A Audio and hi-fi forum. AudioBanter

Go Back   Home » AudioBanter forum » rec.audio » Tech
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Vintage Pioneer SX-838 receiver loses one channel after warmup



 
 
Thread Tools Display Modes
  #1  
Old May 25th 09, 06:16 AM posted to rec.audio.tech,sci.electronics.repair
Readily Visible
external usenet poster
 
Posts: 22
Default Vintage Pioneer SX-838 receiver loses one channel after warmup

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
Ads
  #2  
Old May 25th 09, 10:20 AM posted to rec.audio.tech,sci.electronics.repair
mark
external usenet poster
 
Posts: 2
Default Vintage Pioneer SX-838 receiver loses one channel after warmup

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.
  #3  
Old May 25th 09, 10:54 AM posted to rec.audio.tech,sci.electronics.repair
Arfa Daily
external usenet poster
 
Posts: 136
Default Vintage Pioneer SX-838 receiver loses one channel after warmup


"mark" > wrote in message ...
> 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


  #4  
Old May 25th 09, 12:32 PM posted to rec.audio.tech,sci.electronics.repair
Mark Zacharias
external usenet poster
 
Posts: 35
Default Vintage Pioneer SX-838 receiver loses one channel after warmup

"Arfa Daily" > wrote in message
...
>
> "mark" > wrote in message ...
>> 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 OP just got through saying he had narrowed the problem to the main amp
section...

Granted the controls and switches should be cleaned on general principles,
but inn this case I'd strongly suspect tarnished or pitted speaker relay
contacts. A knowledgeable tech can usually revive these safely; a DIY
attempt may just ruin the relay.

The speaker select switch(s) should be cleaned as well.


Mark Z.

  #5  
Old May 25th 09, 05:17 PM posted to rec.audio.tech,sci.electronics.repair
Dave
external usenet poster
 
Posts: 139
Default Vintage Pioneer SX-838 receiver loses one channel after warmup


"Readily Visible" > wrote in message
...
> 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.
>


The most common affliction older gear like yours gets is oxidized switches
and pots in the signal path. Over time oxidation builds up on the contact
surfaces of switches and pots (and relays), especially switches and pots
which are rarely used (hence the tip about the tape monitor switch(es)).
The oxidation is an insulative layer which resists the flow of current.
When the contacts stop conducting, a voltage differential is set up across
the contacts. By increasing the volume, you are increasing the voltage
differential. When it gets big enough, the signal will arc and blast a tiny
hole through the oxidation and you'll hear sound again... for awhile. If
you'd like to clean it up yourself, it ain't rocket science. For tools all
you'll need is a screwdriver to open up the receiver. You'll also need an
aerosol can of contact cleaner such as Caig DeOxit and an aerosol can of
cleaner/lubricant such as Faderlube (might be called "tuner spray"). You
can buy these, or something like them, at any electronics store including
Radio Shack. Unplug the unit and take off the cover. I haven't done a lot
of work on Pioneers so I'm not sure how accessible the front-panel switches
and knobs are, but you may need to take off the knobs and faceplate...
hopefully not as sometimes these are a real pain to get apart/together. Use
the DeOxit first. Use the thin straw that comes with the can to apply the
cleaner INSIDE each switch and pot (knob)... you need to do ALL of them.
Volume/balance/bass/treble/loudness/high-cut filter/low-cut filter/tape
monitors/speaker selector/tone defeat/turnover frequency switches and any
others you find. Each control should have one or more small holes through
which you can spray the cleaner. Remember, soaking the outside will do no
good, it's gotta get inside the control. After a good squirt, operate each
control 50 times. Don't cheat. You're scrubbing away the oxidation. Allow
the unit to dry out overnight. Now repeat the process with the Faderlube.
Faderlube containes a solvent and a lubricant which coats the innards of the
controls and keeps them from immediately re-oxidizing. After letting it dry
out overnight again, give it a try. If the problem persists, you may try
cleaning the contacts in your speaker protection relay, or just replacing
the relay... they're cheap and available. Most relays of that era have a
plastic removable cover... it may take some fiddling to figure out how it
comes off. The relay will be located near the back of the unit and will
likely be about 1" x 3/4" in size. The relay contains gossamer thin copper
arms with a contact at the ends... they're very easy to bend or break so be
very careful. You can use a fine emery board or a points file to clean up
the contact surfaces. I find that if I put the emery board between the
contacts, I can GENTLY push them closed while pulling the board out.

