJSl.30440$Ws1.9799@nlpi064.nbdc.sbc.com...
After all that aggravation with futilely tearing apart the carb in
frustration, I guess I was due for an easy layup with the Pioneer. I
guess I was tricked by the fading out and fading in into thinking it
couldn't be the speaker relay. I have never been so happy to be wrong!
But next tiem you rip into the carb it will only take you 25% of the time!
Ha ha, too bad you probably will never have to. I had a similar situation
with my car last year, it turns out I had an extra fuel injector in the
throttle body for cold starts which kept coming on when it shouldn't...
that
was a WHORE to track down, it took MONTHS of replacing suspect parts.
Good news on the Pioneer though. Dirty controls and relays are, as I said
from the git-go, by a factor of about 10X the most common ailment these
old
dogs have... oxidiation is unstoppable even in dry climates. The great
thing about this group (and others like it) is that there is probably a
combined 100 years of tech experience monitoring the posts at any given
time...
Dave
In that sort of situation, I'll beat up on the unit a little."Dave" <dspear99ca@yahoo.com> wrote in message
news:wcTSl.28154$Db2.20380@edtnps83...
"Major Debacle" <Major_Debacle@pentagon.mil> wrote in message
news:FNJSl.30440$Ws1.9799@nlpi064.nbdc.sbc.com...
After all that aggravation with futilely tearing apart the carb in
frustration, I guess I was due for an easy layup with the Pioneer. I
guess I was tricked by the fading out and fading in into thinking it
couldn't be the speaker relay. I have never been so happy to be wrong!
But next tiem you rip into the carb it will only take you 25% of the time!
Ha ha, too bad you probably will never have to. I had a similar situation
with my car last year, it turns out I had an extra fuel injector in the
throttle body for cold starts which kept coming on when it shouldn't...
that
was a WHORE to track down, it took MONTHS of replacing suspect parts.
Good news on the Pioneer though. Dirty controls and relays are, as I said
from the git-go, by a factor of about 10X the most common ailment these
old
dogs have... oxidiation is unstoppable even in dry climates. The great
thing about this group (and others like it) is that there is probably a
combined 100 years of tech experience monitoring the posts at any given
time...
Dave
Usually for me, intermittent goes away as soon as I pull the cover off and
comes back when I put the cover back on, so the answer is to just leave the
cover off and use it.
Been busy at what is for me the bread and butter work. I picked up a can"mark" <bk416@lafn.org> wrote in message news:4A1A62FA.7010505@lafn.org...
Readily Visible wrote:
I came into possession of this beautiful old Pioneer SX-838 receiver a
couple of years ago and I would like to fix this problem. After the unit
has been playing for a half hour or so, irrespective of source, the left
channel will drop out. When it drops out, it fades out over a period of
a second or two. It does not cut out instantaneously.
To isolate the problem I switched the preamp-to-main amp connections in
the back so that the left preamp channel feeds the right main amp
channel and vice versa. The left channel continued to drop out. This
tells me that the problem is in the main amp section (speakers are *not*
the problem). I was hoping that the problem was in the volume pot, this
behavior seems to rule that out and points to the left channel of the
main amp section.
If I turn the volume up high enough the dropped channel can be heard
faintly and if I turn it up even higher it will kick back in with a
crackle and play okay for a while before dropping out again. To avoid
blasting the volume out of the speakers when doing this I used the
speaker button on the front to disconnect the speakers. This technique
resurrects the channel as well as when I leave the speakers connected.
This seems significant as turning the volume up past a certain point
resurrects the channel whether or not current is flowing through the
circuit. Apparently, the higher voltage applied to the circuit is enough
to do the trick.
When the channel drops out, I can power down the unit for 10 or 15
seconds and when I turn it on again the channel is still out.
Can anyone suggest to me which type of component(s) might be causing
this behavior, resistor, capacitor or semiconductor? I have the
schematics and I don't see any coils. I don't suspect the power supply
because both channels seem to be powered by the same circuitry and if
one channel went out, both would go out.
I don't have a scope or a signal generator for probing the circuit, just
an analog and a digital meter.
Any help appreciated.
