SX-828 Rebuild - anything to note?

Discussion in 'Pioneer Audio' started by EastPoint, Jun 22, 2018.

  1. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Hi folks. I picked up an SX-828 maybe 5 years ago, and it had the usual low filter pop issue. I knew very little about repair at the time, and some folks on here walked me through it. I replaced the caps but managed to detach a ground wire and lost a channel. It turned out the channel wasn't due to the wire, but was due to me accidentally bending one of the A725 transistors on the tone board and shorting it out. I replaced that (backwards, as it turns out), but managed to snap one of the 33k resistors next to it. I gave up on it and put it away until I felt like dealing with it.

    Flash forward a few years, and I picked up another SX-828 on Craigslist. This one works fine but all of the lever switches are worn out, in that they will not stay up--the springs are either broken or worn out. The previous owner had desoldered two of the switches and then given up. I started working on acquiring replacement switches but put this one aside too.

    Then a few months ago I picked up a parts SX-828. This one is missing the knobs and has a busted antenna mount, but is otherwise complete. No idea if it works, but probably not.

    I figure between the three of these I can put together two working SX-828s without difficulty. I have fully rebuilt (other than the tuner sections) two SX-636s, an SX-535, an SA-7100, and an SX-950 over the last few months, as well as servicing an SX-939. My plan is to start with the first one, get it working first, then rebuild the boards. Then I'll put together a working one from the other two. I didn't see a full recap/rebuild thread on here, but I saw a few repair threads, and it shouldn't be that hard to figure out what to do. Any suggestions or things I should pay attention to? Are there properly-sized main caps out there at all, or should I just put in smaller ones and pad them with foam?
     

     

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  2. john stumpf

    john stumpf AK Subscriber Subscriber

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    unless you know how to work n double sided boards i wouldnt try it. theres a major board, the control amp that needs lots of parts you will have a very bad time with it if you dont know how.or have a solder extractor (motorized). its not a solder wick job for sure
     
  3. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    I have a desoldering gun, I've never used solder wick. I have an extra preamp board to practice on, so that should help.
     
  4. ivandezande

    ivandezande Super Member

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    Just did a 828 a while back that had so many gremlins it drove me nuts. Nothing really to note other than I would replace all semi's on the amp board (be carful with all the grounds), go with the smaller main filter caps and foam. Good luck, I'd use the second one you mentioned as the parts unit. It's not worth it to unsolder those switches. That double sided board sucks.
     
  5. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Finished the preamp board aside from the 1.5uf caps, which I will order. Finished the power supply aside from the diodes, which I will order. Finished the phono eq board. Finished the protect board caps, but I have to order transistors. Preamp board was a pain but not the worst thing ever.
     
  6. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    So two of my 828s are mulit-voltage models, and both were set to 110v, which can’t be good. I adjusted them to 120v.

    The first 828 is mostly done, I just need to do the amp board, main caps, and a half-dozen other little things.

    I fired up the parts 828, which is one of the mult-voltage units. It is totally dead—no lights, no action. It does pull some juice on turn-on, and occasionally the relay will click but then it’ll click off again. One of the six fuses was dead, and I replaced it, but it’s still dead. Should I start checking power supply voltages first?
     

     

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  7. john stumpf

    john stumpf AK Subscriber Subscriber

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    if all the fuses check good yes the power supply is a good place to start. be mindful of the gang switches deoxit the hell out of them.i'd hose 'em down and leave it on the switches for the duration until you get basically sorted.best to do them initially as you can really chase your tale with all those contacts and deoxit takes time to soften the junk.
     
  8. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Power supply voltages are (all in volts unless otherwise specified):

    1. 0
    2. 12.5
    3. 10 mv and rising to 100+
    4. 4.4
    5. 31.2
    6. 32.8
    7. 34.92
    8. -150 mv fluctuating
    9. 48.5
    10. -150 mv fluctuating
    11. 0
    12. -20 mv fluctuating
    13. -20 mv fluctuating
    14. 41
    15. -41
    16. 0
    17. -42.8
    18. -20 mv fluctuating
    19. 3 mv
    20. 3 mv
    21. 3v fluctuating
    22. 3v fluctuating
    Looks like 19 and 20 are issues.
     
  9. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Relay voltages

    1. 34.8
    2. 34.8
    3. 2.75 fluctuating
    4. 3 mv fluctuating
    5. 180 mv rising
    6. 190 mv rising
     
  10. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    That’s starting at the top left and working my way across and then down
     
  11. john stumpf

    john stumpf AK Subscriber Subscriber

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    i'd check the main amp adjustment your looking for 20mv between 11 and 17 for the left ch and 12and18 for the right 7 and 8 are set as close as you can get it to 0 volts to grnd, i think your relay 1 and 2 terminals are the coil? if so the relay isnt energized as its in protection. use the schematic pin numbers to eliminate confusion. or simply go to 9 and 10 on the protection board and read the voltage between the pins and use clips insulated and hook up with the set off same for the main amp pin's.as i recall the amp alignment isnt covered in the service manual. that 22 pin is too high at 3volts it should be zero volts
     
    Last edited: Jun 24, 2018

     

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  12. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    On the amp board I have around 12-15 mv across 11/17 and 12/18. Pin 8 is like 3 mv but pin 7 is around 2.75 volts.
     
  13. ivandezande

    ivandezande Super Member

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    The power supply and protection boards absolutely need to be rebuilt. Normally it's good to find the issue first, but in this case, replacing all caps and semis may solve a lot of issues as long as the amp board hasn't given up the ghost.
     
  14. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    I rebuilt the protection board--new caps, new transistors (except the one SC1384, which I will order). I rebuilt the power supply as well--new caps, new transistors. I replaced the four big diodes with 1N4005s (didn't have 1N4004s, figured these were fine) but I don't think I have replacements for the other weirder diodes. No change.
     
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  15. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    So it came out of protection for a bit. One channel played fine, other one was super-quiet. I started playing with the switches to see if it was an issue of dirty controls, and when I hit the audio muting switch the relay flipped off again. The lighting issue appears to be a bad dimmer switch. When I wiggle it the lights come up briefly.
     
  16. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Well, that was silly. Apparently the caps tied to the audio muting switch are so leaky that when the audio muting switch is off, it stays in protection. All I had to do was power it up with the muting switch on, and it works fine. Both channels work--the previous channel issue was a dirty tape monitor switch. I bypassed the lighting switch so that's all set. Now just to rebuild everything else.
     
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  17. john stumpf

    john stumpf AK Subscriber Subscriber

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    is pin 7still 2.7 volts?as i said amp pin 7-8 to be 0 volts to grnd.pin 7 is 2.7v and its in protection.the problem is in the main amp.
     
  18. john stumpf

    john stumpf AK Subscriber Subscriber

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    let me rephrase this,2.7v on main amp pin 7 SHOULD put it in protection as it is way to high which is NOT good. if it is at 2.7 and not in protection i'd say you've also got a problem with the protection board.
     
  19. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Pin 7 is now at like 3 mV so I think we're good.
     
  20. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Almost done with the first 828. Finished redoing the amp board today—-all new electrolytics and transistors. Set idle and offset and it’s running well. Lots of tricky transistor pinouts on that board.

    Just got the main caps to replace and we’ll be set.
     

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