If none of this works, get a bright light and a magnifying glass and start
looking at solder joints... it's tedious but must be done. Start around the
speaker terminals and pay close attention to any joints which are subjected
to any mechanical forces at all.

If you happen to have an oscilloscope and a schematic for the unit, you can
probably save yourself some time as this appears to be a very reproducible
problem. Apply a sine wave to the input and start probing at the output
working backwards until you find the problem component(s).

  #6  
Old May 25th 09, 06:08 PM posted to rec.audio.tech,sci.electronics.repair
Readily Visible
external usenet poster
 
Posts: 22
Default Vintage Pioneer SX-838 receiver loses one channel after warmup

Mark Zacharias wrote:
> "Arfa Daily" > wrote in message
> ...
>> "mark" > wrote in message ...
>>> 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 OP just got through saying he had narrowed the problem to the main amp
> section...
>
> Granted the controls and switches should be cleaned on general principles,
> but inn this case I'd strongly suspect tarnished or pitted speaker relay
> contacts. A knowledgeable tech can usually revive these safely; a DIY
> attempt may just ruin the relay.
>
> The speaker select switch(s) should be cleaned as well.
>
>
> Mark Z.
>


Thanks for the careful reading, Mark.

I hadn't even considered the speaker relays. If that *is* the problem, I
should be able to confirm with a simple continuity test when the
condition prevails. Power down and test the left side for continuity.

But upon further thought, the fact that the condition persists through a
power down and power up cycle makes me doubtful that it is the relay,
which physically opens and closes during power down/up.

Also, the fact that the sound does not cut out immediately, but fades
over a period of a second or two would definitely rule out the speaker
relay. Plus the fact that when the volume is turned way up, the bad
channel can be heard faintly.

What intrigues me about this problem is that the I can cause the channel
to kick back in by turning up the volume with the speakers muted. That
indicates that the problem can be overcome by merely increasing the
voltages to the amp circuit by way of increasing the volume, without the
flow of current through the circuit.

What is interesting is what is happening as I type!

The volume is fading out more slowly now, over a period of 3 or 4
seconds and then fading back in. It did it twice since beginning to type
about what intrigues me. Now it just faded back in over a period of
about 15 to 20 seconds!

Real time troubleshooting... now it's playing okay.

One of the fade-outs was so gradual that I thought it was part of the
music on the jazz station, but then it faded back in just as gradually
and the same song was playing. It did that long fade out/in twice.

Still playing okay, but those fade-outs and fade-ins were unmistakably
long and drawn out. Now it occurs to me that it might have been the
station fading in and out for some reason. I have the receiver set to
mono and the balance set all the way to the left channel so I can
monitor it while hearing both channels of music. I don't know what was
happening with the right channel during those drawn out fades. But, as I
now recall, I have noticed this same behavior once before with the CD
player driving the unit.

Now the left channel has done a quick fade out. I will wait to see if if
it revives on it's own.

Ten minutes and the channel is still out. Turning the volume up a bit
past half way kicked it back in. It then kicked out more three times in
well within a minute and kicked back in when the volume was turned up to
the same point, just a bit past halfway.

It seems to be cutting out more frequently after about 30 to 45 minutes
of playing. The last time the cut out was abrupt, as if a switch were
opened.

Time to put it on the desk and bust the case open.
  #7  
Old May 25th 09, 06:28 PM posted to rec.audio.tech,sci.electronics.repair
external usenet poster
 
Posts: 1
Default Vintage Pioneer SX-838 receiver loses one channel after warmup

On May 25, 1:08*pm, Readily Visible > wrote:
> Mark Zacharias wrote:
> > "Arfa Daily" > wrote in message
> ...
> >> "mark" > wrote in ....
> >>> 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 OP just got through saying he had narrowed the problem to the main amp
> > section...

>
> > Granted the controls and switches should be cleaned on general principles,
> > but inn this case I'd strongly suspect tarnished or pitted speaker relay
> > contacts. A knowledgeable tech can usually revive these safely; a DIY
> > attempt may just ruin the relay.

>
> > The speaker select switch(s) should be cleaned as well.

