Yard Sale SX-636

Discussion in 'Pioneer Audio' started by Motnick, Aug 12, 2017.

  1. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    153
    Location:
    Wyandotte Michigan
    Today the wife and I decided to hit up some local yard sales since the weather was so nice. Glad that we did, as I scored a nice SX-636 for a good price. I have been working on restoring a SX-737 and figured the SX-636 would be a nice addition to have.

    When I got it home I opened it up and gave all of the controls a cleaning with deoxit. I was surprised how clean this unit was inside! I powered it up on the DBT and checked the DC offset. Everything checked out ok, less than 10mV offset in both channels, so I went ahead and put it on full line power. After letting it warm up for 15 minutes or so I dialed in the bias to 20mV per the shop manuals instruction, and then hooked up some cheap test speakers.

    To my delight the amp played perfectly. I listened to it for about an hour or so and then the left channel started to crackle and pop. I hurried up and turned off the channel A speaker button, and powered down the unit for a closer look.

    Looking closer at the power amp board I can see that someone before me has already replaced the 2SC1451 transistors with two NTE 287 transistors. Nothing else looks to have been touched inside. Are these acceptable replacement parts? I know that on my SX-737 I replaced them with KSC3503, as advised by this forum.

    I cannot read the printing on the differential pair transistors Q1-Q4, so I don't know for sure if they were also replaced or not. I suspect that they were not replaced, and they look to have the 2SA726 pin layout even though the workshop manual suggests that they should be 2SA763. I have some KSA-992 transistors on hand. I was thinking of matching some up and using them to replace Q1-Q4.

    What do you guys think? Is replacing the differential pair transistors a good next step, or should the NTE transistors be changed out? Unfortunately, I do not have any more KSC3503 transistors on hand otherwise I think I would just change all 6.
     
    Last edited: Aug 12, 2017
  2. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,232
    Location:
    Victoria, Australia
    Congrats on your find, very nice line of receivers. It is a great next step and the KSA992's are a good sub. Good idea on getting those NTE's out of there also, but the input diff pair may cure it's current problem. Get those two little blue Sanyo caps out of there also, l think they may be 2.2uF, do you have any films in this value?

    [​IMG]
     
  3. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    153
    Location:
    Wyandotte Michigan
    Well, I was able to find 4 KSA992 transistors that had very close hFE measurements on my harbor freight multi meter.

    447
    448
    448
    449

    I went ahead and swapped them out. Now that I have the original transistors out of the PCB I can read what's left of the original marking on one of the transistors, and it says A726 meaning these are most likely the original transistors.

    I powered it back up on the DBT and all seems fine. Back on full line power I was able to adjust the bias back to 20mV in both channels. I also noticed that the heavy bass sound that the amp had before is gone now. There was no need for the loudness button before, but now it sounds more correct.

    DC offset is 9mV in the Left channel, and 12mV in the Right channel now.

    I think I'll go ahead and replace the NTE 287 transistors when I place my next Mouser order just to be safe. I will also check and see if I have any caps to replace the 2.2uF caps. If not I'll order those as well.
     
    Last edited: Aug 12, 2017
  4. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    153
    Location:
    Wyandotte Michigan
    The other issue that I have noticed with this amp is that the dial pointer light is out. Are these replaceable? All of the other lamps are working fine.
     
    Last edited: Aug 12, 2017
  5. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,232
    Location:
    Victoria, Australia
    Nice work, sounds like you have it sorted :thumbsup:.

    Yes you can replace these, l have done one in my SX-939 and a SX-737 l used to have. They are a grain of wheat globe, l can't remember the exact specs but it will probably be 8v. You are best to remove faceplate and carefully unclip the dial pointer from the tuning string and sliding surface. Then you can bend the little retaining tabs that hold the dial pointer in to the metal frame and access the globe. These pointers are reasonably fragile.

    [​IMG]
    [​IMG]
     
  6. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    153
    Location:
    Wyandotte Michigan
    Awesome! Those pictures were exactly what I needed.

    If I replace that bulb then this amp should be all set. I don't think it was played very much. The power supply shows no signs of heat, and it's as clean as a whistle inside. It is surprisingly powerful for 25W as well!! I'm listening at a comfortable level between 2 and 3 on the dial right now. By 4 it is getting too loud for this time of night.

    It will be interesting to see how this compares to my SX-737 once it's all restored/recapped.
     
    Last edited: Aug 13, 2017
  7. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    153
    Location:
    Wyandotte Michigan
    The garage sale lady told me that she also had had an SX-780 for sale, but that I had missed out on that one by just an hour or so! Oh well, I'm still happy to have found this one. What a good way to spend a Saturday afternoon!
     
    Last edited: Aug 12, 2017
  8. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,232
    Location:
    Victoria, Australia
    Good to hear you are enjoying it, sounds like a very clean unit.

