SX 1280 problem

biggal76

Active Member
Recently picked up an SX1280. Sound is great, better than I remember my old SX1250 that I sold and I think I like it better than my Sansui G901 (which I kept over my SX1250).

Problem originally was once you started to push it a little with low frequencies, it would trip the circuit protection relay. I immediately thought DC offset or Bias problem, so I just adjusted those as per the service manual. The left channel was about 60mV offset, so pretty far out of spec, right was ok.

It's all adjusted now, but now, when it draws a lot of low frequency power, the right channel gets into a pulsating approximately 60Hz loop that quickly increases in volume until it trips the circuit protection relay. I have no idea what the problem can be. Anyone have this issue in the past?

Funny that the problem seemed to be the left channel before I adjusted and now is the right. Could excess DeOxit Gold in the tone control switch contribute to this? I flooded it too much when I did the other adjustment as it seemed to still be dirty after cleaning it a couple of months ago.
 
Big clue just noticed. It does it when the 50Hz tone control is turned up at the same volume setting with or without the loudness control on, so I don't think it's actually the amount of power being pushed but the amount of low frequency signal being put through the preamp stage with the 50 Hz control up. Is the loudness control upstream of the tone controls in the circuit? If so, I'd say the problem is between the 50Hz tone control and the loudness control. A shorting pot wouldn't do that, would it?
 
Sounds like some of your supply decoupling capacitors in the preamp are giving up...

Does it act funny with the pre out main in jumpers pulled? With a different preamp maybe?
 
Thanks ManyMoons,
It would be hard to tell with just another preamp because it only does it when I turn up the 50Hz tone control past 1/2way. Most preamps wouldn't even have a bass boost that low in frequency. Maybe I could put my MP3 player at low volume into an Equalizer and then into the power amplifier input and turn the bass way up and see if it does it. That would be a good test to isolate the problem to the preamp stage.
 
I'm betting it's your regulated power supply. That board runs hotter than any other in your receiver, and therefore, its parts are the first to go. If the regulated supplies to either the preamp or power amp circuitry are flagging, any attempt to push power through the unit (i.e., any kind of bass demands) are going to cause the receiver to choke and shut down.

Usually with these receivers, I just rebuild the entire regulated PS board, because if I find and replace one bad component, it's a sure bet another will fail soon after. However, if you want to try to pinpoint the issue, I would start by downloading a service manual and checking the various regulated supply voltages.
 
just one note: If you are driving 4 ohm speakers seems the 1280 kicks out more... I rebuild the protection board for someone and it worked a little better at low freqs ,,, also there is 2 outputs from the amp boards that go to the protect board that was also kicking out the amp, I don't have the schematic handy right this sec .. but I believe it's a collector output that runs out of it .. might not be the prob. but just a suggestion ..
good luck

Also alot of new music overdrives the bass ... and is quite distorted also ..
 
Anyone have the detailed schematic and/or sealed circuit views (phantom views) or the circuit boards? I have the service manual, but it just has a basic block diagram.
 
I'm really a novice at this but had success (with lots of help from members of AK) diagnosing and fixing my Sansui 5000x which had 2 bad output transistors and a random bad cap in the preamp section and then setting the bias, so hopefully I will have success at this repair as well. All the help is very much appreciated.
 
Be careful inside SX-1280. A slip of an uninsulated screwdriver around the energized power supply board(small heat sinks MIGHT be energized) and you can toast/create - cascading casualties into the 13V feed lines feeding expensive/rare FM tuner PA IC chips. No power-up tests until PS board is properly secured and pinned(can flop & short to the protection board). Be attentive to not destroying solder pads on PS board - they are fragile from years of heat abuse. Strong solder sucking tools can lift and tear them. Recommend solder wick in the high-heat Power Supply areas. Answer to most common questions should be in the AK archives.
 
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If you want it done right, your pockets will hurt. The point, however, is whether you can find anything equivalent to the 1280 for the money you will spend. Actually you will find nothing like the 1280 anywhere at any price. Spend the money and get it done completely and done right. You will not be sorry and years after spending the money you won't remember the pain in your pocket. There are some great repair techs here. I think you have some up in your neck of the woods. I am on a one year waiting list right now to have my 1280 rehabbed.
 
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If anyone knows anyone between Boston and northern Maine who really knows these Pioneers, please let me know. KingBubba, when you say expensive, how much are you thinking you will spend on your 1280 rehab? Also, I see you have a 1250 as well. Do you agree with me that the 1280 has tons more low end power than the 1250?

My plan is to isolate it to the preamp as ManyMoons suggested because that is easy. Assuming it's in the preamp as I believe it is, then get voltages in the power supply to the tone board and try to isolate the problem from there.

I really appreciate the help on this forum. Will post again once I get back into it.
 
While rebuilding the power supply is a good idea, the fact that it is just ONE channel misbehaving points away from the power supply itself.

BUT that leaves the power supply caps ON the tone board to cause mischief. Since THEY are split by channel - to isolate the channels.

Which they are.

Just Recap that tone board, something is badly out of whack and it sounds like it is breaking into oscillation.

plenty of lists for the caps.
 
You can expect to spend from $400 to $600 and shipping will be $75 each way for Fedex. Just replacing the large filter caps is an expensive job. I've done this three times now and had no regrets. I have three more in the lineup, if I can find someone that will take on an Onkyo TX-8500MKII. It is a true monster, but no one has any experience with them. You are lucky that you have a Pioneer because many have worked on them and there is lots of info about them available. Markthefixer has touched base here at this thread. He knows of what he speaks. If you are capable, he might help you to take the fixing on yourself. Only you will know whether you can handle it. You have a great piece of gear and I hope you get it squared away so you can enjoy it.
 
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Yes, testing caps/recapping is something I am familiar with and I thought the problem was directly related to something near that 50Hz control. I will start there. Plus, the tone board is easy to get at. I'll start checking caps there.
 
right

If you want it done right, your pockets will hurt. The point, however, is whether you can find anything equivalent to the 1280 for the money you will spend. Actually you will find nothing like the 1280 anywhere at any price. Spend the money and get it done completely and done right. You will not be sorry and years after spending the money you won't remember the pain in your pocket. There are some great repair techs here. I think you have some up in your neck of the woods. I am on a one year waiting list right now to have my 1280 rehabbed.

totally agree, your pockets will have to hurt. these components are not easy to work with, their sophisticated. no easy way out. i went through the same for my pioneer a-9 and still awaiting the final line straight problem to be resolved
 
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