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SX 1280 high DC offset

mslupczynski

mslupczynski

Active Member
My 1280 has has not been sounding the same lately. So I decided, as a start, to simply check the DC offset at the output terminals. To my surprise, cold, the L channel is 93mV and the R channel is over 115mV. Both drop when the unit gets warm but are still hovering at L channel 75mV and over 90mV for the R channel after 30 minutes. The unit seems to work fine and other than sounding bad and getting pretty warm just above the power supply, pretty warm all over actually. It seems to be in good shape.

Internal inspection shows that the unit has been worked on a little bit before, some caps replaced on the protection board and I can see where both the power supply board and protection board has had some terminals re-soldered because they were disconnected and the boards possibly removed. The work looks fine so I have no suspicions there. Some of the caps all boards are clearly still original, but I haven't really looked closely at any board transistors to see if any were replaced. I suspect some known problem transistors have, but I'll dig into that later.

In the next day or so I'm going to use the schematic and SM I have and check all voltages at the power supply, the heart of the unit. For those of you who follow standard diagnostic procedure, and general methodical process of elimination, I'd like to know where should I first focus my attention on a DC offset issue like this. I don't want to go in and just start replacing stuff, it doesn't end well. Don't ask me how I know.

Thank you in advance.
 
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While you study the diagrams, parts lists, and condition of your unit, it may be advisable to adjust VR2 on each of the power amp sections to bring that offset voltage down as close to zero (after warmup) just to get it under control.

Eventually, checking the idle current will be prudent also. If issue persist after that, it should be relatively safe to take time to analyze while still exercising the unit.
 
mslupczynski said:
To my surprise, cold, the L channel is 93mV and the R channel is over 115mV. Both drop when the unit gets warm but are still hovering at L channel 75mV and over 90mV for the R channel after 30 minutes. The unit seems to work fine and other than sounding bad and getting pretty warm just above the power supply, pretty warm all over actually.
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That's normal voltages and heat. It's a DC feedback design afterall.
Do not think that's the reason for what you say it sounds "bad". A good tech would measure the performance to the specs as a first test.
 
rcs16 said:
That's normal voltages and heat. It's a DC feedback design afterall.
Do not think that's the reason for what you say it sounds "bad". A good tech would measure the performance to the specs as a first test.
Click to expand...
I suppose I could have said it sounds a bit degraded, because it really doesn't sound "bad." My bad.

You have caused me to remember that it was the phono stage that sounded the most "unusual" and less to my liking. Perhaps I should also investigate the phono board while inside and see what may be happening there? While I'm at it, is there a standardized test for the phono stage to see if it meets spec? A 1khz (or other) signal injection to see if the RIAA circuitry is up to par perhaps?
 
So, I have accomplished the following and would love a second (or more) opinion(s):

Signal Tests
• 1 khz sine and square wave thru RIAA compensator into PHONO1 and 2 and got a good waves out thru both PHONO 1 and 2
• 1 khz sine and square wave into AUX and get good waves out thru there also...

DC Offset Adjustments
• When I disconnected signal generator and connected DMM's to R and L channel both channels DC offset were above 80mV even higher at times.
I adjusted the L channel down to about ~38mV and it was incredibly difficult. The pot was nearly useless, in that, whether it's dirty of malfunctioning the offset jumps like crazy and then started tripping the relay. I finally got it down to about the 38mV (picture shows 49) and stopped messing with it.

• Then on the R channel it was even worse. Same issues with the pot and the relay tripping but then I could not get anything back on the RIGHT. No load connected, of course, but no matter what I did at that potentiometer, I could and still cannot get a reading on the R channel anymore. It may read for moment, climb and then the relay clicks.

The unit, even thought initial DC offset measurements were very high, was stable when I got it. Were the pots just waiting to be messed with??

I know those original potentiometers can really get flakey and I suspect that is what I'm seeing. And my guess if that if anything else on the output boards related to the offset balance is going south then the problem would be compounded.

Any observations or suggestions would be appreciated.

1 Square good thru AUX and PHONO.jpg
2 49mV L Channel R nothing.jpg
1280 DC adjust pot.jpg
 
Perform the same test but with the rear panel pre out/main in jumpers removed.
Double check the regulated voltages at the amp board connections. Make sure they are stable and equal +- on each board.
 
Also, once the relay has opened you can't measure the DC offset at the speaker terminals. You have to do it at the point where the emitter resistors in the output stage connect together.
 
dr*audio said:
Also, once the relay has opened you can't measure the DC offset at the speaker terminals. You have to do it at the point where the emitter resistors in the output stage connect together.
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That certainly makes sense. I do believe the relay would trip and then re-connect (most of the time.) I could be wrong, but will keep this in mind for sure.
 
Those blue pots get really bad over time. Replace them with Bourns 10-turn devices. You can then dial in offset real easy and smooth. Same with bias controls.
 
Another tip; on some amps when you adjust the offset, the feedback tries to correct and you get some overshoot. You have to adjust it just a small amount and wait for it to settle. The 10 turn pots would help with this, allowing you to make fine adjustments with less overshoot.
 
dr*audio said:
Another tip; on some amps when you adjust the offset, the feedback tries to correct and you get some overshoot. You have to adjust it just a small amount and wait for it to settle. The 10 turn pots would help with this, allowing you to make fine adjustments with less overshoot.
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Nice, also something I'll need to keep in mind. BTW, I've been researching the Bourns 10 turn potentiometers and the values required for the 1280. But it appears Mouser doesn't have a 150K, they show 100K? Do you happen to have a source for these? And what is semi-fixed? Thanks.

EDIT: Never-mind, I think I found the right part. But would still like to know what semi-fixed means? And would it be possible to verify the trim pots I've selected for DC offset adjustment?

1280 DC offset pot.jpg

1280 vr 1 and 2.jpg
EDIT
 
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zebulon1 said:
Perform the same test but with the rear panel pre out/main in jumpers removed.
Double check the regulated voltages at the amp board connections. Make sure they are stable and equal +- on each board.
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The signal testing through the preamp was completely successful. No problem whatsoever. If I may, do the mylar connectors at the bottom of the output modules house the regulated voltage checks you referenced? The schematic shows the points for the voltages but they don’t exactly correlate to the connectors. Or, I’m not referencing correctly. I’m also assuming that the regulated voltages you mentioned can be referenced at the output pins of the power supply itself. Or, is it essential to test at the modules pins only. Thank you for your assistance.


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They would be the amp board pins 5 and 6. The +-68v's
They should be close to 68v and be very equal (close) and stable.
 
zebulon1 said:
They would be the amp board pins 5 and 6. The +-68v's
They should be close to 68v and be very equal (close) and stable.
Click to expand...
Thank you kindly @zebulon1 After studying the schematic a bit more I could see where to check, but your confirmation is most welcome! I'm heading out to my garage/lab tonight when it's cooler and I'll verify values and stability.

Also, just thinking out loud, I don't want to get my cart before the horse and I'll stay in sequence. But would getting those trimmer pots I mentioned a few hours ago in this thread on order be prudent at this juncture? It's clear they need to be changed out. Doing any adjustments with them is near impossible. But though I'd check with you and see what your recommendation would be. Thanks again.
 
I would wait on ordering components.
Those trimmers are probably okay.
You may end up doing a partial rebuild.
Check those voltages and perform that DC balance (offset) check with the jumpers removed.
Go from there.
Take your time and think through your process.
 
I think you should change the pots. It will make adjusting the offset much easier. Replace the bias pots as well.
 
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