Pioneer SX-990 main amp problems

Discussion in 'Solid State' started by Stink Bear, Feb 6, 2018.

  1. Stink Bear

    Stink Bear Member

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    Looking for advice / help. The original problem was a weak and garbled right channel. I removed the pre-amp jumpers and cross jumped, the problem stayed on the right side. I then swapped output transistors and completely lost the left channel. I cross jumped again and still got output from the right channel. I would guess I took out a transistor.
    I did some searching for the transistors ( 2SC793Y), it looks like old ones are available, I don't know about new ones.
    I also did some searching for a replacement main amp unit thinking that I might just replace the whole thing. I haven't found one yet.
    So I thought I would ask some people more knowledgeable than I for recommendations on continuing to try to troubleshoot / repair what I have, or keep trying to find a replacement main amp unit.
    I have limited ability to read and understand schematics. I have multi-meters. I also have an O-scope, I haven't touched one since tech school in 83 and didn't use one much then, but I think I'm still capable of learning if someone wants to help.
    Any advice / help would be appreciated.
    Thanks,
    Brett
     

     

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  2. Stillhouse

    Stillhouse Active Member

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  3. Stink Bear

    Stink Bear Member

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    Thanks Stillhouse.
    I just had a new development. I found a broken wire at the left channel output and soldered it back on. I must have broken it when I was swapping transistors. My left channel is working again.
    I received a suggestion to check the output coupling caps. One measured .11nf one way and .45nf the other way, the other cap measured .11nf and .36nf. I would guess that since they are not shorted or open that they are okay?
    Anyway, I'm back to just the weak garbled sound problem on the right channel, and I've eliminated the output transistors as possible culprits, maybe the coupling caps as well.
     
  4. Tom B

    Tom B AK Subscriber Subscriber

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    Those large coupling caps should be measuring 1000uf, not .11nf or .45nf
    But if those caps were a problem, the symptom would be no output, not weak and garbled sound.

    sounds like component issues on the amplifier. Check for signs of burnt resistors. measure the emitter resistors (0.5 ohm @2w) and the other small resistors especially like the 150 ohm and 22 ohm ones. Check proper bias voltage on outputs (should be about 0.5v to 0.6v) measue from base to emitter. also check both 3.3uf caps at the beginning part of the power amp. an open cap there can cause low possibly distorted sound.
     
  5. Stink Bear

    Stink Bear Member

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    Thanks for the reply Tom.
    I have looked at the resistors and I don't see any obviously burnt, the unit is quite dirty though as it went through a house fire. I am going to get some simple green today and start doing some cleaning, maybe then I will be able to see a burnt resistor if there is one.
    When you say to measure the resistors, are you talking about de-soldering at least one end and using an ohmeter, or can they be tested hot using a voltmeter? Resistors are R33, R29, and R27?
    The bias voltage you're talking about would be on the output transistor Q11? Measure with DC voltmeter?
    It looks like the 3.3uf cap could be checked without removal. Is that correct?
     
  6. Stink Bear

    Stink Bear Member

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    Update: I bought another SX-990 but it didn't fix the problem.
    Kidding. I did buy another SX-990 that the guy said had only one channel working. It turns out it had the same symptom, but worse. I opened it up and looked at the main amp unit. There was a burn spot, the 150 ohm resistor Tom said to check was burned (R29). I had a 150 ohm resistor that was the same physical size so I soldered it in and fired up the unit. The right channel sounded good, I turned up the volume a little bit and it smoked the new resistor. So was my new resistor not able to handle the power, or is there another component (Q7?) taking out the resistors?
     

     

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  7. Stink Bear

    Stink Bear Member

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    Another update:
    I cannibalized a .5 ohm / 2W resistor out of the 990 I just bought and replaced R31 on the 990 I've been working on. I played it for about an hour with no issues. I didn't put any real power to it, I'll try that tomorrow, but I think it's fixed.
    I looked at the 990 I just got again and saw that the resistor I replaced was smaller and a different base color than the matching resistor on the left channel. I suspect the one that burned was a 1/4 watt, and I replaced it with the same size. Tomorrow I will try to find a 1/2 watt 150 ohm resistor to replace the one that burned, and a .5 2W resistor to replace the one I cannibalized. I think that will fix the new unit.
    Thanks again to Tom B.
     