>
> > Mark Z.

>
> Thanks for the careful reading, Mark.
>
> I hadn't even considered the speaker relays. If that *is* the problem, I
> should be able to confirm with a simple continuity test when the
> condition prevails. Power down and test the left side for continuity.
>
> But upon further thought, the fact that the condition persists through a
> power down and power up cycle makes me doubtful that it is the relay,
> which physically opens and closes during power down/up.
>
> Also, the fact that the sound does not cut out immediately, but fades
> over a period of a second or two would definitely rule out the speaker
> relay. Plus the fact that when the volume is turned way up, the bad
> channel can be heard faintly.
>
> What intrigues me about this problem is that the I can cause the channel
> to kick back in by turning up the volume with the speakers muted. That
> indicates that the problem can be overcome by merely increasing the
> voltages to the amp circuit by way of increasing the volume, without the
> flow of current through the circuit.
>
> What is interesting is what is happening as I type!
>
> The volume is fading out more slowly now, over a period of 3 or 4
> seconds and then fading back in. It did it twice since beginning to type
> about what intrigues me. Now it just faded back in over a period of
> about 15 to 20 seconds!
>
> Real time troubleshooting... now it's playing okay.
>
> One of the fade-outs was so gradual that I thought it was part of the
> music on the jazz station, but then it faded back in just as gradually
> and the same song was playing. It did that long fade out/in twice.
>
> Still playing okay, but those fade-outs and fade-ins were unmistakably
> long and drawn out. Now it occurs to me that it might have been the
> station fading in and out for some reason. I have the receiver set to
> mono and the balance set all the way to the left channel so I can
> monitor it while hearing both channels of music. I don't know what was
> happening with the right channel during those drawn out fades. But, as I
> now recall, I have noticed this same behavior once before with the CD
> player driving the unit.
>
> Now the left channel has done a quick fade out. I will wait to see if if
> it revives on it's own.
>
> Ten minutes and the channel is still out. Turning the volume up a bit
> past half way kicked it back in. It then kicked out more three times in
> well within a minute and kicked back in when the volume was turned up to
> the same point, just a bit past halfway.
>
> It seems to be cutting out more frequently after about 30 to 45 minutes
> of playing. The last time the cut out was abrupt, as if a switch were
> opened.
>
> Time to put it on the desk and bust the case open.- Hide quoted text -
>
> - Show quoted text -


Bad FET's were a problem with some of the older receivers too.
Sometimes a can of cold spray used in small areas at a time can reveal
much. Then reheating the suspect component with a soldering iron can
help nail down the culprit. But a word of caurtion here. Don't
overheat. Lenny
  #8  
Old May 25th 09, 07:13 PM posted to rec.audio.tech,sci.electronics.repair
Readily Visible
external usenet poster
 
Posts: 22
Default Vintage Pioneer SX-838 receiver loses one channel after warmup

wrote:
> On May 25, 1:08 pm, Readily Visible > wrote:
>> Mark Zacharias wrote:
>>> "Arfa Daily" > wrote in message
>>> ...
>>>> "mark" > wrote in ...
>>>>> 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 OP just got through saying he had narrowed the problem to the main amp
>>> section...
>>> Granted the controls and switches should be cleaned on general principles,
>>> but inn this case I'd strongly suspect tarnished or pitted speaker relay
>>> contacts. A knowledgeable tech can usually revive these safely; a DIY
>>> attempt may just ruin the relay.
>>> The speaker select switch(s) should be cleaned as well.
>>> Mark Z.

