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HH Scott 342-B/13

Discussion in 'Solid State' started by paul_ral, May 16, 2018.

  1. paul_ral

    paul_ral Active Member

    Messages:
    258
    I found a rare (in my country) Scott 342-B/13 who came from Germany (from scrap I believe). No sound from right channel, damaged dial housing and broken dial string mount, hum on working channel. But he fights to survive:
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    Outputs transistors and non-original fuse holder on right channel
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    After a basic cleaning of balance switch I found the right channel is working again. With hum and scratches but is working.
    This is a good base for a new restoring project.
     
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  2. NAD80

    NAD80 Super Member

    Messages:
    2,230
    Location:
    Carlisle PA
  3. paul_ral

    paul_ral Active Member

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    258
  4. paul_ral

    paul_ral Active Member

    Messages:
    258
    I give the answer. In the 342 SM emitter resistors are rated at 2W. In 342B SM these are for 4W. I don t know if is a mistake or not.
     
  5. paul_ral

    paul_ral Active Member

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    258
    Waiting for Mouser order
    Power supply board
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    Tuner section: FM I.F. Amplifier Z-PC-IF-4
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    Tuner section: Automatic multiplex demodulator Z-PC-MX14
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    Amplifier board
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    I replaced old ROE 50uF/35V caps in order to reduce hum on right channel but nothing happens. These old caps have around 75 to 80uF.
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  6. paul_ral

    paul_ral Active Member

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    258
    Preparing for recap. Old Callins filter capacitor 1000uF/50V
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    Coupling capacitor 1000uF/30V
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    New caps
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  7. tcdriver

    tcdriver AK Subscriber Subscriber

    Messages:
    5,336
    Location:
    Valley of Heart's Delight
    You are looking good. :thumbsup:
     
  8. paul_ral

    paul_ral Active Member

    Messages:
    258
    Thank you, I read your thread, very good for me. I ll ask you about BIAS adjustment, as SM said is a bit complicated. I should measure voltages across R9 (3.3ohms) at 0.8mA to get 26.4mV?
     
  9. dly66

    dly66 Active Member

    Messages:
    255
    Location:
    Greensboro, NC
    Good work! Good to see someone else saving an old HH Scott. Just got my 342-C back together after the latest round of fix attempts and now it seems to be working really well. It had blown outputs on one channel and not much seemed to be working right when I got it. It has taken a few weeks to diagnose its problems and get them straightened out along with recapping it. Surprised how good it sounds now with my KLH 17s. Nice sounding tuner section. Much better than I expected. It seems like they should get more love than they do. The 342-C came after the B and is a more conventional design and is not cap coupled. Hopefully your 342-B will sound great when done.

    I don't know how the B model is, tcdriver should have knowledge on your model, but on the C I modified how they had the tone board, preamp, and selector knob grounds run and it reduced sizable hum down to a low level. It also took several deoxit cleaning rounds on the controls, particularly the volume control.

    IMG_20180518_232747.jpg
     
  10. dly66

    dly66 Active Member

    Messages:
    255
    Location:
    Greensboro, NC
    Sometimes lack of signal and noise problems requires hunting for bad solder joints or a broken wire or other broken joint. A lot of times they are not obvious and it takes some hunting to find them. On the C a previous owner installed a replacement power switch and destroyed a pair of resistors in the signal path for everything but the tape monitor. Those resistors were soldered to the selector control that took some time to figure out and see the problem. They soldered a number of things to the side of the selector control. Also I had a sizable channel volume mismatch but when I put the receiver on top of my Yamaha amp and tuner to hook up the the KLHs, the top was off, and at that height I looked across and noticed on one amp card a diode they have about 1/2" above the board had been pushed over against the side of one of the transistor sockets catching one of the leads that solder the socket down. I took a pair of pliers and re-positioned the diode and when I fired it up the channel volume mismatch was gone. Little things like that are easily overlooked and can be the cause of problems.
     
  11. paul_ral

    paul_ral Active Member

    Messages:
    258
    The old filter and coupling caps have been removed, new caps are inside
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  12. paul_ral

    paul_ral Active Member

    Messages:
    258
    PS board. All capacitors and carbon resistors will be replaced with new ones.
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  13. paul_ral

    paul_ral Active Member

    Messages:
    258
    Working on PS board
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    Voila!
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    C401 100uF/100V Illinois Capacitor TMA series
    C402 220uF/63V Nichicon TVX
    C404 100uF/50V Nichicon PW
    C405 22uF/63V Nichicon TVX
    I also replaced all carbon resistors with metal film 2% ones.
     
  14. paul_ral

    paul_ral Active Member

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    258
    In amp board I used a mix of Nichicon: KL in signal path, ES in preamp section and KA&Muse for the others increasing capacitance.
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    I measured all transistors and I found big differences (from 101 to 201) in Hfe for small signal QA-14 4700. I replaced them with matched 1% MPSA06. No hum or noise after.
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  15. paul_ral

    paul_ral Active Member

    Messages:
    258
    With new trimpots I set BIAS in order to have 27mV on emitter resistors. Many thanks to Larry and tcdriver for helping me.
    The sound is very good now, I got a big audible difference.
    Today I recapped tuner section, the FM IF amplifier was first. Only one electrolytic here: i use 25uF/25V Vishay/Sprague TVA1205-E3
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    Next step was the demodulator. 4 capacitors here: 2 Nichicon KL and 2 Nichicon TVX series according to pcb layout
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    The tuner works great.
     
    Last edited: May 26, 2018
  16. dly66

    dly66 Active Member

    Messages:
    255
    Location:
    Greensboro, NC
    Looking good :thumbsup:. I see you have a furry helper.
     

     

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  17. paul_ral

    paul_ral Active Member

    Messages:
    258
    She is Leda and she likes smelling inside old units and searching in Mouser boxes.
     

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