Dave M wrote:
"Readily Visible" wrote in message
...
wrote:
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
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.
Jack