>> Thanks for the careful reading, Mark.
>>
>> I hadn't even considered the speaker relays. If that *is* the problem, I
>> should be able to confirm with a simple continuity test when the
>> condition prevails. Power down and test the left side for continuity.
>>
>> But upon further thought, the fact that the condition persists through a
>> power down and power up cycle makes me doubtful that it is the relay,
>> which physically opens and closes during power down/up.
>>
>> Also, the fact that the sound does not cut out immediately, but fades
>> over a period of a second or two would definitely rule out the speaker
>> relay. Plus the fact that when the volume is turned way up, the bad
>> channel can be heard faintly.
>>
>> What intrigues me about this problem is that the I can cause the channel
>> to kick back in by turning up the volume with the speakers muted. That
>> indicates that the problem can be overcome by merely increasing the
>> voltages to the amp circuit by way of increasing the volume, without the
>> flow of current through the circuit.
>>
>> What is interesting is what is happening as I type!
>>
>> The volume is fading out more slowly now, over a period of 3 or 4
>> seconds and then fading back in. It did it twice since beginning to type
>> about what intrigues me. Now it just faded back in over a period of
>> about 15 to 20 seconds!
>>
>> Real time troubleshooting... now it's playing okay.
>>
>> One of the fade-outs was so gradual that I thought it was part of the
>> music on the jazz station, but then it faded back in just as gradually
>> and the same song was playing. It did that long fade out/in twice.
>>
>> Still playing okay, but those fade-outs and fade-ins were unmistakably
>> long and drawn out. Now it occurs to me that it might have been the
>> station fading in and out for some reason. I have the receiver set to
>> mono and the balance set all the way to the left channel so I can
>> monitor it while hearing both channels of music. I don't know what was
>> happening with the right channel during those drawn out fades. But, as I
>> now recall, I have noticed this same behavior once before with the CD
>> player driving the unit.
>>
>> Now the left channel has done a quick fade out. I will wait to see if if
>> it revives on it's own.
>>
>> Ten minutes and the channel is still out. Turning the volume up a bit
>> past half way kicked it back in. It then kicked out more three times in
>> well within a minute and kicked back in when the volume was turned up to
>> the same point, just a bit past halfway.
>>
>> It seems to be cutting out more frequently after about 30 to 45 minutes
>> of playing. The last time the cut out was abrupt, as if a switch were
>> opened.
>>
>> Time to put it on the desk and bust the case open.- Hide quoted text -
>>
>> - Show quoted text -

>
> Bad FET's were a problem with some of the older receivers too.
> Sometimes a can of cold spray used in small areas at a time can reveal
> much. Then reheating the suspect component with a soldering iron can
> help nail down the culprit. But a word of caurtion here. Don't
> overheat. Lenny


I think you be on to something, Lenny.

After reading about FET's on Wikipedia, I came across this:

"The FET controls the flow of electrons (or electron holes) from the
source to drain by affecting the size and shape of a "conductive
channel" created and influenced by voltage (or lack of voltage) applied
across the gate and source terminals."

This jives with the behavior that I mentioned in the post you replied to
about how the channel will kick back in by applying a higher voltage to
the amp circuit by turning up the volume, even with the speakers muted
so that no actual current is flowing through the amp.

Now, how to identify the FET's as opposed to generic transistors... a
quick google image search for 'fet' returns several schematics. The
symbol for FET's look different than the symbol for what I refer to as
'generic transistors'. I don't see any FET symbols in the amp section of
the schematics for the SX-838, however. Could the schematics from this
period use the round transistor symbol for FET's rather than the
squarish looking symbol?
  #9  
Old May 25th 09, 07:18 PM posted to rec.audio.tech,sci.electronics.repair
external usenet poster
 
Posts: 1
Default Vintage Pioneer SX-838 receiver loses one channel after warmup

On Sun, 24 May 2009 22:16:40 -0700, 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

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 he
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
  #10  
Old May 25th 09, 07:18 PM posted to rec.audio.tech,sci.electronics.repair
Readily Visible
external usenet poster
 
Posts: 22
Default Vintage Pioneer SX-838 receiver loses one channel after warmup

wrote:
> On May 25, 1:08 pm, Readily Visible > wrote:
>> Mark Zacharias wrote:
>>> "Arfa Daily" > wrote in message
>>> ...
>>>> "mark" > wrote in ...
>>>>> 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 OP just got through saying he had narrowed the problem to the main amp
>>> section...
>>> Granted the controls and switches should be cleaned on general principles,
>>> but inn this case I'd strongly suspect tarnished or pitted speaker relay
>>> contacts. A knowledgeable tech can usually revive these safely; a DIY
>>> attempt may just ruin the relay.
>>> The speaker select switch(s) should be cleaned as well.
>>> Mark Z.