  8. Stink Bear

    Stink Bear Member

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    I did some more checking of components and found Q8 bad (not Q7, wrong side), I suspect that was taking out the 150 ohm resistor. Can anyone confirm?
    I also found that someone has apparently replaced Q3 & Q4 as they don't match the part number on the schematic and look new, maybe Q5 & Q6 as well. They are supposed to be
    2SC497's by the schematic, but they are 2SC627's. Is that an acceptable substitution?
    Q5 & Q6 are supposed to be 2SC497'sas well, but are 2SC485's. Is that an acceptable substitution?
     
  9. gadget73

    gadget73 junk junkie Subscriber

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    I've got an SX-990 parts unit if you need anything. I bought it with a known weak channel but i did absolutely no diagnostic work and never confirmed that. I needed other parts out of it and it never even got plugged in. I believe its a virginal unit though.

    You may get some more traction with this in the Pioneer sub-forum. Solid state isn't really my strong point so I can't really answer parts sub questions. I always ask smart people who have done this before, even if it is just Google.
     
  10. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    Just had a look at the schematic, not a great copy but it looks like R29 belongs to Q7? The 2SC497's are much beefier devices than the 2SC627's, the 2SC485's are suitable in regard to current and voltage ratings but only have a min ft of 10 where originals are 30. You need something like a KSC2383/KSA1013 for the drivers and just the KSC2383 for the others (Q3 and Q4).
     
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  11. elnaldo

    elnaldo Addicted Member

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    I'd say you need to stop swapping parts without a diagnostic or at least an understanding of the schematic. You can do more damage, hard to fix, or burn the working channel so you loose the reference you can have to compare measurements.

    Post your finds and wait answers and advice from people who understand the schematic. The unit won't fix by itself, so trying and trying will burn more components. When a resistor is burned it's usually because a short at other point in the circuit.
     
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  12. Stink Bear

    Stink Bear Member

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    Gadget73, I will keep you in mind for a parts source. I am getting yet another 990 that I hear is completely dead, I will try to fix it as well. I'm thinking I may need some power parts for it. The unit that I fixed is missing a couple of knobs, so if I get the third unit fixed I will be looking for some knobs. Thanks for the offer. It may be a while though, I have to leave next Sunday and I will probably be gone for about a month.
    OldSansui441, You are right, R29 belongs to Q7. I figured out recently that I was on the wrong side of the schematic. I'm working on the right channel, so the components that I've found bad are Q8 and R30. I also removed and tested Q4, VR4, R28, & R32, Those tested good. I didn't hear from anyone for a while and I thought maybe I wouldn't so I found an old thread with part numbers for a re-cap and got the same part numbers for the transistors that you posted. I ordered KSC2383/KSA1013 transistors to replace the wrong ones that are presently in the unit, along with a 588-WHCR50FET for R34, and 291-150-RC for R30. I would guess this will take care of the problems but if you have any other suggestions on troubleshooting they would be appreciated. Thanks for your help.
     
  13. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    Your welcome, it was late last night when l posted and l forgot to say also check the replacement transistor pinout as l think they are different to the originals, also when you order them make sure you can get a gain ranking that is as close as possible to the original 2SC497's.

    The best advice l can give is to check ALL transistors, resistors and any diodes in that bad channel before replacing any parts. I am a little confused now between the two units you have, did the one with the bad driver ever work in that channel in your possession? The reason l ask is because the driver would normally fail because of a faulty output transistor, good idea to check these as well. Also best to copy whatever you do in the bad channel over to the good channel to make sure both channels are balanced :).

    If it were me l would rectify original concern before preceding with any other work, ie recapping etc as you MAY introduce other problems and then you won't know where you are at.
     
  14. Stink Bear

    Stink Bear Member

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    Oldsansui441: I'm confused as well, I'll try to straighten some things out.
    The first unit I was working on had a weak and distorted right channel, I think the problem started fairly recently, but I'm not sure since it is not mine. I did swap all the output transistors for troubleshooting purposes, the symptom did not change. Using the advice from Tom B I found a bad resistor (R34), and replaced it with a resistor out of the unit that I just bought. I have operated that unit for probably 6 hours now with Advent Legacy I speakers. I would think the Legacy's would provide plenty of load for a test. It hasn't released any magic smoke and still sounds good. That unit belongs to a friend, it is his garage stereo, it went through a house fire, it will not be taken care of and I don't think it's worth any more of my time or effort. I got it going, I learned a little bit from it, and I'm content to leave it alone now.