Thanks
Push in the tape monitor switches when the problem occurs, tape
monitor switches are a source of problems on a lot of old stereos they
often need to be cleaned.
Seconded. Next suspect after that has to be bad joints, particularly on any
devices screwed to the heatsink. You can look for bad joints (and to some
extent, dicky connectors) by pressing and tapping on the PCB with some kind
of insulated rod - a piece of wood dowelling for instance. After that, a can
of freezer spray, and a hair drier are your best friends ...
Arfa
The left channel bugged out again and I double confirmed that it isArfa Daily wrote:
"mark" <bk416@lafn.org> wrote in message news:4A1A62FA.7010505@lafn.org...
Readily Visible wrote:
I came into possession of this beautiful old Pioneer SX-838 receiver a
couple of years ago and I would like to fix this problem. After the unit
has been playing for a half hour or so, irrespective of source, the left
channel will drop out. When it drops out, it fades out over a period of
a second or two. It does not cut out instantaneously.
To isolate the problem I switched the preamp-to-main amp connections in
the back so that the left preamp channel feeds the right main amp
channel and vice versa. The left channel continued to drop out. This
tells me that the problem is in the main amp section (speakers are *not*
the problem). I was hoping that the problem was in the volume pot, this
behavior seems to rule that out and points to the left channel of the
main amp section.
If I turn the volume up high enough the dropped channel can be heard
faintly and if I turn it up even higher it will kick back in with a
crackle and play okay for a while before dropping out again. To avoid
blasting the volume out of the speakers when doing this I used the
speaker button on the front to disconnect the speakers. This technique
resurrects the channel as well as when I leave the speakers connected.
This seems significant as turning the volume up past a certain point
resurrects the channel whether or not current is flowing through the
circuit. Apparently, the higher voltage applied to the circuit is enough
to do the trick.
When the channel drops out, I can power down the unit for 10 or 15
seconds and when I turn it on again the channel is still out.
Can anyone suggest to me which type of component(s) might be causing
this behavior, resistor, capacitor or semiconductor? I have the
schematics and I don't see any coils. I don't suspect the power supply
because both channels seem to be powered by the same circuitry and if
one channel went out, both would go out.
I don't have a scope or a signal generator for probing the circuit, just
an analog and a digital meter.
Any help appreciated.
Thanks
Push in the tape monitor switches when the problem occurs, tape
monitor switches are a source of problems on a lot of old stereos they
often need to be cleaned.
Seconded. Next suspect after that has to be bad joints, particularly on any
devices screwed to the heatsink. You can look for bad joints (and to some
extent, dicky connectors) by pressing and tapping on the PCB with some kind
of insulated rod - a piece of wood dowelling for instance. After that, a can
of freezer spray, and a hair drier are your best friends ...
Arfa
Been busy at what is for me the bread and butter work. I picked up a can
of Super Cold 134 today and I am waiting for the left channel to bug out
again. Yesterday, when I didn't have stuff, it was bugging out fairly
regular. Poking at the main amp components didn't seem to have much
effect at this point like it seemed to have earlier.
I will post any significant developments.
Jack
Presumably "tranny" means transistor in this context. (It used to meanThe left channel bugged out again and I double confirmed that it is
*not* the speaker relay. I also double confirmed that switching right
and left preamp outputs to left and right power amp inputs
respectively *does not* change which channel drops out, confirming
that the problem is downstream of these connections. Then, with the
left channel out, I sprayed some freeze onto the tranny I suspected
of being at fault, with no results. I then sprayed every other tranny
on the amp board, including the output trannies. No change. Then I
sprayed all the resistors and caps. Then I gave the whole amp board a
general spraying.
So sending an audio transient into the amplifier does something thatI got absolutely no reaction.
The channel continues to respond to an increase in volume, that is, it
kicks back in with a crackle when the volume pot is turned just past
the halfway point.
Another possible candidate, under these conditions, would be a badThe left channel bugged out again and I double confirmed that it is
*not* the speaker relay. I also double confirmed that switching right
and left preamp outputs to left and right power amp inputs respectively
*does not* change which channel drops out, confirming that the problem
is downstream of these connections. Then, with the left channel out, I
sprayed some freeze onto the tranny I suspected of being at fault, with
no results. I then sprayed every other tranny on the amp board,
including the output trannies. No change. Then I sprayed all the
resistors and caps. Then I gave the whole amp board a general spraying.
