Technics SA-200: How to adjust the DC offset?

Discussion in 'Solid State' started by kotofei, Jul 9, 2009.

  1. elnaldo

    elnaldo Addicted Member

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    Sometimes, to reach non standard values, I just solder a resistor in parallel on the solder side:

    This pict is from a Technics SA-400, it solved the DC offset in that unit.

    P1140768.jpg
     
  2. rjcrjc

    rjcrjc New Member

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    Thanks all for the help. I am learning a lot here... two steps forward, one step back.
    Here's where I'm at with my SA-200:

    Originally b ought from ebay as 'for parts or not working'. Mainly just needed deoxit. Both channels worked, but both had a dc offset of 80 and a big 'thump' through the speakers when I turned the receiveroff, which i thought might be attributable to the dc offset.
    Following Ecluser's advice at the beginning of this thread, i became fixated on trying to reduce the dc offset. I began to test by adding resistors in parallel to the 614 and 613 resistors. Long story short, one of my probes wandered and I ended up shorting out the left channel power IC, IC601.

    Good news: I was able to get one on eBay that seems to work ok. I replaced the IC and that channel worked again.
    I couldn't get the old resistors out, but i found it easy to solder new ones in in parallel. With a 680 ohm resistor in parallel on 614 and (I think) an 1100 ohm in parallel on 613, I got the dc offset down to .005 on the right channel and .015 on the left.
    And now it doesn't seem to thump when i turn it off (though I have limited data points).

    So here's what's up now:
    The right channel, the one that was fine, works fine when I first turn the unit on (cold). But after a few minutes, it deteriorates, getting more and more staticky until you can barely make out the music on that channel. I checked the related IC, and all appears to be fine.

    The only thing I've noted is that there is a big boxy cement-ish resistor labeled 5W 180 ohm J 2836 that gets very, very warm during operation (I included a picture). I checked resistance when the unit is off and it's still 180 ohms. I did not measure when warm. I also realize I didn't write down the resistor number from the board. I need to trace it on the schematic but I wouldn't think that is related to an issue in just one channel.

    Also, another thing I found very odd - the unit seems to continue to play for about 8 seconds after I turn off the unit. I had my iphone playing through aux on headphones, and after I switched off the receiver, it kept playing. Even more odd, I wanted to check the dc offset when the unit was off - so I plugged in my probes to the speaker jacks after the receiver was turned off - and the iphone picked up volume and started playing through the headphones on the Technics' headphone jack for second!

    If anyone can offer help with the deteriorating right channel issue, I would be very grateful. Also, if the staying on for 8 seconds is a concern, please let me know.
     

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  3. elnaldo

    elnaldo Addicted Member

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    The unit playing for some seconds after power off is normal, Actually it means "healthy capacitors" that hold the charge.

    Sound vanishing after power ON sounds like some leaky component, it could be a capacitor, a transistor or even the IC that can't hold the voltage. I can't check the schematic now, but somebody will for sure.

    You should monitor + and - voltages while this happens. Also, to identify if the problem comes from the preamp or power amp section.

    Don't poke with your probes inside a live unit (you know why). Instead, get alligator clips or mini grabber probes, anr attach them with the unit UNPLUGGED FROM THE WALL, then power on to read the measurements without touching anything. It's a bit tedious to power off and UNPLUG every time, but in the long run it's safer and will save time and money and perhaps your life.
     
  4. rjcrjc

    rjcrjc New Member

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    Thanks Elnaldo!
    I do understand not poking with the probe - I actually was using alligator clips when one of them came off of a resistor and must have shorted something. I am very careful about doing exactly what you say - turn off, unplug, attach, power on. That's when the playing on after shutoff really freaked me out!
    I did check the IC voltages on the vanishing side, they seemed in line with the prior measurements and the other channel (which now works great). I will check that again, since i know how to; maybe I shorted it when I was testing it.
    SInce that channel WAS working, I assume I must have created the problem somehow.
    When you mention checking the voltages - I assume one probe on ground, and the other on the circuit? Can I do that all along the signal path? I assume the two channels should be the same, I can use that as a sanity check, and also to see if the values change when cold/warm on the bad side. Is the expected voltage on the schematic? I'm looking at it right now, but can't see the small writing (will grab my extra strong readers later).
     
  5. elnaldo

    elnaldo Addicted Member

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    I'd start checking (monitoring) + and - voltage on the IC you suspect, or around areas where you see some overheating. What you describe sounds to me that some supply voltage is pulled down by some component failing. An amplifier stage with assymetrycal power supply sounds very distorted since it's amplifying only one half of the signal. I've seen similar problems related to leaky capacitors that started to shunt the voltage to ground after some minutes. But a transistor or an IC could do the same, shunt some voltage to ground... I'm guessing your problem will show a problem on the supply voltages to some circuit, but I'm not 100% sure of course.

    Also, monitoring DC offset when failing could tell something

    You could also start checking at the preamp, supply voltage to the preamp section, compare channels, and check forward at each amplifier stage until find some anomaly or difference between channels.

    Yes, one probe to ground, other probe measuring the voltage against ground.
     
  6. elnaldo

    elnaldo Addicted Member

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    I see you have a 2SA798 on that picture, that double transistor with 5 pins. I'd replace it with 2 single transistors as described in other threads. We can't trust that part, those transistors always fail.
     
    Last edited: Mar 20, 2017
  7. rjcrjc

    rjcrjc New Member

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    Elnaldo - thank you - can you please point out where on the picture that 2SA798 is? I don't know which one it is, and can't count pins from the picture.

    I re-tested the IC, and it checked out OK.

    So if i check the supply voltage along the circuit, and compare side to side, I should find an anomaly and that may indicate the source of my problem?
    Since the sound gets bad after a couple of minutes, will the voltage appear ok at first and then change? I would think that would be the case?

    Curious - what am I looking for? should I test DC offset when I first turn the unit on, then continue to test, am I looking for a change? What would that indicate?

    I will have a better multimeter this weekend to work with.
    Also, I have an MCS3338 receiver that I am also debugging, going to spend a bit more time with that as my first victim and be a little more cautious with this one.
     
  8. elnaldo

    elnaldo Addicted Member

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    Just google 2sa798 images, it's a little rectangle with 5 leads.

    [​IMG]

    http://audiokarma.org/forums/index....nsistor-2sa798-is-there-a-replacement.281614/

    I suspect something pulling down the voltage, so yes, voltage should be OK at power ON, and vanish or change when the unit starts to fail.

    The same, perhaps monitoring the DC at the output, some change could happen when failing.

    I'll download the schematic and try to point you to some test points to monitor. I'd monitor supply to the preamp, and then to the power amp.
     
  9. elnaldo

    elnaldo Addicted Member

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    PLEASE check with the AUX input only, not FM, not Phono, to isolate the power amp section. I don't see any active components before the power amp section if you use the AUX input, so if the problem appears, the problem must be at the power amp.

    I see pins 2 and 9 carry the + and - V to the power IC. I'd monitor the voltage there while the problem happens.

    If OK, and the problem persists, it could be the DC offset going off. Monitor the DC at PIN 1 (signal input, to see if it stays at near zero volt), and at the output, to see if the input is OK, but the output not. (that would indicate a problem at the IC)

    Also, it could be the power amp being OK, and the signal arriving distorted. In that case, I'd check the voltages around Q601 Q602 Q603 Q604. Anyway, I think a shift there should be seen at PIN 1 of the power IC.

    If every voltage is OK, but the sound is not, you'll need a signal tracer, a probe to send the signal to a 2nd amp. But that would be a next step.
     

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