SX-636 hiss, crackle, pop!

Discussion in 'Pioneer Audio' started by cambronne71, Jun 2, 2011.

  1. cambronne71

    cambronne71 New Member

    new to the forum and done some reading and browsing and ready to ask for some solid help here.

    my SX-636 has a constant hiss with snaps and pops (sometimes louder than others) at ALL time in the LEFT channel (regardless of speaker combo etc..)whether an audio signal is present or not.

    the person I bought this from had it serviced last year (paperwork) and an OT was replaced.

    I DO have the cluster of blue electrolytic caps on the left side.

    I am comfortable using a multi-meter and soldering.

    any help is appreciated. I can supply any other necessary info... thank you kindly!
  2. compucat

    compucat Live and in color

    I used to have an SX-636 back in the eighties that developed similar symptoms in the left channel. It would have a crackle and static almost like thunderstorm static on am radio. Eventually the left side went out completely. The cause was defective transistors in the audio driver section. The transistors used silver plated leads and over time the silver tarnishes and the corrosion works its way up the lead and into the junction of the transistor causing the symptoms. These transistors are not expensive and there are two per channel. It is best to replace all four at once as they will all fail this eventually. The 636 is a great receiver and well worth keeping. Good luck.
  3. pioneervato

    pioneervato Lunatic Member

    Have you done the usual cleaning of the pots and switches with DeOxit and a followup with Faderlube? Amazing what dirty contacts will do to the sound which usually fits your description. I would start there and go from there. Plenty of info on using DeOxit. Just do a search here on AK.
  4. larryderouin

    larryderouin AK Subscriber Subscriber

    Look for 2SA725, 2sa726, and 2sc1451's on your schematic and boards. The following lists two different transistors for the 636. Either may be there. But they won't be mixed. It's one or the other. If it has 725, and 726's REPLACE ALL OF THEM. These transistors are problematic and cause these symptoms. I'm with Mark on the 763's. I have a unit with them and haven't had problems with it. but every unit that had 725,726, 1451's I've had to replace them to get rid of the crackling, pops, etc.

    Taken from another recent 636 thread and parsed.

    # in parenthesis is the board Number. The new transistor numbers are Mouser #'s.

    (AWH-034)Q5+Q6 2SC1451 = KSC3503's, 512-KSC3503ESTU to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe

    TONE AMP BOARD (AWE-046-0)
    Q8 Transistor 2SA763P-5 or 6,(2SA725-F or G)
    Q9 Transistor 2SA763P-5 or 6,(2SA725-F or G)

    AF Amp BOARD (AWK-035-0)
    Q1 Transistor 2SA726-F or G
    Q2 Transistor 2SA726-F or G

    PA Amp (AWH-034-0)
    Q1 Transistor 2SA763F-6 or 5,(2SA726-F or G)
    Q2 Transistor 2SA763F-6 or 5,(2SA726-F or G)
    Q3 Transistor 2SA763F-6 or 5,(2SA726-F or G)(2SA640-K or L)
    Q4 Transistor 2SA763F-6 or 5,(2SA726-F or G)(2SA640-K or L)

    Q1-Q4 2SA726 = 512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe

    2sc1312 use a KSC1845 This is the complement to the ksa992.

    2sa725 and 2sa726 use the ksa992

    2sc1451 use the ksc3503

  5. cambronne71

    cambronne71 New Member

    THANK YOU all very kindly for the great advice. I have kept my pots cleaned with Deoxit and suspect a more techinical problem. I will start in the direction you have guided me.

    is there a source for schematics here on AK?
  6. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

  7. cambronne71

    cambronne71 New Member


    another big THANK YOU to those who pointed me in the right direction!!! I was able to get the schematic/manuals. once I located everything and took inventory, I ordered new transistors. received them and set to work...I took my time and once I noted the pinouts/orientations of the new parts to the boards I had them swapped out in no time. I replaced ALL of them as suggested.

    if anyone else is having a similar problem, I can vouch for this fix!

    my unit now functions fantastic, all unwanted noise is GONE. I look forward to browsing the forums for more great advice!
  8. backmd

    backmd Active Member

    Intermittent L channel problem with -15 volts at the L speaker output

    Pioneer SX636 1-25-12 L output shows –25volts
    All power output transistors have large megohm resistances to the positive speaker outputs; ruling out blatant collector emitter shorts
    Outputs and PNP and NPN drivers are symmetric between the working R channel and the defective L channel.
    Did some reading; it could be the differential pair. Will pull outputs and measure voltages after testing the outputs
    Transistor outputs on the L channel as follows:
    Transistor HFE
    2SB531 75
    2SD371 23

    So the outputs are good!
    -25 volts at the speaker outputs with the outputs pulled!
    So the problem is likely upstream
    2SC1451 checks good with the diode tests
    2N3906, 2N2907, and 2N4403 are the generic radioshack PNP types
    Mysteriously everything works now and bias set at 20mv bilaterally
    1-29-12 –15 VOLTS at the L speaker output?