I got absolutely no reaction.
The channel continues to respond to an increase in volume, that is, it
kicks back in with a crackle when the volume pot is turned just past the
halfway point.
I guess now is the time to begin probing with a voltage meter on the
right and left sections of the amp board to see where I might find
discrepancies.
Agreed. This is the sort of problem that often goes away after someone goesSo sending an audio transient into the amplifier does
something that restores the signal path. My suspect list
would be: solder joints, capacitors, resistors.
Ayup. That's exactly what I'd do, starting with the heatsinked parts."Richard Crowley" <rcrowley@xp7rt.net> wrote in message
news:5MednVkAwpaJVbzXnZ2dnUVZ_rydnZ2d@posted.pcez
So sending an audio transient into the amplifier does
something that restores the signal path. My suspect list
would be: solder joints, capacitors, resistors.
Agreed. This is the sort of problem that often goes away after someone goes
through and reconditions every solder joint in the machine.
As we both no doubt know, the failure mechanism is that an insulating or
semiconducting oxide or sulphide film forms in some signal or supply path,
which has a very low break-over voltage. The path usually involves a
soldered joint that has either cracked in use, or was defective all along,
but took years to fail.
Applying enough voltage to "punch though" the insulating film, will create a
temporary signal path, which eventually gets interrupted again by corrosion.
A hot, tinned soldering iron in skilled hands, is applied to every soldered
joint, with a little flux and solder being applied where needed to ensure a
good soldered joint. Sometimes it is is necessary to remove the existing
solder with a solder pump, and resolder the joint completely.
Back in the day I replaced a fair number of TO-220 case powerArny Krueger wrote:
"Richard Crowley" <rcrow...@xp7rt.net> wrote in message
news:5MednVkAwpaJVbzXnZ2dnUVZ_rydnZ2d@posted.pcez
So sending an audio transient into the amplifier does
something that restores the signal path. My suspect list
would be: solder joints, capacitors, resistors.
Agreed. This is the sort of problem that often goes away after someone goes
through and reconditions every solder joint in the machine.
As we both no doubt know, the failure mechanism is that an insulating or
semiconducting oxide or sulphide film forms in some signal or supply path,
which has a very low break-over voltage. The path usually involves a
soldered joint that has either cracked in use, or was defective all along,
but took years to fail.
Applying enough voltage to "punch though" the insulating film, will create a
temporary signal path, which eventually gets interrupted again by corrosion.
A hot, tinned soldering iron in skilled hands, is applied to every soldered
joint, with a little flux and solder being applied where needed to ensure a
good soldered joint. Sometimes it is is necessary to remove the existing
solder with a solder pump, and resolder the joint completely.
Ayup. That's exactly what I'd do, starting with the heatsinked parts.
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
Arny Krueger wrote:
"Richard Crowley" <rcrow...@xp7rt.net> wrote in message
news:5MednVkAwpaJVbzXnZ2dnUVZ_rydnZ2d@posted.pcez
So sending an audio transient into the amplifier does
something that restores the signal path. My suspect list
would be: solder joints, capacitors, resistors.
Agreed. This is the sort of problem that often goes away after someone
goes
through and reconditions every solder joint in the machine.
As we both no doubt know, the failure mechanism is that an insulating or
semiconducting oxide or sulphide film forms in some signal or supply
path,
which has a very low break-over voltage. The path usually involves a
soldered joint that has either cracked in use, or was defective all
along,
but took years to fail.
Applying enough voltage to "punch though" the insulating film, will
create a
temporary signal path, which eventually gets interrupted again by
corrosion.
A hot, tinned soldering iron in skilled hands, is applied to every
soldered
joint, with a little flux and solder being applied where needed to
ensure a
good soldered joint. Sometimes it is is necessary to remove the existing
solder with a solder pump, and resolder the joint completely.
Ayup. That's exactly what I'd do, starting with the heatsinked parts.
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
Back in the day I replaced a fair number of TO-220 case power
transistors that would open the base lead when they warmed up. Does
that Pioneer use any of those?
G˛