SA-7800 restore

Discussion in 'Pioneer Audio' started by loweran, Nov 19, 2017.

  1. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
    Austin, Texas
    All, I have an SA-7800 which arrived this weekend. I pulled the cover and did the initial inspection. Looks like a few burned resistors.

    From what I have read, I need to do a full test of each component on this unit. I did fire it up on a DBT, which might have not been the best approach, but the seller had it powered up for pictures, so I assumed if any damage was done, it was already there.

    I have voltage on the left side of the amp test points, but not the right. Burned components are on the right.

    Any advise on how to approach would be appreciated. This one is going to take some time.
     

     

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  2. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
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    Here are some pics of the foil side and top side.
     

    Attached Files:

    Last edited: Nov 19, 2017
  3. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
    Austin, Texas
    Also, I noticed in the manual that some of these resistors have an exclamation next to them indicating 'safety factors' need apply. Any idea what that means?

    Edit: Looks like those may be flameproof. Looked at mouser and they don't have all the values. Any ideas what I should put in here and where to source them? I am going to at least need:

    R52 - 220ohm 1/4w
    R34 - 560ohm 1/4w
    R46 - 1Kohm 1/4w
    R56 - 82Kohm 1/4w
    R58 - 150ohm 1/2w
     
    Last edited: Nov 20, 2017
  4. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
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    Started unsoldering components to lift one leg and test. Looks like I have to take out the heat sink to loosen the wires enough to flip the eq board up for access to it and the amp board under it.
     
  5. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    Location:
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    Some ! mean don't change the wattage, as opposed to flameproofing. The 1/4w are meant to pop like fuses - 1/2 watt would let too much energy through.

    There is much more to say, yet I can't get it in fast enough.

    R56, 82k ohms bad? Burned? 8.2k ohms...
     
  6. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
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    Hey Mark, right looks to be 8.2K the schematic is difficult to read. Are you suggesting that 1/4 watt can be replaced with regular old resistors? The type specified is RD1/4PSF and RD1/2PSF.

    And, don't worry. This one is going to take some time, and time I will give it.
     
    Last edited: Nov 21, 2017

     

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  7. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
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    Ok, managed to get the phono out of the way. On to testing.
     
  8. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
    Austin, Texas
    Only resistors that tested bad on the bad channel were:
    R34
    R56
    R58
    R60

    None of them gave a reading and they are physically burnt.

    Now I am off to test the transistors.
     
    Last edited: Nov 21, 2017
  9. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
    Austin, Texas
    Also went ahead and replaced the 100ohm trimmers.
     

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  10. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
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    So far I have not been able to find anything else wrong with it. Tempted to put it on a dbt and see if I can get the voltage readings. I am going to wait and see if anyone chimes in before I do.
     
  11. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

    Messages:
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    Location:
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    Transistors can test good yet still be waiting to bite you when they get real power.
    WEAR EYE PROTECTION.
    I had SHRAPNEL go flying past my nose the first time all my old transistors in a NSA amp tested good and I repowered it.
    It is a most unforgiving beast.

    My formula for success in that arena is 100% silicon replacement and 100% lifted leg resistor testing. Just say'in.

    My favorite saying is " the transistors were racing to destruction and these guys lost the race - they are waiting for their chance to finish the race."
     

     

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  12. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
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    OK, thanks for the advise Mark. I lifted every resistor and diode on the bad channel. All tested good except the afore mentioned ones. I did not have a 560ohm on hand and used two tied together to get that value. I will need to order one in my next order. I did find flameproof for the .5W resistors that needed to be replaced. I do have some questions for you if you don't mind:

    I do have all the transistors except for Q25, Q26 2sa913 and q23 q24 2sc1913 as we discussed in my recap list thread. Q24 and Q26 happen to be in the path of R56 and R60 which were burnt. I tested them and they show to be OK.

    Are you suggesting I shotgun the transistors? Should I also shotgun the diodes?

    When I originally got the amp, I did power on a DBT and it did not indicate a short. Would that mean the outputs are OK?

    Just to be damn sure, I need to set up the trimpots to read 0ohms initially (fully counter clockwise) and then move up from there correct? I didnt have 68K ohms, but do have 100K ohms. Can I use that in place of the 68Ks?

    Last question, should I pull the heat sink and regrease the outputs? Looks like that may be the best way to get at them without pulling them from the board. If I use sil pads, what size should I get for those?

    Thanks for your help and Happy Thanksgiving!
     
    Last edited: Nov 22, 2017
  13. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
    Austin, Texas
    Did a little more research on this amp. Looks like the issues I have are common. The same resistors are burnt in the following threads:
    http://audiokarma.org/forums/index.php?threads/sa-7800-another-one-bites-the-dust.751276/
    http://audiokarma.org/forums/index.php?threads/pioneer-sa-7800-help-replacing-transistors.601238/
    http://www.audiokarma.org/forums/index.php?threads/sa-7800-with-burnt-out-resistors.487176/

    I still need to disconnect the outputs and emitters and test. I suspect the ones in the bad channel are shorted and the reason the DBT test showed good is because the burned resistors were open.

    Now, the question I have is what causes this condition to happen?

    Once I am done testing, I suppose I will need to order 4 new outputs so the sides are symmetric and I will go ahead and replace most the transistors in the both sides. The drivers are not available, but it looks like I can source the Toshiba replacements on ebay if I need to. The two in the bad channel tested OK so I re-installed them.

    Is it OK to repair the bad channel and power up on a DBT to test before I change anything on the good channel?

    Would these insulators work for the MJL1302 outputs?
    https://www.mouser.com/ProductDetai...JOF5Pn68Ca%2bobR6FRfM8RmADQwCgkHRt8e%2bgICA==
     
    Last edited: Nov 23, 2017
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  14. redk9258

    redk9258 Super Member

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    1,416
    Location:
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    I wouldn't source any transistors from eBay unless you want to buy fakes. What are the numbers you need to replace?
     
  15. redk9258

    redk9258 Super Member

    Messages:
    1,416
    Location:
    Illinois, close to St. Louis.
  16. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
    Austin, Texas
    Thanks for the links! I did pull them and tested them and they tested OK. I am starting to think the source of the issue is a dirty trimmer that was fiddled with by the previous person. Not sure what other damage I will find. I may just go ahead and order these for peace of mind.
     

     

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  17. leesonic

    leesonic Hold on, here comes the bass. Subscriber

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    Use the transistors listed in my thread. These were from Mark's own recommendations.

    Lee.
     
  18. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
    Austin, Texas
    Thanks Lee. I did use your thread as a reference. I have all the parts except the outputs and drivers. Gotta put an order together.

    Did you use pads or mica to mount those new outputs?
     
  19. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
    Austin, Texas
    OK, back at it. I pulled the ouputs on what i thought was bad side and took readings. Turns out it was the good side. Moving on to the others. Here is how they measure:

    2SA1075 PNP
    -B +E: .529v
    -B +C: .521v
    +B -E: OL
    +B -C OL
    #4
    2SC2525 NPN
    +B -E: .543V
    +B -C:.540V
    -B +E: OL
    -B +C: OL
     
  20. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,001
    Location:
    Austin, Texas
    And on the bad side they are:

    2SC2525 NPN
    +B -E: OL
    +B -C: .0V
    -B +E: OL
    -B +C: .0V

    2SA1075 PNP
    -B +E: .003V
    -B +C: .003V
    +B -E:.003V
    +B -C: .003V
     

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