Pioneer SX-450 restoration

Discussion in 'Pioneer Audio' started by racer2086, Apr 13, 2018.

  1. racer2086

    racer2086 Member

    Messages:
    67
    Location:
    Tampa, FL
    Hi AK

    Was visiting my parents this weekend and rummaging around the attic for some carburetor parts when I spotted this SX-450 receiver. It used to run the sound in my dad's office back in the 80s. It was covered in dust and dirt from roof work. My dad still has the manual to it also :)

    Anyway, brought it home with me, cleaned it up, and it still works surprisingly. It did have the flickering lights on power on, so I cleaned the power switch as all the threads talk about. I cleaned all pots with D5/F5. It sounds pretty good! DC offset is 3mv on the right channel and 13 on the left.

    The biggest issue I have with it is woofer excursion and/or popping when turned on and when changing the function switch to or past phono.

    On power on, the "A" speakers appear to fully bottom out before rebounding as if I push the cones all the way in once. I'm afraid of damage to any speakers hooked up to A. I put it on B now, as some suggest online, but are there bad caps that would cause this?

    With the function switch, there's no noise going AM to FM. There's a pop going FM to phono and then past phono to Aux. Then a pop when going back to FM through each. I think there may be something wrong with the phono section? Because also when in phono, it is quiet, but if I turn the volume anywhere, only the left speaker crackles/scratchy (and only in phono!). If I turn the balance knob left, everything is silent, but turning the balance knob anywhere right causes noise in the left speaker. ??

    Can any of you guide me on how to take care of the popping on power on and when changing functions so I don't damage my speakers? The scratchy phono stuff is less important to me since I don't have a turntable. I just want to know that if I use this thing it won't damage whatever is hooked up to it.

    Thanks!
     

     

    Please register to disable this ad.

  2. ivandezande

    ivandezande Super Member

    Messages:
    1,593
    Location:
    Twin Cities
    Last edited: Apr 13, 2018
  3. paul_ral

    paul_ral Active Member

    Messages:
    258
    After recap, transistor replacement and controls cleaning no pop, hiss, hum in my SX-440
     
  4. racer2086

    racer2086 Member

    Messages:
    67
    Location:
    Tampa, FL
    Hmm ok so it likely needs a recap and maybe new transistors... Are we talking recap in the amp section or everywhere?

    Also, does anyone have e a part number for bulbs to replace the power and stereo lights? Those both appear out.

    Thanks for the help! Listening to it now...sounds good :)
     
  5. paul_ral

    paul_ral Active Member

    Messages:
    258
    I did a full recap.
     
  6. mbz

    mbz AK Subscriber Subscriber

    Messages:
    2,026
    Location:
    Melbourne, Victoria
    Recapping "might" fix the poping/dc thump at power on.
    Do you also get a pop/thump at power off? If no then I'd wait on the recapping, doubtful
    it would be main filters. Have not looked at the poping when switching functions.

    First up, it's a vintage piece and "should" be refurb'd if you have diy skills.
    This includes a recap, replace known problematic transistors and other bits
    (bias diodes? trimmers, out of spec resistors...)

    Poping can be caused by a mismatch in startup between the positive and negative
    voltage rails. This inbalance causes a few volts on the speaker line. The root
    cause can be out of spec main filters where one has drifted low and consequently
    charges up a lot quicker than the other. The mismatch may also be cause by a rails
    resistor that has drifted high or faulty/dying components in the regulated supply.
    A pop can occur at power off when one voltage rail collapses quicker than the other
    again due to an out of spec cap...

    If you have diy skills and want to learn then do the recap. It's an entry level
    piece so I wouldn't be paying someone huge $$$$'s to do the work. The power ON/OFF
    pops can be muted with a protection circuit, the chinese will sell you one for
    about $10-, probably need a 12V transformer to power it, another $10-, and a fuse
    and some wire.
    https://www.ebay.com.au/itm/2-chann...D-assembled-/121317580107?hash=item1c3f175d4b
    plenty of others to choose from
     

     

    Please register to disable this ad.

  7. Watthour

    Watthour Electron Rancher - JS3600

    There is no protection relay on the SX-450. I have several of them, as well as it's bigger brother the SX-550. They all perform the "speaker cone excursion" on power-up and on power-off as you describe, creating a mild thump. One of them I had new in 1977. It did this right out of the box and has done it ever since. They all do it to some extent. It's just the design of the unit.

    Popping is a different matter. Clean switches are critical. Drifting cap values can contribute to this.

    The noisy Phono function is potentially the failing of the 2SA725s in the preamp section. You'll want to replace all eight of them in the 450.
     
    AJD1964 likes this.
  8. Watthour

    Watthour Electron Rancher - JS3600

    Incidentally, the Power lamp and Stereo indicator lamps are both the typical 8V/50mA 0.2" lamp. You can substitute a JTL9P lamp (8v/35mA) or get the correct lamps form Dwojo.com
     
  9. mbz

    mbz AK Subscriber Subscriber

    Messages:
    2,026
    Location:
    Melbourne, Victoria
    Certainly new items can exhibit this behaviour one cause
    being uneven loading of the pos/neg power rails as a cost
    saving measure during design, eg, the neg rail might feed
    the preamp while the corresponding pos rail feeds the preamp,
    phono and tuner so higher loading, longer time to charge up
    (not saying this is the case for the SX-450)
     
  10. racer2086

    racer2086 Member

    Messages:
    67
    Location:
    Tampa, FL
    No popping on power off. I realized that the function switch has contacts on BOTH sides of each of the two boards. I cleaned the sides of the boards I could see from the back of the unit, but probably have to get back in there and clean the other sides that are harder to see. Hopefully that's all it needs.

    The phono section may be the transistors you mention. I could definitely DIY this, and it's a great unit to do it on, but too many projects going on at the moment. I would like to at least get the bulbs. Thanks for the part numbers!

    Cool little unit this is. Love the clean looks. Nice and simple design, but with clean sound and decent headroom on the 15wpc
     
  11. mbz

    mbz AK Subscriber Subscriber

    Messages:
    2,026
    Location:
    Melbourne, Victoria
    Another long shot would be checking/replacing C217,8 in the phono stage. Maybe they are leaking some dc volts.
    You could try measuring dc voltage at R225(150k) and C217 node. Repeat for R226 and C218 node.
     

     

    Please register to disable this ad.

Share This Page