Pioneer SX-838 -- random no sound output, clicking relay - diyAudio
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Old 16th February 2004, 12:32 AM   #1
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Default Pioneer SX-838 -- random no sound output, clicking relay

Hello All,

I'm new to this board. I have been looking for a place on the internet to get some help with this problem for a few weeks now, and this site looks the most promising. I wish I had known about it earlier.

Hopefully, you guys can help me out.

I have an old Pioneer SX-838 that i've been using for a few years now. I think my dad bought it new in '76 or so. I love this amp, but lately it's been really annoying.

You know that relay that clicks on when you first turn on the system, letting you know the sound output is working...well mine seems to randomly click on and off. The sound will go off and on with the clicking. I used to be able to turn the volume all the way down, then the relay would click back on and everything would be fine for another few minutes. Right now, I can't get the relay to click on at all.

Does anyone have any clues to what this problem is? Somebody had mentioned maybe the output transistors on the heat sink. I see where they are, but don't know how to test them. Could it be anything else?

I have found another unit for sale which I was thinking about using for parts, but it would be great i f I knew exactly what was wrong with mine.

Thanks for your time guys,
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Old 16th February 2004, 06:24 AM   #2
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K.I.S.S.

No, I'm not making a pass. Hopefully you know what it means.

Look at the driver board...the one in the rear of the unit mounted vertically. There are four pots on it..the two close together in the center are the DC offset pots.

Put a meter between the speaker leads and set the input to 'Aux' with nothing plugged into the aux jacks. Select the 'A' speakers if that is what yours are connected to, and turn the volume all the way down. Now adjust these pots to read '0V' (left pot for L chan, right pot for R chan...yes you have to move the meter to the channel you are adjusting). Make sure your meter is on it highest resolution scale (200mV or therabouts). The pots will be touchy as hell, and if you accidently turn one so far as to kick in the protection circuit, you will have to move your leads to the driver board to make the adjustment (pin5 and chassis for the L chan, pin 20 and chassis for the R chan). Let the unit sit for a while and recheck.

If this does not cure it, chances are the protect circuit has problems and should be dug into...(probably old caps, or weak transistor that drives the relay).
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Old 16th February 2004, 07:06 AM   #3
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I had the same problem with my Pioneer SX-1280.
On each driver board there is one blue pot, and one white.
I turned the blue one all the way up, relay kicked in, turned all the way down it seems fine. With the left channel, with the blue pot turned all the way down, the left channel wattage meter shows about 1 watt, with no load/signal, and the speaker selector off. At low levels, the left channel seems a tiny bit louder, at high listening levels, you can't tell the difference. I noticed that as I turned the white pot all the way up, over time the heatsink temp increased, without music signal. I am assuming this is the bias pot. I thought as you increase bias you'll get better output, I havent noticed any change. EchoWars since you have knowledge on pioneer products, maybe you can shed some light on the subject.

The sound from the SX-1280 is astonishing, I am currently building an Aleph 2, since I have never heard any Pass products, I am wondering what I can expect from the A2 vs the SX-1280

Thanks in advance.
JH
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Old 16th February 2004, 09:34 AM   #4
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You're changing the bias and offset pots....it's really easy to adjust properly, I'd suggest you do so... ...else the poor amp may die a premature death...
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Old 16th February 2004, 09:03 PM   #5
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Default thanks for the replies

I folllowed your instructions very well. I'm new to all this stuff, but it's beginning to make sense. How close to 0 do I have to get the Pots?

OK...so I've been playing around with the protection relay. If I jump the pins that connect the circuit for speaker output, I get sound out of the system. This is awesome, haha, because now I can rig it so I'll always have sound.

But, I am interested in seeing if I can fix the protection circuit, which I assume has gone bad. There are two wires that I assume control the on/off function of the relay. There is an orange wire, and a white wire with an orange stripe.

Both wires read exactly 36V. The resistance between them reads .655kOhms. I've traced the wires to a small board, located through the bottom panel, on the right side. Is this the protection circuit board? I guess I can remove it and start checking for broken/blown resistors and transistors. What should I do next?
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Old 16th February 2004, 10:09 PM   #6
mish is offline mish  Israel
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HI KEVIN!
I HAD A SIMILAR PROBLEM WITH MY SA-8100 PIONEER
IT WAS A PREAMP TRANSISTOR ON AMP BOARD.
YOU CAN LOCATE IT ONE BY COOLING THEM ONE BY ONE-(THERE ARE SOME OF THEM). USE THE FREEZER SPRAY OR HEAT THEM.
IN MY CASE IT WAS 2SA762 SMALL TRANSISTOR.
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Old 17th February 2004, 03:43 AM   #7
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Mish, can you tell me where the amp board is? Is it the board mounted on the large heat sinks? What are the round things that fit through the heat sink?

So are you saying that I should cool each transistor, and when I find the one that is bad, it should make the Protection Relay come on?

For some reason, the relay does not move to the closed position now. It had been intermittent, but now it just sits there. I can jumper the wires across the relay, and I will have music like normal. Just something is preventing the relay from working. Could a bad transistor cause this?

Thanks Alot,

PS MegaAmp, aren't those old amplifiers wonderful? I have a pair of classic Advent speakers hooked up to the Pioneer, nothing can beat that natural sound
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Old 17th February 2004, 07:17 PM   #8
mish is offline mish  Israel
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HI KEVIN!

You can recognize the amp board by the symmetric components layot.
The suspicious transistors are the small ones.
Cool them, heat them and watch what happens.
You can do the same to protectin circuit board transistors,
maybe one of them is bad.
I am going to change ALL of them on my amp.

GOOD LUCK

mish
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Old 18th February 2004, 02:09 AM   #9
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Quote:
Originally posted by VTbimmer
For some reason, the relay does not move to the closed position now. It had been intermittent, but now it just sits there. I can jumper the wires across the relay, and I will have music like normal. Just something is preventing the relay from working. Could a bad transistor cause this?
Measure the DC voltage across the speaker when you jumper the relay. If there is more than a volt or so DC, then protection circuit is just doing its job. You mentioned previously that problem was intermittant, which can be due to varying DC voltage on output. I have seen this condition a number of times and quite often is due to open circuit input ground reference resistor at power amp input.


Cheers
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Old 18th February 2004, 04:35 AM   #10
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I have found the the grounding side of the relay operation cicuit does not work correctly. Giving ground to the orange/white wire will close the relay, letting the current through to the speakers.

On the protection circuit board, I bypassed the first transistor (or diode, i don't know how to distinguish) with an alligator clip. The relay will close when I do this. I actually have everything working perfectly now.

So I assume this is what I need to replace. It has these number on it-- C1383 R .74 What exactly is this, and where can I find one?

What does it do in the circuit? does it work like a miniature relay, since its third leg doesn't seem to control the thing.

Thanks...SO happy to be getting at the bottom of this problem.

Kev
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