>> Thanks for the careful reading, Mark.
>>
>> I hadn't even considered the speaker relays. If that *is* the problem, I
>> should be able to confirm with a simple continuity test when the
>> condition prevails. Power down and test the left side for continuity.
>>
>> But upon further thought, the fact that the condition persists through a
>> power down and power up cycle makes me doubtful that it is the relay,
>> which physically opens and closes during power down/up.
>>
>> Also, the fact that the sound does not cut out immediately, but fades
>> over a period of a second or two would definitely rule out the speaker
>> relay. Plus the fact that when the volume is turned way up, the bad
>> channel can be heard faintly.
>>
>> What intrigues me about this problem is that the I can cause the channel
>> to kick back in by turning up the volume with the speakers muted. That
>> indicates that the problem can be overcome by merely increasing the
>> voltages to the amp circuit by way of increasing the volume, without the
>> flow of current through the circuit.
>>
>> What is interesting is what is happening as I type!
>>
>> The volume is fading out more slowly now, over a period of 3 or 4
>> seconds and then fading back in. It did it twice since beginning to type
>> about what intrigues me. Now it just faded back in over a period of
>> about 15 to 20 seconds!
>>
>> Real time troubleshooting... now it's playing okay.
>>
>> One of the fade-outs was so gradual that I thought it was part of the
>> music on the jazz station, but then it faded back in just as gradually
>> and the same song was playing. It did that long fade out/in twice.
>>
>> Still playing okay, but those fade-outs and fade-ins were unmistakably
>> long and drawn out. Now it occurs to me that it might have been the
>> station fading in and out for some reason. I have the receiver set to
>> mono and the balance set all the way to the left channel so I can
>> monitor it while hearing both channels of music. I don't know what was
>> happening with the right channel during those drawn out fades. But, as I
>> now recall, I have noticed this same behavior once before with the CD
>> player driving the unit.
>>
>> Now the left channel has done a quick fade out. I will wait to see if if
>> it revives on it's own.
>>
>> Ten minutes and the channel is still out. Turning the volume up a bit
>> past half way kicked it back in. It then kicked out more three times in
>> well within a minute and kicked back in when the volume was turned up to
>> the same point, just a bit past halfway.
>>
>> It seems to be cutting out more frequently after about 30 to 45 minutes
>> of playing. The last time the cut out was abrupt, as if a switch were
>> opened.
>>
>> Time to put it on the desk and bust the case open.- Hide quoted text -
>>
>> - Show quoted text -

>
> Bad FET's were a problem with some of the older receivers too.
> Sometimes a can of cold spray used in small areas at a time can reveal
> much. Then reheating the suspect component with a soldering iron can
> help nail down the culprit. But a word of caurtion here. Don't
> overheat. Lenny


I think the type of transistor that I am referring to as a 'generic
transistor' might more accurately be referred to as a 'BJT':

http://en.wikipedia.org/wiki/BJT

This uses the symbol that I am used to seeing for a transistor.

The FET's seem to be represented by this type of symbol:

http://commons.wikimedia.org/wiki/Fi...nal_symbol.png

Could the FET's, if any, in the Pioneer SX-838 be represented in the
original schematics by the symbol commonly used for BJT's?
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
ELECTRONICS CLEARANCE SALE- PIONEER-JVC-KENWOOD-SONY-EMERSON-6 CHANNEL DISCRETE SACD/DVD-A RECEIVER-VINTAGE TAPE DECKS-MORE-CD DISK CHANGERS-CASSETTE DECKS-RECEIVERS-ALL ITEMS STARTING AT ONE CENT OPENING PRICE ! duty-honor-country[_2_] Marketplace 2 May 30th 07 04:11 PM
ELECTRONICS CLEARANCE SALE- PIONEER-JVC-KENWOOD-SONY-EMERSON-6 CHANNEL DISCRETE SACD/DVD-A RECEIVER-VINTAGE TAPE DECKS-MORE-CD DISK CHANGERS-CASSETTE DECKS-RECEIVERS-ALL ITEMS STARTING AT ONE CENT OPENING PRICE ! duty-honor-country[_2_] General 0 May 30th 07 01:08 PM
FA:Pioneer VSDX810S 7.1 channel input receiver Marketplace 0 February 7th 04 11:31 PM
FA:Pioneer VSDX810S 7.1 channel input receiver Marketplace 0 February 7th 04 11:31 PM


All times are GMT +1. The time now is 03:02 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright 2004-2014 AudioBanter.
The comments are property of their posters.