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Technics SU-X102 Amp Repair

Discussion in 'Troubleshooting and Repair' started by DaveW_nz, Dec 27, 2016.

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  1. DaveW_nz

    DaveW_nz

    4
    0
    Dec 27, 2016
    Hi All

    I have a Technics Midi system from the early 1990's (all that remains of my first paycheck after graduation) currently sitting on my bench. It started playing up a while back, I took it down to a service agent but he advised that it wasn't worth his time to fault find and repair so it has gathered dust under the desk but now is the time to give it another look.

    The protection cut out is tripping the speakers, in some cases as soon as it is turned on while in others it runs for a couple of hours before failure. Yesterday I spent most of the afternoon rocking to the 80's at 50%+ volume no issues before tripping.

    Turning the volume to zero, turn power on, slowly increase volume, if speakers 'Pop' and protection trips, turn off and try again.

    I have followed the Service Manual instructions and disassembled the unit on the bench so it can still be used.
    http://elektrotanya.com/technics_su-x102_sm_1.pdf/download.html

    My reading on the internet has indicated that Caps or Dry Joints tend to be an issue with this model. The main board looks clean, no discoloration or leaking Caps, the through whole solder joints appear secure.

    Looking through the schematic I have traced a few of the voltage lines and the only thing I noticed was the that higher voltage lines to the Power Amp look low;

    Pin Spec As Read
    1 0v 0v
    2 38.7 37.3
    3 -39.4 -37.5
    4 0
    5 14.2 13.3 (starts at 14.2 but voltage drops away as reading )
    6 0
    7 4.4 4.3
    8 -14.8 -14.3
    9 0 0

    I have also found that as I read the 0V signal pins (10, 11 & 13, 14) I get an interference buzz except reading pin 14 trips the protection every time.

    I am open to any and all suggestions on how I should best fault find to resolution.

    Regards
    Dave
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,219
    2,201
    Nov 17, 2011
    Electrolytics don't have to leak to loose capacitance. What masuring equipment do you have acess to? An oscilloscope maybe? That would be ideal to look at the DC supply voltages and check them for AC ripple voltage. A high ripple voltage is s sure sign of defect capacitors. Alternatively use your multimeter in AC range and check the DC supplies for AC ripple.
    Another option: preemptively replace the electrolytic capacitors in the powre supply by new ones. This will not hurt the amp (only your budget ;)).
    The service manual indicates there should be a fan. Is it there? If so, does it work? If not, overheating could also trigger the protection circuit.
     
  3. DaveW_nz

    DaveW_nz

    4
    0
    Dec 27, 2016
    Hi Harald, Thanks for your reply.

    Here is the actual board, no fan in this model. I think the 102-EX has the fan.
    power board.jpg
    The oscilloscope is showing about 150mV of ripple on the 37v DC lines
    Screen Trace.jpg

    Using the multi-meter I am seeing the following AC on each DC line
    Negative Line AC.jpg negative line DC.jpg


    Positive line AC.jpg Positive line DC.jpg

    Looking at the regulated lines 5v & 14.8v look clean.

    Would you say it is the big blue 3300μF Caps on the power supply? Would they also explain the lower than expected voltages?

    Regards
    Dave
     
  4. DaveW_nz

    DaveW_nz

    4
    0
    Dec 27, 2016
  5. DaveW_nz

    DaveW_nz

    4
    0
    Dec 27, 2016
    After checking on the internet the consensus was that for power filter Caps an increase in capacity is ok so I have now replaced those big caps. The DC voltage has not changes but the AC ripple has dropped, not as much as I would have expected, 0.040 on the negative line and 0.024 on the positive line.

    The oscilloscope is now showing under 100mv of AC ripple.

    While reassembling it I 'Pop'ed the protection & noticed that it could be repeated via a small tap to the front panel.

    A visual inspection showed what looked to be a dry joint on the Balance Pot. I reflowed solder over the Balance, Bass, & treble pots.

    Taping and knocking the boards now does not cause the protection to trip, a qualified fix it appears!

    Will reassemble and leave running on the bench to confirm.

    Thanks
    Dave
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    10,219
    2,201
    Nov 17, 2011
    That's good, not excessive. The caps should be ok.

    A good find. I'll keep my fingers crossed for you.
     
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