    Yeah, will be interesting to compare the two although l suspect the SX-737 should sound noticeably better if you are restoring it.

    Ahh well, l think you got the better unit anyhow :).
     
  9. john stumpf

    john stumpf AK Subscriber Subscriber

    Messages:
    417
    Location:
    philippines
    considering the problems with 780 un-obtainium parts you probably are the winner in the long run with the 636!
     
  10. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    153
    Location:
    Wyandotte Michigan
    Here it is. It's been playing all morning.

    [​IMG]
     
    Outta Ammo and Oldsansui441 like this.
  11. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

    Messages:
    20,143
    Location:
    Glen Burnie Md.
    The 636 doesn't have a protection circuit in it, just the two fuses on the back. Just so you know, I would twist and tin any speaker cables you want to use, or get some speaker pin connector's that solder to the wires so you don't get single strands trying to play touchy-feely with the other polarity and have some high current fun which will cost you. It's a good unit, but it lacks 1 or 2 of the circuits of the Higher ended x3x series.
     
  12. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,232
    Location:
    Victoria, Australia
    Looks to be in very nice condition.
     
  13. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    153
    Location:
    Wyandotte Michigan
    I noticed this. I also noticed that my amp has two 20amp/32V fuses where the 4amp/125V fuses should be. I assume that's all the previous owner had handy. I went ahead and ordered some fuses of the proper rating as replacements.

    I played the amp all day. From about 11 this morning until about 7 tonight. Mostly FM, a couple of CD's, and also listened to the first quarter of the Lions game on AM.

    Since replacing the differential pair transistors the amp seems stable. I checked the bias a few times throughout the day and it was solid at 20mV. The offset stayed the same, around 9mV in the Left and 12mV in the right. I wonder if replacing the NTE 287 transistors would zero these out?
     
  14. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

    Messages:
    20,143
    Location:
    Glen Burnie Md.
    Might be cheaper just to drop in @ Home Depot's electrical Dept. They have glass fuses in the sizes you need. The voltage rating is 125/250V which isn't a factor here. The 32v fuses are for autos. Find a Pinto and stick them in the light circuit! Guaranteed Weenie roast at the next corner. :naughty::naughty::naughty:
     
  15. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    153
    Location:
    Wyandotte Michigan
    Good to know. I have been wondering about a good local source now that radio shack is gone from my area entirely.

    I had to order some caps and speaker termainals for the EPI 120B speakers I picked up a week or so back anyway, so I just added the fuses onto that order from Parts Express.

    I will give a look at HD next time I'm in there. Thanks for the tip!
     
  16. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    153
    Location:
    Wyandotte Michigan
    I recently placed an order with Mouser for parts to restore my SX-1010, and I thought I would add a few pieces to be able to finish up the power amp board in this receiver. This SX-636 has been playing fine since I replaced the differential pair transistors, but I thought I would go back and change out the NTE 287 transistors that someone before me had used to replace the 2SC1451's. I also picked up a couple of WIMA caps to replace the little blue Sanyo's as you suggested.

    On my power amp board the blue Sanyo's were .33uF/25V. I replaced them with 505-MKS20.33/63/5. Hopefully these are acceptable replacements? They fit well and they do look nice on the board. I used KSC3503 to replace the NTE 287 transistors.

    I think I'll order a replacement pointer bulb when I order bulbs for my 1010, and then this receiver should be all set. I'm not going to recap the entire unit since it is in such nice shape inside, and It sounds really good as is.

    [​IMG]
     
  17. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,232
    Location:
    Victoria, Australia
    Nice work, yes those films are a good sub and the KSC3503's are a good sub also. Don't know why l thought the little Sanyo's may have been 2.2uF as now that you mention .33uF that value sounds very familiar :). Sounds like you are good to go.
     
  18. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    153
    Location:
    Wyandotte Michigan
    I finally got around to replacing the dial pointer bulb. I also ordered some deep blue LED bulbs to replace the dial and meter bulbs. Im really happy with the way this turned out!

    I went back and used some faderlube on the pots that I had cleaned with deoxit, and I also cleaned the tuner with the CRC spray from Home Depot. The tuner is nice and quiet now when moving between stations.

    I am going to pass this receiver along to a friend who is getting interested in vintage audio. Hopefully it will do him well.

    [​IMG]
     
  19. Aaron99

    Aaron99 AK Subscriber Subscriber

    Messages:
    210
    Location:
    Vancouver WA.
    Very nice work.....

    Tell your friend.... "The first one is free".... We all know where it goes from there.
     
  20. Blue Shadow

    Blue Shadow I gotta get me a new title

    Messages:
    18,120
    Location:
    SE PA
    Always an excellent way to make the final decisions once you have selected the right electrical characteristics. I know I pick replacement caps by the color, liking those black and gold or gold and black Nichicons for the lytics and those Wimas are nice for a film cap.

    Nice work on that receiver and your friend should be happy with the unit for sure.
     

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