SA-7800 restore

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

  1. loweran

    loweran AK Subscriber Subscriber

    Messages:
    923
    Location:
    Austin, Texas
    It’s alive!
     

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  2. loweran

    loweran AK Subscriber Subscriber

    Messages:
    923
    Location:
    Austin, Texas
    Wow, I went back and re-read my thread and seems like I am rambling and just thinking out loud on my posts. Sorry if this is confusing and hard to follow. I will re-cap (pun intended) whats been done so far:
    • Visually inspected the set and found burned resistors
    • Tested all resistors diodes and drivers on the amp board. R34,R56, R58, R60 tested bad
    • Replaced the bad resistors
    • Asked a lot of dumb questions
    • Lifted the leads of the outputs and tested them. Right channel outputs were dead. This was the channel where the bad resistors were
    • Replaced outputs with On-Semi MJL3281 and MJL1302
    • Replaced trimmers
    • Checked for bad solder joints and did some reflowing
    • Powered up on DBT and got no shorts and the relay clicked
    • Checked power board voltages. All checked good
    • Re-capped the power supply, pulled, tested, re-greased and reinstalled the large transistors on the power board
    • Powered up on DBT and got nothing
    • Fuse checked good but was loose. Tightened it up and cleaned it. Powered up on DBT fine
    • Checked power supply voltages and all checked out
    • Replaced Q11 - Q16 and Q19 - Q22 on the amp
    • Powered up on DBT
    • Display went dark and I traced it down to R104. It tested bad so I replaced. Display came back as normal
    • Set the idle current and tested on speakers. Sounds and functions great
    • Ordered the rest of the caps I need and am waiting on time to finish
    Much more to do on this unit including cleaning the pots and switches, finishing the board work, replacing the power cord (it's got a crimped on temporary since someone cut the cord), building a new walnut case, and general clean up.

    At this point I think I have too much in this unit. After all the orders, since I keep finding issues that require another order, I am up to $260 on it. Case will probably be another $20 in materials. I was thinking of selling this one when I am done, but I may second guess that and hold on to it for a little while.
     
  3. loweran

    loweran AK Subscriber Subscriber

    Messages:
    923
    Location:
    Austin, Texas
    Got more done last night. I finished all the boards except for the display board. Notes from the work:

    The two 2AS912s in the eq board had desoldered themselves and left very little pad to work with. Replacing with KSA1220AYS requires drilling out the mounting holes. This combined with the bad shape the pads were in required scraping away the green board masking to copper and soldering them to the traces.

    Everything else went fine and a quick test on the DBT showed now shorts and I get relay click.

    The last board will be the display.
     
  4. loweran

    loweran AK Subscriber Subscriber

    Messages:
    923
    Location:
    Austin, Texas
    Done with the boards. Deoxit and lubed the controls, put the thing back together and am breaking it in. I am still a little worried about that phono amp transistor. It dont look so good. The connection seems solid though. Check out the pics and let me know what you think about that transistor.
     

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