Technics SU-CH7 / Panasonic SC-CH7 amplifier issue

Discussion in 'General Electronics Chat' started by Alex_C, Jan 28, 2016.

  1. Alex_C

    Thread Starter New Member

    Jan 28, 2016
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    There are a few threads about the Technics version of this, all with a similar issue, but I think mine is in the early stages. I have the Panasonic version, bought recently second-hand. I think the two versions are electronically identical.

    It looks perfect, barely used. When I first switched it on it immediately switched itself off again, with a relay clicking sound. I fiddled with the interconnecting cables and it worked... but after leaving it overnight with mains power off, the same thing happened again. Here's what I've worked out:

    1. When the amp has been left off for a while (hours), it won't switch out of Standby mode until it's had mains power for about two minutes: almost as though it's charging up (capacitors?).
    2. It then works fine, but the display of the tuner module and to a lesser extent the amp module flicker at what could be AC frequency. The tape and CD module displays are rock solid.
    3. If left permanently connected to the mains, the problem is not apparent, though I think it will get worse and the amp will start cutting out as other users have described.
    4. The issue remains even if just the amp and tuner are connected together; no speakers, tape deck or CD. It's not possible to isolate the amp entirely as the tuner holds the power button to get out of standby.
    5. Even in standby mode the amp is quite warm, so much so that I wouldn't use this as an alarm clock due to waste of power. That might just be 1992 engineering, but might be due to the fault.
    6. There is a mild burning smell. Initially I thought that might just be dust, though I have cleaned the unit. Now I think otherwise...

    See photos attached (the fourth is of the tuner), in particular the burn areas. It's perhaps not coincidental that these seem to be around diodes.

    Amp power amp burn.jpg Amp PSU.jpg Amp PSU burn.jpg Tuner PSU.jpg

    So, capacitor failure in the rectifier circuit? Or something else? I've ordered replacement capacitors for the larger ones because they're cheap, but those big caps look to be in perfect condition.

    Weirdly, the tape, CD and tuner modules each take a 23VAC supply from the amp itself, so each needs its own transformer and rectifier circuit. Seems very wasteful.

    Any thoughts?

    Thanks.
     
  2. Alex_C

    Thread Starter New Member

    Jan 28, 2016
    8
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    Here's a section of the schematic relating to the two burnt areas. Looks like they are both involved in voltage regulation.

    Schematic section of burnt areas.png

    The two big capacitors are C703 and C704, part of the bridge rectifier (I think). That seems to supply 33V/34V to the two regulator sections that have burnt components. I wonder if I should also replace C705, C706, C708, C709... in fact all up to C718 just to be sure.

    Does anyone have any suggestions or ideas? Am I on the right track?
     
  3. #12

    Expert

    Nov 30, 2010
    16,345
    6,831
    Don't bother with replacing ceramic capacitors. I've see 2 bad in 40 years and they had exploded.
     
  4. Alex_C

    Thread Starter New Member

    Jan 28, 2016
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    OK. I think most of the ones I've listed are electrolytic.
     
  5. Alex_C

    Thread Starter New Member

    Jan 28, 2016
    8
    1
    Fixed in April 2016, as follows.

    I replaced C703 and C704 in the amp. That made a slight difference, in that the unit would turn on from 'cold' after about 30 seconds rather than a couple of minutes. But the display on the tuner was still flickering.

    Then I replaced C702 and C713 and C714 in the tuner. That fixed the problem completely! Instant power-on from cold, no flickering on the tuner display. Everything works fine.

    As well as the burn marks near C714 and C709 in the amp, I also noticed that the area around R704 in the tuner looks burned, so that was probably struggling to cope. R705 and the bank of eight diodes nearby are also showing signs of heat.

    I'll replace a few more related capacitors in the amp and tuner, just to make sure, but I'm declaring this fixed.
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    Some large (cheap) electrolytic caps have a working life rated at less than 5000 hours.
    1992 - means replace the all.
    We referred to this operation as a "shotgun" replacement.
    20 plus years is more than enough time for even unused new equipment to manifest problems from a bad electrolytic cap. The liquid in the electrolyte dries up.
     
  7. Alex_C

    Thread Starter New Member

    Jan 28, 2016
    8
    1
    Yes, so it would seem. I'd estimate there are about 200 electrolytic capacitors across the four units, though, so for now I'll stick to the obvious ones relating to power circuits.

    There's slight browning of the PCB in the CD player near its transformer, and to a lesser extent in the tape deck. I'll replace the caps near those as a precaution, but I'll only replace others if problems occur.

    It sounds remarkably good now. Bi-amp, bi-wired, very clear sound for something 24 years old.

    Edit: Can I add "[solved]" to this thread's title for the benefit of anyone searching for this topic? I can't see how to do it.
     
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