SX-3700 Recap/Restore Thread

Discussion in 'Pioneer Audio' started by ohgoshjosh, Mar 11, 2018.

  1. ohgoshjosh

    ohgoshjosh New Member

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    First time posting, short time lurking. Found a lot of very helpful info to repair and recap my SX-3700. This all started when I noticed the speaker protection relay loudly clicking, and eventually failing completely to push power to the speakers. I thought replacing that relay would have done the trick, but if I had read these forums more closely, I would have found out my problem likely lies elsewhere.

    That brings me here, where I'm going to attempt to recap the Power Amp, document my process, and hopefully learn a few things in the process. I haven't seen a ton of SX-3700 recap threads on AK, so I hope this might be helpful to others in the future. I've read the 3700 Service Manual and the Tuning Fork guide on Pioneer caps.

    First, here's a draft of my cap list. I'd appreciate any feedback folks have here. I'm a bit unclear about the difference between Nichicon caps that end with 1TD,1TA vs no suffix after M[x]D.

    2x ACH-215; 8200uF/50VDC; 647-UKW1H103MRD
    2x ACG-004; 0.01uF/150VDC;
    2x CEA R47M 50L; 0.47uF/50VDC; 647-UKW1HR47MDD
    3x CEA 010M 50L; 1uF/50VDC; 647-UPW1H010MDD1TD
    1x CEA 470M 10L; 47uF/10VDC; 647-UPW1A470MDD
    1x CEA 470M 25L; 47uF/25VDC; 647-UPW1E470MDD
    3x CEA 470M 50L; 47uF/50VDC; 647-UPW1H470MED
    3x CEA 101M 50L; 100uF/50VDC; 647-UPW1H101MPD
    1x CEA 221M 35L; 220uF/35VDC; 647-UPW1V221MPD
    1x CEA 221M 50L; 220uF/50VDC; 647-UPW1H221MPD
    1x CEA 331M 10L; 330uF/10VDC; 647-UPW1A331MPD
    2x CEANL 100M 50; 10uF/50VDC; 647-UKL1H100MDDANA
    2x CEANL 2R2M 50; 2.2uF/50VDC; 647-UKL1H2R2MDDANA
    2x CEANL 4R7M 50; 4.7uF/50VDC; 647-UKL1H4R7MDDANA
    2x CEA 4R7M 50L; 4.7uF/50VDC; 647-UPW1H4R7MDD
    2x CEA 100M 50L; 10uF/50VDC; 647-UPW1H100MDD
    2x CEA 220M 25L; 22uF/25VDC; 647-UPW1E220MDD
    2x CEA 101M 35L; 100uF/35VDC; 647-UPW1V101MPD
     
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  2. jbrainey

    jbrainey ticking away the moments that make up a dull day Subscriber

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    Good luck in this endeavor and welcome to AK.
     
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  3. quaddriver

    quaddriver 120 What's per channel Subscriber

    replacing these 38 year old caps with just about any modern caps will be an improvement, for the caps, but its likely not your problem.

    the relay unclicked because you either have and overload or big DC, something broke. but its fixable. the manual might be here, but it is on hi fi engine. you will need it for reference. our resident pioneer sultan may be showing up shortly to tell you exactly what broke, but if it were mine, I like to isolate the outputs, then go and verify all the different +/- voltages from the power supply area. if by any chance you disable the outputs and it clicks and stays clicked...that points to something..
     
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  4. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    As Mark (The Sultan) would say "Danger Will Robinson!!!!!"
    The SX-3700 is an NSA amp design and is a very tricky. High frequency transistors that are hard to find, although not impossible.
    "Quaddriver" is right - the caps won't fix it.
    It may only need an adjustment to the amps. Start with the set on the DBT and read the service manual, page 19 for the procedure.
    Ask questions and don't attempt any adjustments. Record only and report.
    What's a DBT?
    http://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build-and-how-to.808399/
    Build the DBT before you do anything else.
    Use micro grabbers for the DMM leads. Never remove or install the DMM leads when the set is on.
    Black probe to the chassis, red probe to the amp test pin(s). DC voltage, auto range is fine. Readings will be in the millivolt range.
     
    Last edited: Mar 11, 2018
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  5. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Also the power supply has some transistors that like to unsolder themselves.
    Probably look at the back of the PS before moving ahead.
     
  6. MBuras

    MBuras Restoration is an Addiction Subscriber

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    All good advice above. +1

    I can address this question. The 1TA, 1TD, etc suffixes are only on products that come on a tape applicator designed for machine insertion. Not that you can’t use them but the suffix doesn’t apply to your application.

    On the main filter caps I might tend to go up in voltage a little. You’re running at least 41v through those caps and 50v doesn’t give much margin. Higher voltage replacement will likely yield longer lifespan of the capacitor as it’s not running on the edge of spec. Another critical measurement is lead spacing. These filter caps are board mounted and need to be fairly accurate with the spacing or you risk awkward installation, leads bent too far, and bad mechanical connection.

    Enjoy the project!