    The unit that I just bought had a weak and distorted right channel when I got it a week ago. I bought it from a record store owner, he said he tried to figure out what was wrong with it but I get the impression that he knows considerably less about electronics than I do. I doubt he replaced any components. Apparently at some point in it's life though, someone repaired, or tried to repair it, that's all the history I know on that one. That unit had an obvious burn spot, R30 was fried. I replaced the resistor and powered up the unit, it worked for a few minutes but when I turned up the volume R30 smoked again. That's when I checked Q4, VR4, R28, & R32, they are all good. I was going to stop testing there. I planned on replacing Q3, Q4, Q5, Q6, Q7, Q8, R30, and R34. Q3-6 will get the 512-KSC2383YTA's, and Q7 & Q8 will get the 512-KSA1013YBU's.
    Are you saying you think I need to check every component in the right channel of the board, and the output transistors?
    I don't plan to re-cap this one. I just want to get it functioning.

    I will be picking up the third unit tomorrow from the same record store. The owner says it's completely dead, no sign of power. If I can get that one functioning I may re-cap it. I've been looking for a unit to re-cap that won't be too difficult and isn't worth a lot of money. I have an SX-939 as my main stereo that I would like to re-cap, but I don't want to start on that one. I'll see how an easier and cheaper one goes first.
    Since you're apparently a Sansui fan, I'll let you know that I recently acquired an Eight Deluxe that I sometimes use as well. It's in good condition, and I like it almost as well as the 939.
    Thanks again.
     
  15. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    I would yes, but up to you of course. Most resistors can be checked in circuit, use the other channel as a reference.

    That makes sense and is a good plan. I too have an SX-939 that l have rebuilt and it is prob my favourite unit also.

    Lucky you, an eight deluxe would be very nice to own.
     
  16. Stink Bear

    Stink Bear Member

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    Checking most components in circuit would certainly be easier and less time consuming than de-soldering to isolate components. I'll give that a shot.
    It looks like I will have to put the repair job on hold for a while. I have to go south next Sunday for work, I don't think I'll get the parts in time to install them before I go. I'll be gone for about a month.
    Was there a noticeable improvement in your 939 after you rebuilt it? Mine sounds pretty good right now, I just figure with it's age it probably has some dried out caps, and who knows what else.
    With your username I would have thought your favorite would be a Sansui.
    Thanks for all your help. I really do appreciate it.
     

     

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  17. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    Yes the resistors can be checked in circuit unless they read an odd value then you will have to lift a leg and recheck. Diodes you will prob have to lift one leg although if they test eg .550v one way and ramp up to a high value eg 1.500v the other way IN circuit they will be ok. Transistors would have to be removed and checked with a 6 way diode test on your multimeter, but it looks like you will be replacing these anyway.

    My SX-939 showed a massive difference after rebuild (not an exaggeration) l checked it on the same speakers and against other units before and after. In original state it was very sloppy in the bass and the mids were very recessed, it is much more detailed and controlled now. I have rebuilt many of my other units and whilst they all improved somewhat none so much as the big Pioneer, maybe the parts in mine were particularly degraded.

    I do like Sansui very much also, the little 441 was my first vintage piece and l still have it (rebuilt also). I don't really have a favourite brand, l just use my ears :). Your very welcome.
     
  18. gadget73

    gadget73 junk junkie Subscriber

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    Don't have knobs but if you need anything else shoot me a PM.
     
  19. Stink Bear

    Stink Bear Member

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    Roger that gadget, thanks. Like I said though, it may be a while. I have to leave Saturday, for probably a month. I don't know if I'll get a chance to troubleshoot the other unit before I leave. I looked at the schematic, it looks like a totally dead unit could only be so many things. If it's not something simple like a bad fuse or cord, it could only be the transformer, power switch, or power supply unit.
     
  20. gadget73

    gadget73 junk junkie Subscriber

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    well I have all that stuff too. its not going anywhere, let me know if you need anything.
     

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