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Technics CH-540 VF Display - troubleshooting help

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    Technics CH-540 VF Display - troubleshooting help

    Hi guys,

    i saved this HiFi from trash. It only problem last is the VF display - it shows nothing. I searched for simmilar topics and found one, but still havent figure it out.

    I have schematics, replaced caps C615-C620, i have negative -25V to -30V on all display pins (including fl1, fl2 - heater) but no AC voltage there.

    AC voltage is present on c615 cap (around 10v as i remember)

    Between FL1 and FL2 i have ~6 ohms, no AC voltage. From FL1 or FL2 to GND i have 3,5V AC (and -29V DC as i wrote) or jumping 12V to 0V (short + protection?)

    Schematics: https://tinyurl.com/3s3b4zks

    Searched for dry solder - no luck.

    Any help would be great, thanks!

    #2
    Re: Technics CH-540 VF Display - troubleshooting help

    Have you checked C917 & C916? I would also check if D622, R621 & R622 are ok
    D622 is a 3.9 volt zener
    Also check if you can see the heater lines in the display in a dark room, If they are working they should glow.
    Last edited by R_J; 05-17-2021, 08:46 PM.

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      #3
      Re: Technics CH-540 VF Display - troubleshooting help

      R_J thanks for reply.
      Without display board connected (so without c916 and c917) AC voltage also jumps from 10 to 0v.

      Just replaced D622 to be sure. Checked R621 622 - both ok 150ohm.

      Lines do not glow - no ac voltage
      Last edited by Radekqr; 05-18-2021, 05:34 AM.

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        #4
        Re: Technics CH-540 VF Display - troubleshooting help

        Whats more - i put external +5v dc on heater and display works... don't know what to think

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          #5
          Re: Technics CH-540 VF Display - troubleshooting help

          The Technics SC-CH540 service manual is missing (width of scans) pages, on pg. 10.
          I can't see where the VFD filament power comes from.
          The power transformer has 12-0-12VAC going to all the units, but where it gets dropped down to 3VAC for the display I cannot find. That part (transistor or capacitor) is likely dead, or a bad connection.

          Most common VFD problem is cold solder joints. The tube's pins are Kovar and look for fractured solder joints there at the tube's (ends) pcb pads, then follow the VFD filament wiring, it does many hops through connectors.
          VFD->CN901B->CN901A->JK603->JK201->SE-CH540 ??? then it's lost. You can also use an ohmeter to verify continuity.

          Both filament leads FL1, FL2 float at -ve DC bias voltage which can be confusing.

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            #6
            Re: Technics CH-540 VF Display - troubleshooting help

            Thanks!

            5,7V AC comes from separate trafo wires.
            I found the issue - i was measuring also the flex cable and connectors many many times and it was ok. It turns out that after connecting flex to socket, one wire (of course, AC) slided off and that's why AC was missing after connecting to board... ehhh...

            Cut the flex, made new connection and et voila! it works

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              #7
              Re: Technics CH-540 VF Display - troubleshooting help

              Good to here it's working, I thought that schematic looked odd and was missing something as the FL1&2 on Jk603 went nowhere. It goes to a different service manual, the SE-CH540 (page 10) - page 11, AC5.2V from the transformer.
              Attached Files
              Last edited by R_J; 05-18-2021, 08:15 PM.

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