    Mike
     
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  7. ohgoshjosh

    ohgoshjosh New Member

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    Thanks to everyone for getting me going here. Went out and built a DBT after work today. Using a 75W bulb, it’s very dimly lit when power is on. Following the procedure on Page 19 (8.3 Idle Current Adjustment), here’s what I’ve got:

    Per the instructions on p. 19, between the test pins:
    TP48-49 (L ch) 14.7mV (fluctuating between 5-14.7mV)
    TP47-46 (R ch) 2.2mV

    Test pins to chassis:
    TP48: 6mV
    TP49: 20mV
    TP47: 36mV
    TP48: 34mV

    Visually inspected the pack of the PS. Didn’t see any alarming solder joints.

    One more thing I observed: When the unit was last plugged into AC, I would often hear a subtle “vibration” noise coming from the speaker protection relay. Now that it’s plugged into the DBT, I don’t hear it at all anymore. Is that because the DBT is taking up that power that was formerly making it to the relay?
     
    Last edited: Mar 12, 2018
  8. ohgoshjosh

    ohgoshjosh New Member

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    Mike, thanks for your recommendations about the caps here. Once I can figure out the power issue, I’ll return to the caps. The lead spacing is something I had not considered. I guess I thought these were of some “standard” sizes. Should have known better!
     
  9. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Pull the jumpers from the back of the receiver and re read the idle millivolts once again.
    Look for the white jumper and report whether the jumper is cut or not. Both channels.
    Just do the check in the service manual.
    One more time.
    You want a fairly stable reading between those pins.
    Per the instructions on p. 19, between the test pins:
    TP48-49 (L ch) ?
    TP47-46 (R ch) ?
     
  10. ohgoshjosh

    ohgoshjosh New Member

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    Didn’t see any white jumpers, but saw two red ones that look like they had already been cut (I didn’t cut them) — they look like the same ones in the manual — are they?

    Now both the power meter and the frequency displays on the front don’t light up anymore, but the bulbs on the tuner panel do. What did I do?
     
  11. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Its not in tuner mode?
    Red wire right next to the resistor R319, R318?
    The connectors for the outputs are very cheesy. Check for good contact and security.
    Would like to see the Idles with the jumpers removed.
    You can keep it on the DBT.
     
    Last edited: Mar 13, 2018

     

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  12. ohgoshjosh

    ohgoshjosh New Member

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    Even in tuner mode, it’s not lit. Neither is the watt meter.

    Yep, red wires right next to R319,R318.

    Idle voltages w/o jumpers:
    TP48-49: 14mV
    TP46-47: 2.2mV
     
  13. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Power supply readings?
     
  14. ohgoshjosh

    ohgoshjosh New Member

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    I’m going to have to ask how to take those readings, so that I don’t assume I know what I’m doing.
     
  15. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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  16. ohgoshjosh

    ohgoshjosh New Member

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    Here are the readings for those pins. The board diagram from the manual doesn’t exactly line up with the arrangement of the pins on the board. I couldn’t find a pin on that board labeled 18, 32, or 24.

    20: 18.35V
    22: 6.1V
    25: -20.2V
    26: -12.9V
    23: 12.8V
    24: No pin

    18: Cannot locate
    19: 0V
    21: 0V
    27: No pin (empty hole in board labeled 27)

    30: 25.9V
    31: 26.0V
    29: 26.1V
    32: Cannot locate
     

     

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  17. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Power supply is good.
    The Idles are OK.

    The DC balance needs to be checked. We should of looked at that earlier but the display turned our attention.
    The difficulty in checking it is the relay is not engaging. so checking at the speaker terminals wont give a reading. We have to locate a point between the amp board speaker output and the relay. I don't have a set here to look at.
    There's a Henry coil at the output of each amp labeled L1 for the left and L2 for the right. Can you grab at those points and check the DC Balance?
    Black probe to chassis and the red probe to the relay side of the coil. Set on the DBT. The reading should be close to zero volts.
     
    Last edited: Mar 14, 2018
  18. ohgoshjosh

    ohgoshjosh New Member

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    Yep, both at or around 0.2mV
     
  19. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Must be a protection issue.
    Everything is checking out.
    Although that right amp could use a little more Idle/bias. :dunno: 2.2mV's is right on the edge and no jumper to cut.
    Did you find a rebuild list for the protection circuit?
     
  20. ohgoshjosh

    ohgoshjosh New Member

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    The SPC and the Power Amp are combined in the Parts List in the service manual, so I’m making a guess based on the circuit diagram. Should I also be swapping out the resistors and diodes?

    Transistors
    Q21,Q22 (2SC1914A): 512-KSC2383YTA
    Q23,Q24 (2SC945A): 512-KSC945CYTA ** I got this from another AK post for part no. 2SC945, missing the “A” — what’s the difference? Seems like maybe the leads are switched around for “A” models?
    Q25 (2SC1384): 512-KSC2690AYS

    Caps
    C111,C112 CEA R47M 50L 647-UKW1HR47MDD
    C113 CEA 331M 10L 647-UPW1A331MPD
    C114 CEA 101M 50L 647-UPW1H101MPD
    C115 CEA 010M 50L 647-UPW1H010MDD1TD
    C116 CEA 470M 50L 647-UPW1H470MED
     

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