Vintage Pioneer SX-838 receiver - UPDATE

Of course you can, that's when you use a hot air gun and "freezer" spray. In fact heat related issues are usually the easiest of intermittent faults to find IMO.

MrT.

I agree. Most probably heat *and* position related. As I said originally, look for bad joints on items screwed to the heatsink. When the thing is the right way up, heat expansion on a bad joint may well cause it to become intermittent. When it's on it's side, the change in mechanical force on that joint, could easily keep it *good*, electrically.

Arfa

I will often take a BIC pen or a wooden chopstick or the like and push, poke, tap around etc to help find intermittent connections.

BTW, the fact that you may have tended to the relay contacts does not eliminate the relay completely. This is where the chopstick or whatever plastic object you have comes in handy.

Mark Z.

Gosh this is one you want in front of you, I have been reading the posts to this and thought I would throw in my half pennys worth. When I first read your description I was going to say get out freezer spray, but the fact it started working for hours after cleaning relay contacts - makes me suspicious of them still. Maybe others will have same experience, if they are not big meaty relays often cleaning is not good enough, and replacement is only sure fire way of making good repair. If you are feeling adventurous you could bridge across relays

- to eliminate them from being the problem, until you have found fault. Remember it is a hazard if left open no kids near please. Regards Martin

But it was on its side with the top and bottom off when it played for hours after cleaning the relays. I was reluctant to put it back in its proper place until I was sure it was playing okay. It could be that the relays were fine and the fact that it was open and on its side may have been the factors that allowed the left channel to play for hours without bugging out.

I was waiting for the left channel to drop out to try that on the spot with a jumper to see if that kicked the bad channel back in, but I may have to jumper it and reassemble it to tell for sure as it only seems to bug out when the case is fully assembled and it is properly oriented.

Or run the temporary relay jumper wires out of the case through an air vent so you can open/close to test the theory with the case closed.

Good one, Jim. I like it.

It has been playing on its side with the top and bottom off for over 10 hours now and no problem. If I do as you suggest and I find it is not the relay I guess I will have to resort to heating up the amp board and output trannies to see if I can replicate the problem with the case and bottom off. I have a cheap hairdryer that I bought to use as a heat gun on the job when I need one. That should be sufficient to heat things up enough to trip the problem if the problem is heat related.

Yesterday with the unit on its side and the top and bottom removed, it played for 10 hours without the problem left channel dropping out.

This was using 16 ohm speakers.

Today I hooked the unit up to 8 ohm speakers, still on its side and the top and bottom removed, and after less than two hours of play, the left channel dropped out. It could have been sooner, I set it to stereo and set the balance to the midpoint. I checked it after about two hours of play and the left channel was gone.

Here is where it gets pinpointy. First I tried jumping the speaker relay solder joints and got nothing, so that rules out the speaker relay as the problem.

Then I got a nice Japanese style pine chopstick and started poking on the amp board. First, since the bottom was oriented to me, I started poking all the cold solder joints that I had earlier hot soldered. No change. The left channel remained dead. Then I rotated the unit, still standing on it's end and began poking all the trannies, resistors, coils, caps and whatnot. First time through, nothing. Then, when I poked the leads of one of those flat trannies screwed to a heat sink on the left side of the amp board, the left channel kicked back in.

For those who are really interested, here is a link to the service manual with parts lists and schematics:

The tranny in question is 2SD358 on the left channel side. It is one of those that drives one of the output trannies. The solder joints are hard to access, but look to be well soldered, so I am going to assume, at this point, that poking the leads corrected, for the moment, an internal fault in the tranny. If the local electronics supply shop has the NTE equivalent, I may swap it out at some point, but for now I will just let the unit play to see what may develop.

Wow, just after I posted this, I got up to check the unit. Left channel has dropped out again so naturally I am going straight to the 2SD358 tranny on the left side.

Here I go, with my exquisite Japanese style pine chopstick...

Well here's what happened. The unit is on the floor and as I lay down next to it and positioned myself to poke the tranny (damn, that sounds suggestive!) I hit the power on/off switch and cut the power. I switched it back on and the left channel started playing. I then poked the tranny (this is definitely *not* what it sounds like) and the left channel dropped out.

Well, after a few minutes of poking the tranny with the left channel playing and the left channel dead, I can't say for sure what is happening. The left channel is playing now and no amount of poking this specific tranny makes it drop out. When it does drop out, the trick of turning the volume up to a specific point kicks it back in.

If it weren't for the fact that the left channel continues to be the one to drop out after connecting the left preamp output with the right power amp input and vice versa, I would bet money on the volume pot.

Well, it's continuing to play without a hitch so far, but at least I have a suspect.

You need to get some deoxit D5 and clean all the pots and switches, especially the tape monitor switches and if it has one, external processor loop switch. You most likely have an oxidized contact somewhere in the left channel which is being disturbed when you touch the volume control, causing it to play for awhile until it gets tired and quits again.

Fred

The left channel continues to drop out even when I switch the right preamp output to the left power amp input and the left preamp output to the right power amp input.

That rules out all the switches and pots... which I have already cleaned with copious amounts of deoxit D5-like substances.

The problem *has* to be after the signal reaches the power amp circuitry.

(snip)

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It sounds like a speaker relay problem. I had an AM/FM receiver of the same vintage that dropped one channel randomly (always the same one) but it would sometimes come back if you "bumped" the volume up to "too loud" and then back before the speakers/ears protested to much. I had already changed the volume control - not a fix. At this point I suspected the speaker relay contacts, i.e. higher speaker voltage spike breaking down an intermittantly open relay contact. I unsoldered the relay very carefully, removed the plastic case even more carefully (these things are not designed to be user maintained!), cleaned the contacts (fine abrasive paper plus switch cleaner) and put it all back together. Problem solved, no more drop-outs. Hope this helps. Cheers, Roger

Thanks, Roger, but I don't think the speaker relay is at fault. As I posted earlier in this thread, when the left channel dropped out I shorted the soldered terminals going into the speaker relay and got nothing. I wish it were the speaker relay, but it definitely is not. I bypassed it when the channel was out and got nothing.

Chopstick will work nicely especially if its plastic. You could have also tapped the output relay and avoided attaching leads.

greg

If I understand it correctly, poking the tranny (something I would never do!) caused the corresponding channel to drop in and out.

So... are you ready to swap trannys?

(I'm a bit confused. Does this unit actually use driver transformers? It's hardly a recent model, but I thought driver transformers had disappeared a few years before this series.)

I think he meant trany istor.

Used to have bad transistors in some Tek scopes that ran hot. over time the internal connection would become unstable and poking would bring it back. Probably just the solder joint.

greg

Is it just me, or is this getting (made) ridiculously complicated ? It's an old amplifier with a well defined repeating intermittent problem, that can be provoked pretty much at will, most of the time, by poking with a chopstick. It has now reached the point where different thread respondents, are offering the same advice. Just solder up all the joints on all the devices attached to the heatsink, and if that doesn't work, swap the suspect transistor with the corresponding one from the other channel. if the problem moves to the right channel, then you know it's the transistor, so just stick a new one in. If neither of those things cure it, then you need to apply some more detailed fault finding. This sort of problem is a fundamental 'bread and butter' one that you would put the workshop junior on. I don't mean to be unkind, but if you are having this much trouble tracking down a straightforward problem like this, I'm not sure that you should actually be inside it in the first place ?

Arfa

On heavy parts - especially heatsinked ones - it's entirely possible for a good looking joint to be dry internally. I'd desolder all the pins & resolder them from scratch.

He means transistors, not transformers.