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Technics SU-Z2 acting strange...

Discussion in 'Solid State' started by MattiaM, Jun 13, 2018.

  1. MattiaM

    MattiaM New Member

    Messages:
    28
    Location:
    London
    I got a non working Technics SU-Z2 that I have been unsuccessfully trying to get back to work. I am not an expert and I have been self learning about electronics on books and forums, and trying to apply that knowledge hands on with a few amps I got from friends or at carboot sales.

    This one had a lot of problems, from a very loose connection to ground, to filter caps in the power supply that measured 2pF, to one of the germanium diodes in the VU meter circuit that was shorted, together with some bad soldering joints in the cables sending the AC to the meters.

    I replaced all the caps, checked all the diodes and transistors and got new ICs just in case. Diode checking the STK2038 from pins 1 and 2, 4 and 5, 12 and 13 and 15 and 16 (collector base according to a schematic I found here on the forum for the STK2038) all measured OK about 0.6 volts.

    So I decided to turn the amplifier on, and after about 5 seconds the meters go full scale, and then slowly go back down to 0. The STK2038 becomes hot quite quickly. I checked all the voltages and what I found is the following:

    Pin 1, 3, 5, 6, 11, 12, 14, and 16 of the STK2038 jump to about +22V or more when I turn the amplifier on, and then slowly drop to about -4/-6V. All other pins measure OK.

    Q401 and Q402 measure OK, whereas the base of Q403 and the collector of Q404 jump around 1.5V and never stabilize. The collectors of Q301 and Q302 behave as the pins of the STK2038 that do not measure right in their voltages (jumping to about +22V when power is applied and then slowly dropping to -4/-6V)

    Both AN7060 have the same behaviour for pins 1 and 2 and 4, pin 5 measures -20V instead of -6V, and 6 to 9 measure OK. I also have the right +-15V for pins 4 and 8 of the 4559DD, but all other pins also jump to 22V and then drop.

    I thought that maybe the problem was in the AN7060, so I replaced those, but no difference. I am basically stuck, and even replacing all the other ICs has made no difference. There is something I am obviously missing here, but I can't seem to be able to figure out what it is. Does anyone have any guesses?
     
    Last edited: Jun 14, 2018

     

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  2. SVI2004A

    SVI2004A Synchro Bias

    Messages:
    2,014
    Location:
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    Here’s a look of schematic of inside STK

    Check pins 2-3, 3-4, 13-14 & 14-15 for Collector-Emitter short/open

    EF5B528E-2E16-4FB2-B3FC-6BBDE8B9BED9.png

    I’m curious as pin 11 is protect ref for the channel that you say is fine.....

    It’s been a long time since I’ve had one of these on the bench... and I hazily remember these behaved strange when it was dismantled- check that all ground points have continuity to eachother.... when main pcb is screwed to frame, things were fine...

    Did you fit new STK?
     
  3. MattiaM

    MattiaM New Member

    Messages:
    28
    Location:
    London
    Thanks, I will check that later today.

    Yes, I installed the new STK, as well as new AN7060. I might have burnt them for other problems though, or I might have used fake AN7060 (I am confident that the STK is original as I got it from a usually very reputable source).
    To avoid problems with ground, I wired a second ground connection to the chassis (I removed that thick insulating paint that is all over the interior of the chassis where I established the new ground connection), from the ground pad where the screw goes.
    Both channels behave in the same way, and last time I checked, the amplifier actually worked (it amplified the signal I injected from a signal generator), even with those unusual voltages. If I remember correctly, I had about 12V DC at the speaker output; I will check that again later, but I would expect the same fluctuation there given that pins 3 and 14 go to the speaker outputs and the meters.

    Checking again, pin 4 of the AN7060 should be the ground, maybe I don't have ground continuity there as I measure there the wrong voltages. Could that explain that erratic behaviour? :idea:
     
    Last edited: Jun 14, 2018
  4. SVI2004A

    SVI2004A Synchro Bias

    Messages:
    2,014
    Location:
    Albany Western Australia
    Definitely... make sure pin 4 of them are grounded- this most likely is the result of no ground there
     
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  5. SVI2004A

    SVI2004A Synchro Bias

    Messages:
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    Location:
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    AN7060

    CBB5FC9C-EA4D-4B96-902B-98D1D904D133.png
     
  6. MattiaM

    MattiaM New Member

    Messages:
    28
    Location:
    London
    Update: it was indeed pin4 of the two AN7060 floating. All the right side of the board was floating, with all the inputs the phono stage and the two AN7060. I added another ground wire to connect the two sections and have better ground continuity, and now it works just fine.
    Once again, thanks for the help! :beerchug:
     

     

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  7. SVI2004A

    SVI2004A Synchro Bias

    Messages:
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    I remember grounding doing funnies with these... it was 1996 I think I last looked at one of these in the flesh...
     
  8. SVI2004A

    SVI2004A Synchro Bias

    Messages:
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    Location:
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    Glad to see you got it all going again... enjoy the music :music::music:
     
  9. MattiaM

    MattiaM New Member

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    Location:
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    Thanks! I have to say, I am not impressed, the stage isn't particularly wide and it could use a bit more clarity and control.
    Definitely fun restoring it though! :)
     
  10. SVI2004A

    SVI2004A Synchro Bias

    Messages:
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    Location:
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    I’d be going through it in detai... sounds like some things to resolve..

    In form these should have clarity... maybe check main filter caps - and tone board...
     
  11. MattiaM

    MattiaM New Member

    Messages:
    28
    Location:
    London
    After some more listening, I believe I was just setting the bar a bit too high, given that I use as a main amplifier in my system a Sony vfet.
    If the lack of clarity is an actual problem and not my impression switching from the Sony to the Technics, I don't think it comes from the caps as I replaced all the electrolytics in the audio path with Nichicon audio grade ones, and those in the power supply with Panasonic high ripple ones.
    Maybe generic STK ICs do not sound as good as the Sanyo ones? :idea: I can't tell whether the one I got is an original Sanyo or not.
     

     

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