Pioneer SX-737 electrical gremlins.

[Arty30]

Hello all, I recently brought back to life a pioneer SX-737 I picked up off the curb. I didn't do any electrical work to it because it sounded great already, and still does. the problem arose when I hooked up a set of nice fisher STV-410m's and noticed that at idle the sub at the bottom was gently waving back and forth randomly, meaning DC across the terminals. I've been looking into it lately and cant figure anything out, I've only determined that it is independent of volume, or the settings of the base/treble knobs, my analog meter shows about 0.4v max on occasion. I have found however that having the "low" filter on cuts it out, but not the "high" filter. I also sampled capacitors on all boards and I would say they are all okay, not great, sometimes 15% above value in extreme cases, but it doesn't seem to effect performance in the sound department. I'm just not sure if that could be the cause of my issue. I ran out of ideas on this one so I came here for help, any suggestions are welcome.

[TimFox]

If the "low" filter reduces the subwoofer excursion, that implies that something before the power amp has very-low-frequency (quasi-DC) content, possibly due to a bad capacitor somewhere in the preamp or line amp section.
Do you have an oscilloscope?

[bdunham7]

Does it do it for all sources?  And is it on both channels?

If it is all sources and the volume control has no effect but the low-pass filter eliminates the issue, it looks like it would be an issue on the control amplifier board.  If it is both channels, then I'd have to think about it...

[Arty30]

It is perfectly equal on both channels and does not matter what source is selected. I do have access to an oscilloscope as well.

[TimFox]

I'm not sure how low your annoying frequency is.  You may need to make a test probe with, say, 1 to 5 uF feeding your (DC-coupled 1 megohm) 'scope input and probe the power supplies in the control amp section.
The AC-coupling high-pass filter on your 'scope probably cuts off below 5 or 10 Hz, depending on model.  1 uF and 1 megohm cuts off below 0.16 Hz.

[bdunham7]

It is perfectly equal on both channels and does not matter what source is selected. I do have access to an oscilloscope as well.

OK, can you verify that it is actually working in stereo by disconnecting R and L inputs one at a time, and also that the balance control works?

If all that is good, set your DMM or scope up to look at the power supply to the control amplifier on both sides of R45.  It is possible that R45 has gone bad (noisy) but C23 is able to filter out all but the very lowest frequencies.  A simple test would be to poke R45 and see if your speakers go crazy.

[floobydust]

Given the age of the receiver, I shotgun the electrolytic capacitors, replace them all. Because it's cheaper than the hours spent troubleshooting. The power amp only has 5 per channel. Testing capacitors in circuit with a multimeter is not useful, the grey Elna's just dry out by now and your reading high value +15% can mean a cap is leaky.

Common to both channels is the +0.7V/-1.5V rails which feed the DC offset trimpots VR3, VR4. If the Low Filter is affecting it, then I would suspect the input coupling caps C1/C2 4.7uF 25V are seeing reverse bias or otherwise unhappy. I would measure the voltages at those trimpots, they should not be moving around. Schematic has an error showing two D1, and the one at R42 might be a problem.

[SpecialK]

Check out Audiokarma's "Top Ten Worst Transistors - noisy, failure-prone, whatever... and replacements" list.  I notice your schematic had 2SA726, which you might want to evaluate.


https://www.audiokarma.org/forums/index.php?threads/top-ten-worst-transistors-noisy-failure-prone-whatever-and-replacements.731653/

[floobydust]

OP says the problem appears in both channels, so I would not suspect them just yet.

Common problem with vintage Japanese amplifiers is the stabistor diodes for the bias D1, D2 STV4H going bonkers. They're mounted to the heatsink and failing they will make bias current move around and sometimes pops and noise as well. You'd have to keep an eye on bias current to see how stable it is, although spec is 50mA and 0.4V/8ohms is pretty much a 50mA imbalance.
I would measure the voltage drop across them, without bending and trashing them because if they go open circuit you will blow the amp.
But again, these parts are specific to each channel and a problem common to both channels is rare outside of power supply problems I think.

[Arty30]

ok, thanks for all the suggestions, I will investigate some of these things and consider replacing caps if I can find a reasonably priced kit or just use the parts list to make one myself. the other option is a 75$ rebuild kit that includes everything but that's no fun.

[floobydust]

Try to have fun with it, you could start small and just recap the power amps (8 parts), or connect clip probes to various places and watch some voltages to narrow it down. It's a classic receiver and worth the repair.
The loudspeaker-protect relay should engage if the DC offset goes too high.
I've seen some amplifiers the loudspeaker cone moving around due to mains voltage moving around. Others it was the STV4H stabistors.
If it's the same gremlin in both channels- then the common elements are the power supply for the DC offset trimpots and preamp.

website has extensive pictures of the boards http://amp8.com/radio/receiv/sx737.htm  slow loading, lots of closeup pics

[bdunham7]

Try to have fun with it, you could start small and just recap the power amps (8 parts), or connect clip probes to various places and watch some voltages to narrow it down. It's a classic receiver and worth the repair.
If it's the same gremlin in both channels- then the common elements are the power supply for the DC offset trimpots and preamp.

