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

Pioneer SX-750 hum issue repair???

Discussion in 'Pioneer Audio' started by DirtyDog, Dec 6, 2018 at 5:25 PM.

  1. DirtyDog

    DirtyDog New Member

    Messages:
    26
    I just recently purchased a Pioneer SX-5560 (SX-750 in black) and it has the hum issue that seems to be fairly common in these models. After reading some threads it looks like there are some capacitors that go bad and need to be replaced in particular C17 & C18.

    On ebay I found a listing for a Pioneer SX-750 hum repair kit. Link below:

    https://www.ebay.com/itm/Pioneer-SX-750-Hum-Repair-Kit-Capacitor-Upgrade-High-Quality-Receiver-Recap-Set/113423664131?hash=item1a6893c403:g:AxMAAOSw9g1cBu9N:rk:1:pf:

    Do you guys think its worth it? How difficult would it be to install the kit? Is there an easy way to get to the underside of the power board without disconnecting a lot of wire connections?

    [​IMG]
     

     

    Please register to disable this ad.

  2. Watthour

    Watthour Electron Rancher - JS3600

    I'd bet a nickel that the power supply caps are failing. Most of the 750s I've been through had already developed hum or were on their way, not to mention other supply-related problems they can exhibit.

    The "kit" on eBay may be a good start, but may not be ideal. Get the recommended appropriate replacements from a reputable distributor, along with a few other minor items, and repair it properly. To do it properly, you will need thermal paste, probably some mica washers, and potentially other supplies and hardware. You may also need both rectifier and Zener diodes, which don't appear to be in the kit. The price for that "kit" is not unreasonable, but the two larger caps included really need to be different from what is supplied. If you have doubts, the following is why we change things such as this.

    The unit was designed to operate at a nominal 117VAC when it was sold (KC/KU types). The power supply that is filtered by those 330µF/50VDC caps was originally 48V. As if that were not already dangerously close to the WV rating of the caps, most utilities have been increasing voltage over the years to help stretch the distribution systems. 125VAC is very common today. If you do the simple math, that 48VDC circuit is already probably at or over 50V (51+). They need to be 63V caps at a minimum, and bumping them to 470µF is not a bad idea.

    Similarly, experience has proven that the original protection relay driver is also a bit too close to its ratings for reasonable reliability. That needs to be replaced with a slightly more robust device.

    The regulator for the 37V supply operates hot, and can benefit greatly from a clip- or bolt-on heat sink, something which doesn't appear to be included.

    If you research the threads on SX-750s you'll likely find a lot more useful information. For about the same cost, a complete set of parts plus some useful additional pieces can be obtained.
     
  3. Watthour

    Watthour Electron Rancher - JS3600

    As for the ease of access, the board only needs to be removed from its mountings, It can be swung up and into full exposure without disturbing any wrap connections.

    [​IMG]
     
  4. Watthour

    Watthour Electron Rancher - JS3600

    Incidentally, if you zoom in on that photo, you can see about halfway across the bottom of the board a solder pad which has begun to disconnect itself. I believe this should be sufficient evidence that Q12 can use a little heat added sink.

    Similarly, if you want to keep the unit in good working order, this may be an ideal time to address the power switch issue. For a small expenditure, the power switch can be made to last nearly indefinitely.
     
    Last edited: Dec 6, 2018 at 9:20 PM
  5. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,662
    Location:
    Milwaukee, WI
    Just recap the power supply and the hum will go away.
     
  6. DirtyDog

    DirtyDog New Member

    Messages:
    26
    Thank you. Now that is exactly what I wanted to know and the picture makes it perfectly clear. :)
     

     

    Please register to disable this ad.

  7. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,662
    Location:
    Milwaukee, WI
    I just recapped a pair of 750s. I used the BOM here, which calls for 330 uf 50v caps. I’ll take a look at the one I still have open and see what the voltage is on them.
     
    Last edited: Dec 6, 2018 at 9:29 PM
  8. Watthour

    Watthour Electron Rancher - JS3600

    Originals are indeed 330/50. No service factor at all.
     
  9. Watthour

    Watthour Electron Rancher - JS3600

    Sounds like another nickel bet on the caps weakening.
     
  10. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,662
    Location:
    Milwaukee, WI
    i swear half the 750s out there hum. It’s always the ps caps.
     
    Last edited: Dec 6, 2018 at 9:45 PM
  11. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,662
    Location:
    Milwaukee, WI
    Well that was interesting. I fired up my 750 on a dimbulb tester with a 100 W bulb in it. On the dim bulb tester I only got 43 V on each capacitor. However, when I took it off the damn ball and plug it straight into an outlet, I got a nice solid 50 V per cap, which is bad.

    So I went to replace the 330 uf caps with 470 at 63V, but all I had were Panasonic FC caps and the wires on those are so thick that they barely would fit through the circuit board holes. I had to jam them in. I will have to order some Nichicon caps with thinner wires.
     

     

    Please register to disable this ad.

  12. tsd71

    tsd71 RIP Tom Petty

    Messages:
    2,569
    Location:
    New Jersey
    Jamming leads in is not a good idea you can rip the solder pad right off the board then you have more work. Take an exacto knife and slowly bore out the hole slightly on both sides to get the leg to fit properly. If you get 470uf 63v caps the legs on those could be thicker.
     
  13. Watthour

    Watthour Electron Rancher - JS3600

    They are indeed thicker. As much as I don't like low-buck equipment, I've found that a cheapo "micro" rotary tool kit is a handy way to safely open those holes. For about the cost of a real drill bit, you can get one of these kits and work your way up to the correct hole size:

    https://www.harborfreight.com/rotary-tool-kit-80-pc-63235.html

    I've encountered the same situation when refitting transistors, such as using a KSA992 to replace all those 2SA7-From Hell devices. Same solution.
     

Share This Page