SA-7800 restore

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

  1. loweran

    loweran AK Subscriber Subscriber

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    It’s alive!
     

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  2. loweran

    loweran AK Subscriber Subscriber

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    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

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    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

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    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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  5. loweran

    loweran AK Subscriber Subscriber

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    Interesting note. I have restored an SX-737 and have it next to my newly restored SA-7800. The damn SX-737 kicks the SA-7800s ass in the sound department. Maybe it's the color the 737 adds, but for me, it is much more pleasing. Its a little fatter on the bottom end, and just pleasing. I haven't listened to it for a while since I had my SA-7500II hooked up, and then my SA-7800. It's a bit eye opening since the specs are so not in it's favor.
     
    Last edited: Jan 25, 2018
  6. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    Nice job on the new outputs....
     

     

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  7. loweran

    loweran AK Subscriber Subscriber

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    Thanks Fuxtor.

    I can't help but think something may not be right with this amp. When I was listening to it on its own, it seems fine, but next to my SX-737 it's falling short.

    Maybe I just prefer the sound of the 737, but I may try and find a way to measure the amp to see if something isn't right. I only have an oscilloscope and think I may need more equipment to get proper measurements of the specs. I suppose I could take it to someone and have them run the tests.

    I have noticed that it sounds thinner at higher volume. I ordered filter caps for it and will be replacing those this weekend. We will see if that makes a difference to my ears.
     
    Last edited: Jan 26, 2018
  8. QSilver

    QSilver Super Member

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    You might find a recap of the audio boards really makes it sit up.
     
  9. loweran

    loweran AK Subscriber Subscriber

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    I recapped all the entire unit. Also put in new outputs since two were shorted. We will see if the filters help. Those are the only ones I didn't replace.
     
  10. loweran

    loweran AK Subscriber Subscriber

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    Swapped out the filters. Had to extend the leads with some hookup wire to make them fit. There is a bit of difference in the bottom end. The amp is more detailed than the SX-737. I think it's just not as warm and tube like. Sounds good but different.

    All thats left is to replace the power cord and build a custom walnut cabinet for it.
     
    Last edited: Jan 26, 2018
  11. loweran

    loweran AK Subscriber Subscriber

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    OK, so been running it through it's paces. Much better. Must have had some weak filters. Very clear and detailed. Not as much bass set flat for my taste. Bumped up the bass and its nice. I will post some pics when I get the case built. Hoping to get that built tomorrow and then have to finish the wood.
     

     

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  12. tarior

    tarior Dirty pool, old man? Subscriber

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    Nice save on that amp.
    Just for future reference, MT-200 transistors will fit (outputs, naturally) without modifications.
    Near as I can tell, there are two primary failure points with these. The bias trimmer briefly goes open, usually when someone has to twiddle it, smoking the outputs. And less likely, the heatsink grease dries out, and they go into thermal runaway when driven hard.
     
  13. loweran

    loweran AK Subscriber Subscriber

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    Thanks Tarior. I referenced your post about these in my research. This one had the typical three or four burned resistors and shorted outputs on one channel. I suspect this was due to a trimmer failing open. Those got replaced as did a lot of other stuff. I am working on the power cord now. I have it installed, but not sure I have it oriented the correct way. I put up a post specifically for that. I wired the neutral side of the polarized plug to the side that goes to the brown wire on the transformer. The hot side was wired to the side with the switch and the fuse.
     
  14. loweran

    loweran AK Subscriber Subscriber

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    Case is done. It’s a bit tight, might have to shave the blocks where the screws go. All in all it was a fun project. BTW Walnut is expensive!
     

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    Last edited: Jan 28, 2018
  15. loweran

    loweran AK Subscriber Subscriber

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    More pics
     

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  16. Motnick

    Motnick AK Subscriber Subscriber

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    Wow you knocked that case out quick! That looks great. Good job.

    I’d love to do something like that for my SX-737.
     

     

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  17. loweran

    loweran AK Subscriber Subscriber

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    Yeah, I have a bit of ADD. It took me more time to dig out the tools than to build the case. Started with wood working a while ago. I get into things and immerse myself. The latest is vintage audio. I have a shit ton of tools including a cabinet table saw and built in router table. I know my way around wood working stuff. Here is a built in I did recently.

    Its a bit of a curse. I have myself in all kinds of crap including photography, astronomy, wood working, audio, etc.

    Listening to the amp now. Sounds sweet and looks great! Smells like stain :)

    I have an SX-737 too. LOVE it! I kind of like how it looks. Unassuming sleeper. It kicks butt! The SA-7800 I have came with the veneer stripped and the "wood" was chipped. Needed some help.
     

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    Last edited: Jan 28, 2018
  18. Motnick

    Motnick AK Subscriber Subscriber

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    Wow that built in is beautiful!

    I tend to immerse myself in things as well. It’s always great to push yourself and see what your capable of. I always surprise myself. You never know what’s possible until you get off of the couch and try! Life is no fun just sitting on the bleachers.

    The walnut case that you have built looks great and it makes that unit truly your own. Nice job!
     
    Last edited: Jan 28, 2018
  19. loweran

    loweran AK Subscriber Subscriber

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    Thanks man.See that SX-1010 in the bottom :) If you are seriously interested in a case for your 737 I will take a look at it and see what it involves. Since I have one, I bet I could whip out something for it and might as well do two since I have one.
     
  20. Motnick

    Motnick AK Subscriber Subscriber

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    I have the vintage audio bug bad! I’m almost done with my SX-1010. Waiting in the wings is a SX-950 and a Sansui 9090DB. The SX-737 is special though, as it was my first recap project. Although it lacks the power of the others it has a special sound that’s all it’s own.

    The worst part of the SX-737 is the vinyl wrapped case, and mine has had a bad life. The rest of the receiver is very nice. I’d love to have a walnut case. Let me know if you decide to make some. I would be interested in one.
     

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