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Pioneer SA-7800 phono input problem

Discussion in 'Pioneer Audio' started by vin13, Jul 6, 2007.

  1. vin13

    vin13 New Member

    Messages:
    23
    Hi

    I recently aquired an SA-7800. It works perfectly apart from the phono input, which only works through the right channel. Does anyone have any idea if this is an easy or difficult problem to fix.

    Thanks

    Vin
     

     

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  2. markthefixer

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

    Messages:
    21,054
    Location:
    Bensenville,Illinois
    Have you cleaned the switches using deoxit? That is an ESSENTIAL first step.

    Have you tried switching in and out the phono interference filter, in the back, near the phono plugs?

    Have you tried switching inputs ( L - R ) and seeing if the problem stays put?

    After that is settled, there is little else except some sort of component failure in that channel of the phono amp that must be diagnosed, since a lot of circuitry IS working, to give you one channel. The board is mounted foil side up on the top of the unit by the function and tape duplicate switches, and to replace components on it would mean the faceplate would have to be pulled to get those switches loose. To pinpoint the problem without a scope would mean taking some voltage readings, testing the transistors and perhaps replacing six of the electrolytic capacitors on that channel. It IS possible that the voltage readings could pinpoint the problem, and the voltage readings can be done WITHOUT doing anything other than removing the cover, but the repairs in all likelyhood would involve that disassembly I mentioned.

    now, is that easy or difficult?
     
  3. vin13

    vin13 New Member

    Messages:
    23
    Mark
    Thanks for your reply. No I haven't cleaned the switches using deoxit? I'd never heard of it 'til now, I'll try to get some. Yes I switched the phono interference filter and switched the inputs (with 3 turntables). The problem stays put. I'm sure the rest of it easy if you now how!! I'm not an electronics guy, just someone who wanted to pick up some gear from the 70s that I was too young to afford at the time. I'll look for someone who know what they're doing to look at at.

    Cheers

    Vin
     
  4. Timothy Flath

    Timothy Flath Active Member

    Messages:
    287
    Location:
    Victoria, BC
    I am having the same issue. Have re-capped the entire amp and problem stays put. Everything on the unit works as it should except phono left channel is out. Done all of the above and problem persists. Not sure if it is the little sub board at the back or the board it is tied to at the front of the unit - any specific transistors I should try swapping?
     
  5. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,009
    Location:
    Austin, Texas
    I think those 2sa912s run hot. Mine desoldered themselves from the board. You can pop the cover off and reach them without removing the faceplate and board. Wiggle them and look to see if they are soldered well, or try reflowing them. You can replace them with KSA1220AYS. You will need to drill out the holes and then pad are small. Mine were gone so I had to scrape some green off and solder to the trace.
     
    Timothy Flath likes this.
  6. Timothy Flath

    Timothy Flath Active Member

    Messages:
    287
    Location:
    Victoria, BC
    You sir are a true Gentleman and a Scholar. I tried re-flowing the transistor contacts and it was a no-go. By sheer blind luck I had mis-ordered the transistors on this refurb effort as per the recommended list on this thread (Post #14): http://www.audiokarma.org/forums/index.php?threads/sa-7800-with-burnt-out-resistors.487176/

    It recommended the following transistors and substitutes:
    Transistors,,,,,
    "1,2",2SA1114,KSA992,,,
    "3,4",2SC1117A,KSC1845PBU,,,
    "5,6",2SA912,KSA1142 + heatsinks,,,

    This is actually my first re-cap and repair effort on my own (previously I had installed a number of old Quad kits like the 34, 44, 306 and FM4 which came with everything and instructions - kinda like recap by numbers). The repair part was the u/s left channel of the Phono which I didn't think I would be able to do so was going to take it in to a tech if I couldn't sort it out myself.This was really my first ever attempt at finding a suitable order list and then ordering from an online parts seller (Digi-key and now I am looking at Mouser Canada). I had not messed with transistors before this and only ordered two of the above not realizing it called for 6 Transistors. One of the two that I received was the KSA1142 which also appears to be a replacement for the 2SA912 you mentioned as per the order list. I thought they were one in the same and didn't realize the 5,6 referred to Q5 and Q6 and i should have ordered two. It was Q5 that was duff. I had bought some thumb drills on a recommendation from the local electronics store and sure enough I used one to bore out the holes as you mentioned. It was a good buy! I plugged the amp back in and hooked up the Turntable and "POW", it frikkin' worked on both channels with the phono!
    This was the final thing to compete the refurb/repair and I am absolutely tickled pink. I will of course order the balance of the transistors and install them when they arrive to finish it up. It sounds real real good now....
    Next I will be doing my matching TX-7800 and then a SA-8500II so thank you very much for getting me past this final hurdle.

    :beerchug:

    [​IMG]
    [​IMG]
     
    Last edited: Jan 14, 2018

     

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  7. loweran

    loweran AK Subscriber Subscriber

    Messages:
    1,009
    Location:
    Austin, Texas
    Good deal. Glad you got it working!
     

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