Pioneer SX-1250 amp pops / crackles

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Recently inherited a somewhat broken Pioneer receiver. I replaced the four large (22000uf 80v) capacitors with 22000uf 100v units, as two of them had exploded in its previous life. With a speaker connected to the right channel, the relay will remain open for several seconds, then arc visibly, close, the speaker will pop loudly, and the relay will open again. On the left channel, there is simply a good deal of popping, although music can be heard through it, unlike the right channel. After removing the preamp / amp jumpers on the rear, the noises and tripping relay continue. I've looked at several posts but cannot quite figure out what to do. I have cleaned all of the contacts and checked to make sure that none of the parts were obviously destroyed, as well as tested all of the resistors. Any ideas about what to invesitgate next would be greatly appreciated.
 
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I have a sx-1280, and had all kinds of noise problems, and eventually it would just kick the relay all the time. It would still cut out after i hooked the pre to another amp, having no effect on the music. Got it fixed, and apparantly the transistors that they used for the SX series had problems. The coating on the leads would eventually corode, and work its way into the case of the transistor slowly causing problems, mainly weird noises, cracks, pops, and causing the protection relay to activate. I think they replaced the input driver transistors, and they said it was fairly common in the SX series.


JH
 
Hmm yes, for the left channel a faulty input pair does sound likely now I think about it. The right channel sounds like it has blown outputs though if the relay comes on straight away and you can see arcing between the contacts.

Exploded main filter capacitors sounds bad. Is it possible the unit was hit by a surge at any point ? You may end up checking everything in the poweramp stage as it's quite possible parts will be either leaky or dead due to teh surge
 
Thanks!

Had a hunch it was the transistors. Thank you very much to everyone who suggested this. I have access to a curve tracer through a lab at school and will be testing them at some point this coming week. As for a surge, it could be entirely possible, although from looking at the thing, it seems like it may have just been abused at some point and then left sitting in a garage. Thanks again for all of your suggestions!
 
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Hi 8Track,
Those also had problems with the input coupling cap. Use 16V caps minimum. Diff pairs were normally not matched well, I used to use dual transistors (2SC2259, 2SA798).
jaycee is right about the DC offset, I disagree about the blown outputs. I'd expect a blown fuse in that case. Input caps are good for that fault.
-Chris
 
Another tidbit that may help: When a blown (shorted) speaker with essentially no resistance is connected to the right channel, the tweeter will play clearly without any strange noises. As soon as a functioning speaker (8ohms) is attached and the unit is turned on again, the relay stays open for a few seconds, arcs, the speaker pops, and then the process repeats.

Finally, the input transistors, or what I figured were the input transistors, were Toshiba models S5151 and S5151R. Is this correct, and if it is, is there another part that is more easily available that I could substitute?

Thanks again!
 
I guess I read it to fast and missed the relay arching problem.
I will agree with Chris, no blown outputs or the fuse would have gone. In addition to what Chris has mentioned I would check the bias section and replace the pot and bias transistor after you get the bugs out of it. Time and time again you can count on a dirty bias pot going south and taking out the output stage. Two bucks worth of bias pot and a T0-92 transistor are cheap insurance.
 
I got the service manual and adjusted all of the bias pots as per the procedure in the manual. However, this did not make any progress towards fixing the problem. I tested the input transistors with my multimeter and they have the proper resistance and so on as far as I could tell. Does anyone have any other suggestions of things to look for? Could the transistors still be causing the problem?

It looks like at some point in the life of this beast, its input caps were replaced with 16v units as per one of the suggestions.

Thanks again for all of the help!
 
Chris, I emailed you the schematic and PCB layout this evening.

I measured the voltages on the speaker terminals, and found some interesting results.

On the terminal that simply crackles and pops but will play music, the DC voltage measured between -.015 and +0.06 volts, while on the terminal where the relay arcs, the speaker pops, and the process repeats, the voltage read 28.8.

Another interesting note, with the multimeter in series with the speaker wire, the relay did not arc, but simply closed, and the speaker remained silent on both terminals.

The speaker I am using to test this is just a little bit >8ohms.

As for replacing the transistors, the originals were models 2sd610-r and 2sb630-r. I found some data on the 610, both are made by NEC. A suggestion for replacements was made earlier in this thread, but I couldn't find either of those parts at any of the major vendors (DigiKey, Mouser, etc). Any suggestions about where to look, what parts, or even how to go about finding something with the same values, would be greatly appreciated.

Thanks again!
 
sx-1250 repair

I bought a sx-1250 in decent but non-working shape a while back.. Got original transistors from datadart.com. The amps didn't like some of the nte subs I tried to use first. I ended up replacing all the to-92 case transistors and an open resistor on one amp board. Test for all the voltages listed in the amp schematic in the service manual. First thing is prolly to check the voltage regulator board out and make sure its working. I found some dried out caps and bad transistors on mine.
 
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Hi Mike,
The original 2SD610 driver transistor is rated at 200V 2A 25W with a gain of approx 80. A suitable pair today would be 2SA1667 / 2SC4381. These are rated at 150V 2A 25W. Your circuit voltage +-70V, for a swing of 140V total. Not much safety but the 2A rating may be needed for low impedance loads.

If you feel better with a higher voltage you can go 2SA1837 / 2SC4793, 230V 1A 20W. Or 2SA1006A / 2SC2236A, 200V 1.5A 25W, or 2SA968A / 2SC2238A - 180V 1.5A 25W. The B version could be used for a breakdown of 200V but that's a little overkill.

You are driving a pair of outputs, going with MJ15024 / MJ15025 or MJ21193 and MJ21194 have higher hFE than 2SB600 and 2SD555. This may allow more saftey with the 1.5A drivers.

-Chris
 
Chris,

Thank you again for all of your assistance!

I looked at the stabilizer board more closely and found what looks something like dried electrolytic fluid under one of the capacitors, and it did not behave the same as its twin when I checked it for resistance, so I think that this may be part of why there is 29 vdc on the speaker output.

I think its time to start selectively removing and replacing some of these bits and seeing what progress I can make. Thanks again to everyone who helped me! I will report back once I get some parts and begin the replacement process.

-Mike
 
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Hi Mike,
That's very normal for Pioneer in these years. I can't tell you how many I did years ago with a similar condition. Change them all in the power supply, the main electrolytics may still be okay to get it running.

As for your DC offset, check what looks like C1 and C2. C1 you can disconnect to check (input coupling cap). C2 filters the DC level on the offset pot, VR1. Also check the voltage on board connection pins 2 & 3.
The last fault I've seen some of is defective transistors at Q1 & Q2, 2SA750. They can go open B-E. That could also cause your cracking noise on the other channel.

-Chris
 
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