1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

Cuda's SX-727 Refurb Thread

Discussion in 'Pioneer Audio' started by cuda440, Jul 27, 2018.

  1. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    I have begun the 727 refurb and am going to document the effort here, with plenty of pictures (more coming soon).

    First I have a question about what I found on the power supply board, namely a scorched area (see photo). The unit works just fine and the voltages checked out to spec, but should I do something about this?

    Thanks,
    Cuda.
    IMG_9118.JPG
     

     

    Please register to disable this ad.

  2. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    Okay, I completed the rebuild of this board using the parts listed here:
    http://audiokarma.org/forums/index.php?threads/help-me-create-an-sx-727-bom.826931/#post-11706334

    but when testing the results I'm getting incorrect voltages per the service manual (and what I measured before swapping the parts). Of course I've checked and rechecked all the parts, placements, solder joints, etc. I even lifted a leg on the 330 ohm 2W resistor and verified it's value. Nothing looks amiss.

    The only part of this board that's acting right is the bridge rectifier (pins 14 & 15), which is isolated from the rest of the circuit on this board.

    Note: the nylon screws holding the heat sinks to the KSC2073s snapped right off during disassembly so I substituted some slightly larger ones after drilling out the heatsink holes and tapped them with a metal screw. The odd, clear, glue-like thermal compound was cleaned off and new mica insulators and grease were applied.

    Here's a list of voltages:

    pin spec measured
    2 12.5 5.9
    6 30 16
    7 36 19.1
    12/13 ?? 52.6 AC
    14 36(31.5) 35
    15 -36(-31.5) -35

    Q1 C 41 48
    Q2 C 35 19.7
    Q3 C 31 6


    Please help!
     
  3. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    Found it! Broken trace leading to C6 positive side. See photo.
     

    Attached Files:

    dlucy likes this.
  4. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,729
    Location:
    Victoria, Australia
    Nice find on the broken trace. I bet your 727 sounds very nice and really looks the part with it's real veneer cabinet :thumbsup:.

    That resistor heat discoloration on the board in your photo is fairly common on early Pioneer's, they probably should have lifted those high wattage resistors off the board to allow for some air circulation.
     
    dlucy likes this.
  5. ZR9B

    ZR9B AK Subscriber Subscriber

    Messages:
    400
    Location:
    MI
    Damn... I'm thinking the 727 in the cuda.....unless it's a manual.....I miss my cuda.......sorry to stray...
     
    UncleBingo likes this.
  6. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    Moving on to the protection board. My unit has the newer version, AWM-027, so I downloaded the service manual supplement that covers it and ordered parts accordingly. Looks like there's more to it since the manual calls out .056uf mylar for C1 and C2 (which I did not order), but my board has .22uf 10v electrolytics. I do have some Wima film caps I can use instead.

    More scrutiny reveals the board layout is quite different and, incidentally it has a rev D designation. Does anyone have a manual update that covers this rev?
     

     

    Please register to disable this ad.

  7. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    But wait, there's more. Q1 and Q2 are listed as 2SC857 in the manual, but I have "C86921C" as labeled. I'm planning to proceed with replacing them with KSC1845.

    Any reason not to?
     
  8. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    Went ahead with the 1845s and all seems well. Power amp is probably next.

    I'll post some picts tomorrow of the progress so far.
     
  9. ivandezande

    ivandezande Super Member

    Messages:
    1,703
    Location:
    Twin Cities
    You have to dig around for the different boards on HiFi, I think that 828 manual has it. Or the 727 has one near the end of it, it's been a while since I worked on either of them so I can't really remember.
    Be sure to get a bigger resistor for that fried one as well, as well as replace the one to the left of it just for caution.
     
  10. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    Thanks. I did locate the 027 board in the 727 supplement, but it must be the first version of the board and an initial AK search didn't turn up any reference specific to the rev D. I'll have a look at the 828.
     
  11. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    Next up is the power amp (AWH-011). Everything went well up until the very end. I had to disconnect one orange wire from the filter caps to allow the power amp assembly to swing up so I could get access to the boards and the heatsink hardware. Removed the output transistors and heatsink, set them aside. I then made all the part swaps and then tested my work by plugging in the output transistors, but without the heatsink, letting the board dangle. Powered it up and got the relay to click.

    I then completed the rebuild of this section by cleaning the heatsink and the output transistors, added thermal grease and new insulators, and reassembled everything by swinging the assembly back into position. Lastly, reconnected the orange wire.

    Fired it up, but no relay click. What the!

    Measured all the voltages around the perimeter of the power amp and all seem good.

    I've read here that this problem can be caused by incorrect bias/offset settings, but that wasn't changed during the final assembly, so that seems unlikely (plus there's no adjustment procedure listed in the service manual). I've inspected the board top to bottom and tugged on the point to point wiring to confirm a solid connection.

    Out of ideas. Tell me what measurements to make.
     

     

    Please register to disable this ad.

  12. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    Broken wire. Wasn't visible until I swung the assembly back up for troubleshooting.

    o_O
     
  13. ivandezande

    ivandezande Super Member

    Messages:
    1,703
    Location:
    Twin Cities
    Ha happens glad you got it going. The pot on the protection board can also cause that issue, there's really no adjustment method but it's the DC cutoff slope I think.
     
  14. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    I have moved on to the AF amp board (AWK-010) and have run into another mismatch of components from I got found in the service manual. The manual lists a total of six 2.2uf 25v caps, but my board has four of those and two of them as 1uf 25v. I can't really tell which ones they are since the layout page in the manual pretty much unreadable.

    Should I proceed with the 2.2uf or go with 1uf? I don't have any 1uf low noise parts on hand, but I do have some film caps and am inclined to use them.

    Yes, no?

    Note that this board is double sided, the first I've run across in my, admittedly limited, time doing refurbs.
     
  15. ivandezande

    ivandezande Super Member

    Messages:
    1,703
    Location:
    Twin Cities
    Stick with what was in there. If just DC signal coupling caps 2.2 wouldn't hurt, but eh.
     
  16. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    Went with the film caps.

    Moved on to the Head Amp (W21-002) this afternoon, which is really a preamp for the phono and radio sections. Due to pilot error when creating the BOM, I spent most of the day trying to figure out why it didn't work. Finally found the problem an hour ago.
     

     

    Please register to disable this ad.

  17. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    The electronic portion of the restoration is complete so I have moved on to refinishing of the veneered case. It's in pretty good shape, so some sanding and staining should do the trick.

    Here are some photos (as promised) of the resto process, starting with how it looked when I got it.
    front view.jpg rear view pre.jpg front oblique pre.jpg front above view pre.jpg
     
  18. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    Looks like someone didn't use a coaster.
    top case damage.jpg
     
  19. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    And the inside.
     

    Attached Files:

  20. cuda440

    cuda440 AK Subscriber Subscriber

    Messages:
    181
    Location:
    San Jose, CA
    Now in no particular order, the boards.
    W21-002 pre.jpg W21-002 post.jpg
     
    dlucy likes this.

Share This Page