Pioneer SX-950 repair question

[glankford]

My first post on this forum, so please be gentle.

I have two vintage pioneer receivers (SX-839 and SX-950) that I love, but cannot use either due to them both having amplifier problems.

I will talk about the 950 since I know the most about it. It randomly goes into overload protection. I thought the overload/startup circuitry was going bad, so I jumpered across the relay and hooked up a set of disposable speakers. I heard some serious crackle and pop from one of the channels, and the other was just fine. I could hear the relay kicking in and out during this snap, crackle, and pop. I know this has to be an amplifier problem, since the problem occurs with the pre-amp jumpers out as well. I have a set of schematics for the 950, so I can look anywhere. My current theory is that either an electrolytic cap is going bad, or an output transistor is dying. Does anyone have any experience fixing this problem? I sure do miss the brushed aluminum and amber backlighting.

Thanks,

[opt80]

I can't help you with your problem,but I can welcome you to AudioKarma. We are a gentle people,no flaming or belittling from us.We try to help each other on the forum and in the real world also.

Enjoy your stay,post often.

opt80

[EchoWars]

The typical protection circuit has a few electrolytics that take a bit of a beating, as well as relay turn-on transistors that have developed considerable C-E voltage over the years.

Check DC offset at the speaker terminals. If it looks OK, then the protection circuit needs a rebuild. I do know that some Pioneer's used an IC for protection and turn-on delay which is probably NLA, but a 950 may have discrete parts.

[stereofisher]

That makes two of us. I have a Pioneer QX-949 with the same problem. Its tripping the protection relay. Its a 4 channel. I replaced two of caps in the power supply. Played fine for 3 days and its back. The reason I replaced them was because they were swollen.. a tell tale sign of failure. Have not gotten around to fixing it. Have a SX-850 in its place for now. If you find out whats the cause please post. Welcome to Audiokarma, nice folks...lot of info...Eric:D :D :D :D , one smile for each turntable hooked up..et

[SolderIron]

The protection circuit is a place to look as EW says. Sometimes you can have a flaky transistor, not just the output trans, in the amp. Also could be a diode or even bad contacts on the zero bias pot. Intermittent problems are hard to pin point.

[glankford]

Thanks for all the helpful info so far. I will get my repair area all cleaned up and run it for a while. I will measure DC offset and report back when I have facts, figures, and data. The poor beast has been sitting apart for a few years until I could find an information resource to get it fixed. It appears that day has come!

Thanks :)
-Garrick

[glankford]

I have powered both up sx-950 and 839 and measured dc offset in mV at the speaker terminals. I measured them unloaded, with no source.

SX-950 7-9mV on left, and 9-10mV on right
SX-839 12mV on left, 12mV on right

Does that sound ok? I am now running the 950 at low-medium volume level on a radio station, to get the problem to re-appear. It may be a few days of continuous operation for it to show up. I will keep you posted.

EDIT: I have run both for a few hours now, and the 839 the protection will kick out at 20mV on the output. I get the same offset both channels when the relay kicks out. Once it stays below 20mV for a while the relay engages and we're on. Then if I turn it up a little (just past 1/3 way up) the relay kicks out and the whole process starts over. It also kicks out at zero volume, but I can force it to happen with a moderate/low volume. I jumpered across the relay contacts and didn't hear any nastiness, only a pop when the relay kicked out. The offset jumps all over the map when the relay is dis-engaged. (60mV to -10mV) I am sure it moves more, my meter isn't all that fast.

The 950 is behaving. I can crank that up high enough to heat the room with my load resistors and the DC offset is not changing, and the relay is staying engaged.


Thanks,
Garrick

[EchoWars]

Nothing wrong with the offset. Most protection circuits are designed to kick in if offset hits a volt or so (and you're a long way from there). So offset here is not the problem. Weak caps or transistors in the protection circuit are where the problem most likely lies.

[glankford]

Thanks for the tip. Both receivers are running now and both are behaving. :rolleyes:

I have the same protection circuit board for both receivers, and I see that I have about 5 electrolytics, and 7 transistors. I am going to first change out all the electrolytics, and then the input transistors. I hope that fixes them. Those protection boards are pretty simple, so it won't cost much to shotgun all the suspect components.

Thanks A TON for all the help so far, and I will update with my solution (IF I get there). :)


-Garrick

[EchoWars]

One of the transistors is used to turn on the relay...the relay coil provides the collector load. If you can see which one it is, measure the C-E voltage. That transistor should be saturated. If not, you found your culprit.

Also, take a look at the circuit below. This is a very simple protection circuit, but all the basic functions are there. Note specifically the 10µf capacitor connected through the diode to the base of the D669A. That and the 1N4004 form a simple half-wave rectifier. The idea here is that, on power down, AC is lost at the transformer, so the 10µf cap discharges very quickly. This pulls down the base of the D669A and opens up the relay contacts pronto. Because of the (sorta) large ripple current across the 10µf cap, it is a prime candidate for replacement.

Your circuit almost certainly has a similar mechanism to facilitate a quick turn off. Find the cap...

[glankford]

I finally got the schematic scanned. Here it is.

[EchoWars]

Seeing the remainder of the print might have helped, but basically here Q6, Q7, C5 and C6 are the most likely candidates for replacement.

[glankford]

Thanks for all your help, EchoWars. I'll put an order in to MCM for all the transistors and caps on that board.

If you have any interest in my schematics for your collection, I'll make full size copies on the drawing copier at work and send them to you. I'll pick up postage. Send me an email if you want them. No strings attached, I just want to express my gratitude.

-Garrick

[Grinder]

Hey all, I own a sx-5570 wich is an european model of the sx-950.

The tuner is dead but the preamp and power amp were working fine....Untill...

The protection circuit was also giving me problems...From time to time, the speakers would just cut off for no reason and later come back on again...

Today, I was playing a game and the speakers went off again........

Time to do my yearly maintenance on my "favorite" amp !!!

As I was looking on the power board where the 5 fuse sockets and diode brige are located, I noticed that 1 transistor was just loose as hell!!!

Weird, I know that amp is old but strange...

I unwelded the transistor, wich is located besides one that lookes like it, wich have individual black heat sinks (not the 2 that are screwed on a bigger heat sink).

It had a broken pin, one donut from the board was still attatched to one pin but was not attached to the board and the 3rd pin was sorta ok...

I checked it, read .590 and .589 on my tester set on diode.

It was still good, I soldered a peice of wire to compensate for it`s broken pin, soldered a jumper for the broken donut and soldered it to somewhere on the board where it was supposed to go...

I didn`t have lots of room to work...

I plugged it in the wall and blew two 1 amp fuses :mad:

I doubled checked and silly me I had soldered the jumper of the transistor pin to a bad place!!!

I tripled checked and resoldered it to the correct place and BINGO!!! :p:

My favorite amp is still working better than ever!!!

I noticed that that transitor, over the years, really heated up and loosened up.

I you have protection circuit problems, check for loosened parts, it worked for me.

Thank you

[glankford]

Thanks for the info, I will check that. I haven't done a thing with the amps in the last year, but I did order all the transistors on the protection board.

-Garrick

[joey1355]

Im also having problem with my qx-949 it has some squack and noise problem. the protection circuit also clicking and went of again and again. i suspected that my eq has something to do with this cause the my sx 950 has also a symtom like that. do you guys think its the eq? my tech guy needed a output schematic and the protection circuit board also for sx-950 & qx-949 . can u lend me the service manual for this? thanks a lot!