Pioneer SX-1250 amp pops / crackles - diyAudio
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Old 20th March 2005, 07:22 AM   #1
8Track is offline 8Track  United States
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Default Pioneer SX-1250 amp pops / crackles

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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Old 20th March 2005, 10:41 AM   #2
jaycee is offline jaycee  United Kingdom
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Check for DC on the speaker output terminals. I'll be willing to bet some of the output transistors have gone short.

It may also be worth testing the preamp stage seperately with another power amplifier to check it hasn't been damaged.
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Old 20th March 2005, 10:55 AM   #3
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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
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Old 20th March 2005, 11:33 AM   #4
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Usually the problem is faulty diff input transistors. When they are defective you will get poping, relay opening and closing because the DC on the output exceeds the limit designed into the relay circuit(safety issue). Replace these and your problem will go away.
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Old 20th March 2005, 12:18 PM   #5
jaycee is offline jaycee  United Kingdom
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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
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Old 20th March 2005, 07:48 PM   #6
8Track is offline 8Track  United States
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Default 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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Old 20th March 2005, 09:07 PM   #7
anatech is offline anatech  Canada
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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
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Old 21st March 2005, 06:27 AM   #8
8Track is offline 8Track  United States
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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!
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Old 21st March 2005, 07:37 AM   #9
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Quote:
Originally posted by 8Track
When a blown (shorted) speaker with essentially no resistance
Terminology Tip: If a speakers measures no resistance across its terminals it is OPEN, not shorted.
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Old 21st March 2005, 10:41 AM   #10
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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.
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