    Pulled outputs and still have the aberrant voltage.
    Emitters of Q1 and Q3 have –3.2 volts instead of 0.6 volts
    Inbetween the 3.3kohm resistors we have 9 volts instead of 14.4 volts
    -12volts instead of 1.2 volts at the base of 2SC1509 NPN driver
    -15 volts instead of –1.2 volts at base of 2SA777 PNP driver
  9. backmd

    backmd Active Member

    Mystery; please help

    B+ is 29 volts instead of 31 volts
    B- is –29 volts instead of –31 volts; so they are equal!
    B2+ is 31 volts instead of 30 volts
    B2- is –37 volts instead of –40 volts

    Pull the base lead of 2SA1451 class a PNP amp Q5; speaker output L side is still –21 volts; therefore predrive differential amp circuit is defective? Whoops the Pioneer OCL amp tech guide circuit is not the same as my Pioneer SX 636 circuit
    2SC1451 checks OK on diode tests
    Last edited: Jan 29, 2012
  10. markthefixer

    markthefixer ESCAPED:armed & dangerous Subscriber

    it does NOT matter if the 2sc1451's test good, GET RID OF THEM.
    also use the ksa992 and it's complement ksc1845 for low level replacements and not that RS "stuff".

    "regulated" voltages on a power supply are acceptable if they are within ten percent.

    i cannot concentrate more deeply tonight.
  11. backmd

    backmd Active Member

    Thanks Mark

    Sorry I put the order in already for the diffential pair and MCM electronics will have my NTE 234 in quantity of ten by Wednesday and I will match the HFE as close as possible. Pulled the stock differential pair and they were off by 20 percent. Could a variance of HFE of 20 percent cause the aforementioned problem? I will also get a new 2SC1451 or equivalent. Got a Frey's Electronics that has the 2SC1451 NTE equivalent off the shelf. Thanks for all the advice from the gurus ie Mark the fixer. PS out of curiousity, did you take circuit analysis from Timothy Trick at the University of
    Illinois ie EE101? It is quite obvious that you are are an electrical engineer with at least a bachelors and likely a masters or Phd.
    Last edited: Jan 31, 2012
  12. backmd

    backmd Active Member

    Low level differential PNP matched pairs

    I googled the above fairchild PNP transistor and these would obviously be for the differential pair of PNP transistors Q1 and Q3. In the future I will choose Fairchild over generic NTEs.
  13. backmd

    backmd Active Member

    2sc1451 leaky junctions and noisy sputtering

    Thanks a ton Mark for pointing me in the correct direction. I did some extensive reading ie 5 hours worth on Audiokarma about the interesting failure mode of the 2SC1451. Evidently they are famous for failing and taking out entire channels with them. The abnormal voltages can be explained by this single high voltage NPN transistor. Too bad TriState electronics does not have them in stock and I will have to wait for the mail man and USPS. But at least this is making sense!
  14. backmd

    backmd Active Member

    Comparing the continuous collector current of 2scs1451 vs. NTE2501

    The NTE 2501 video transistor and the 2SC1451 specs are similar in the 300 volt emitter collector absolute one time maximum. However the collector current continuous maximum is 500ma in the 2sc1451 (NTE replacement NTE287) and only 100ma in the NTE 2501. Would this be a concern in the sx 636 application?
    Addendum: Question answered, as the old 1981 Japanese spec sheet shows that the 2SC1451 has absolute one time maximum collector current of ONLY 50ma. So the NTE 2501 should be appropriate as carefully selected by Mark the fixer!
    Last edited: Feb 1, 2012
  15. backmd

    backmd Active Member

    Close to finished!

    2-1-12 2pm
    Hfe of is 2SC1451 is 78! However it is EXCISED AND REMOVED!
    Soldered the NTE2501 after enlarging the holes for a custom fit.
    Reinstalled the Q1 and Q3 transistors to see if problem solved or not.
    Problem solved the DC offset is 12mv and STABLE!

    Will install the output transistors now that the drivers have 12mv offset and sound great through headphones!
  16. backmd

    backmd Active Member

    Thank you Mark the fixer

    Well the DC offset is 13mv and STEADY, after installing the output transistors, checking for shorts and listening through headphones.
    Thanks to Mark the fixer; since the transistor 2SC1451 HFE was 78 I would never have known that this transistor was bad!

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