Vintage Pioneer SX-838 receiver loses one channel after warmup

I would not completely rule out the possibility of a bad speaker relay.

It's been 6 hours now and the left channel appears to be fine. I think that was it! Your story of finding the solution in a place you never thought of looking reminds me of when my truck wouldn't start last week. The first thing I checked was the spark on my after market MSD Blaster ignition unit and I had spark all the way to the end of the plug wires so I figured it *had* to be the fuel. I spent two days tearing apart the carburetor four different times. In desperation I tried changing the MSD ignition back to the stock unit which was still in the truck and BAM! it started right up. I changed back to the MSD ignition and it had spark but no start. Changed back to the stock ignition and BAM! it started right up again. I chucked the MSD ignition.

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!

That was the easiest thing to try so I tried it first and that seems to have done the trick. I used a small strip of 600 grit sandpaper to buff the contacts and the receiver has been playing for over 6 hours without dropping the left channel. Usually it would drop out after 15 to 30 minutes, guaranteed.

I was quickly reading through the posts and I don't see Bang on IT. Thats a method I always use.

Yesterday I was drilling some holes in the bathroom wall with an inpact drill, and I did have some reserve being some AC and other stuff in the wall, but off to the side. I was sweating and I go arounfd to the other side and find in horror the thermostat screen blanked out. I start sweating even more knowing the AC is off. I banged on it and installed new batteries. I was afraid the wires might be shorting and some thermostats get some power from the line. I finally ended up banging on it again, this time in the proper method aparently and it came back on. I don't know if I can trust this thing now. I think it reset to a default temp last night cause I set it to 69 F and found it 78 this morning. I got a back up thermostat to install.

greg

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

The 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.

Presumably "tranny" means transistor in this context. (It used to mean transformer in electronics.) It doesn't seem as likely that a transistor opening (or shorting) would have a reversible effect. In most modern direct-coupled power amplifiers, a transistor fault will take out most of the other transistors in the channel and will NOT be reversible after cooling off.

So sending an audio transient into the amplifier does something that restores the signal path. My suspect list would be: solder joints, capacitors, resistors.

Another possible candidate, under these conditions, would be a bad inter-stage coupling capacitor - an electrolytic located in the signal path. These are used in some designs to block DC offset voltages in one stage from affecting the next (or previous) stage.

At this point I think you really need to compare signals on the two sides of the circuit... but with an oscilloscope rather than with a voltmeter. You're likely to find the problem faster that way.

If you don't have a scope, you should definitely acquire one. Even a slow, old scope will have plenty of bandwidth for looking at audio circuits. As a crude example, I picked up an old-style Tektronix scope (bandwidth is around 5 MHz) at a local ham-radio flea market for about $15 or $20, as a "learning and practice" scope for a friend. Works fine within its limits... that sort of scope would be adequate for your needs.

--
Dave Platt                                    AE6EO
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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.

Sometimes the bad connection is inside a part, particularly coupling capacitors. Then the bad part needs to be replaced.

Ayup. That's exactly what I'd do, starting with the heatsinked parts.

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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=B2

Pioneer used TO-220's for drivers in this model. Also, the SX-838 used flat-pack output transistors, some of the first ones. 2SC2525 and 2SB755 as I recall; certainly they were of that type.

Impractical and unnecessary to resolder all connections - there are simply too many.

These models really didn't suffer from solder problems in the small-signal circuits. Power supply and amp driver cards are another matter.

Once again, controls and switches - and those notorious Sanyo blue coupling capacitors - should be at the top of the list of possible culprits.

Mark Z.