I would suggest starting with the power supply board if he is going to do a shotgun recap trying to fix this.  I'd like to know what he measures at terminal 7 of the control amp or terminal 16 of the PS board.

[Arty30]

alright so here is where I am after a few hours of poking around today, my thought process was finding what stopped the DC offset fluctuations. I started by disconnecting the audio input to the control amp board, there was no change. next I disconnected the power from pin 7 (sitting at about 29v) and obviously it stopped then, but the voltage rail (pin 7) was still fluctuating in the same way so I went over to the power supply. I checked transistors and zeners for that section (pins 15-17) and found no faults. pin 17 is a 24v source through a 1K resistor from pins 15/16 and it was bouncing around, meaning fluctuating current through the resistor, also a bit low at 19v, current is supposedly 8mA but was jumping around from 7 to 11mA. That 24v goes to the "equalizer amplifier" which I don't even know what it does, all it has is the phono input straight from the back panel, a +24v (pin 7), a -16v(pin 2) supply, and an audio output to the function selector. Long story short I unhooked both power pins and the problem went away, the only difference is the phono input is disabled, no DC fluctuations anymore, still works just as nicely. now I just need to find out why it was causing this long chain of cause-and-effect voltage fluctuations. phew, I'm done for today.

[floobydust]

It is likely a capacitor getting leaky and loading down the rail or a cranky transistor, did you ever listen to the phono input? It must be noisy and poppin'.
The phono-preamp "equalizer amp" board has C17 100uF 25V fed from #17 (1k to #15), but also C18 220uF 16V fed from #13.

[Arty30]

I replaced all the electrolytics on the phono preamp which resulted in no change. on the right channel output however I measured -10v, and it only goes to the function switch so it must be coming from this board somewhere, I jut don't know where, I measured some of the listed voltages on the schematic and they seemed all off, the others are too blurry to read. after probing around the -10v on the output magically seemed to disappear, but the problem is still there. I'll test the transistors but I'm not confident my tester will see a problem like this.

[bdunham7]

I replaced all the electrolytics on the phono preamp which resulted in no change. on the right channel output however I measured -10v, and it only goes to the function switch so it must be coming from this board somewhere, I jut don't know where, I measured some of the listed voltages on the schematic and they seemed all off, the others are too blurry to read. after probing around the -10v on the output magically seemed to disappear, but the problem is still there. I'll test the transistors but I'm not confident my tester will see a problem like this.

Just FWIW, every transistor on your phono preamp board is a known troublemaker.  As in they would get replaced on sight on my bench.  I can't assure you that they are the problem and quite frankly I'm not sure how they would be, but something sure is.  Onsemi KSA992 and KSC1845 are good subs.

[Arty30]

yeah, I saw the thread SpecialK linked, I did use my little component tester and it gave no obvious faults which isn't surprising, but there's nothing else it could be except resistors.

[floobydust]

Just FWIW, every transistor on your phono preamp board is a known troublemaker.  As in they would get replaced on sight on my bench.  I can't assure you that they are the problem and quite frankly I'm not sure how they would be, but something sure is.  Onsemi KSA992 and KSC1845 are good subs.

What's wrong with the 2SA725/2SC1313's used, do all these small Japanese transistors simply not last? I notice recap kits on eBay include new transistors. Old phono preamps have no ESD protection and commonly get nailed with the RCA jacks.

Notice the phono-preamp "equalizer amp" runs from (common) +25V and -16V supplies, the schematic gives typical voltages for the board.
It's got three transistors all DC-coupled with feedback, so any one of the three can upset everything on a channel. A bad transistor could reverse-bias a capacitor and cause it to fail, example C11/C12 normally see +1.6V but would go -ve if Q5/Q6 or the others Q1/Q2 go bad.
So I'm saying the new caps could get damaged (leakage current) if there is still a problem with the transistor(s).

I would measure and post the voltages at the transistors, or pull them and test.

[mzacharias]

Mostly the problem with the old transistors is tarnish on the leads which migrates into the semiconductor itself.

It's a known problem. If the transistors leads are not visibly tarnished there still could be noise caused by impurities, aging, etc.

[Arty30]

All the transistors in question looked like their leads were dipped in a black liquid, probably to protect them, but now its a black powder that comes off onto my finger like soot, not sure if it was doing anything any more.

[bdunham7]

All the transistors in question looked like their leads were dipped in a black liquid, probably to protect them, but now its a black powder that comes off onto my finger like soot, not sure if it was doing anything any more.

That's the crap that grows on them.  I would simply replace them all before doing too much more thinking about it.  b/t/w I'm actually working on a few similar projects and I'm out of KSC1845, as is Mouser.  The show some 400k+ 'on order' but zero stock.  So now we're scrounging for ordinary transistors.

[Arty30]

yup, it tried to order those yesterday, I got BC337s instead since they are supposedly a replacement other than lead arrangement being different.

[Arty30]

I tracked down some KSC1845s on eBay and got KSA992s on mouser, they finally arrived and I installed them, problem fixed. I guess Floobydust was right, cranky transistors. which makes me think its the same problem somewhere in my bottomless Sanyo plus 75, Floobydust may remember that rabbit hole that is still not plugged up . I'll get to it someday.