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Faulty CRT AKF12/Philips CM8500: bad flyback?

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    #1

    Faulty CRT AKF12/Philips CM8500: bad flyback?

    Hi all,

    I have a CRT (Acorn AKF12, based on the 240V version of the Philips CM8500) which has stopped working. It's making a high pitched noise but not too high. I know this is symptom of a faulty flyback.

    What happens when a flyback fails? I see that the main voltages out of the PSU are not reaching rated values, as if there is a short.
    There are supposed to be 125V/26.5V/15.5V outputs but I am reading something like 48V, 10V and 8V. If I unplug the PSU from the rest of the PCB, I can measure the expected values.

    Can it be a short somewhere else? Or does the flyback short when it fails?

    Also, where can I find something that explains me the principle of operations of a TV, particularly how the horizontal circuit works with the flyback? I've been staring at schematics for a while and I am not sure I understand why the horizontal pulse ends up in the flyback!

    Thanks!
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    #2
    Re: Faulty CRT AKF12/Philips CM8500: bad flyback?

    Check the horzontal output transistor and damper diode one may be shorted, usually is the flyback shorts the horz. o/p transistor will also short. There is also a electrolytic cap 22µf (C483?) on T401, it may be bad as well
    I would also replace the small electrolytics in the primary switcher, they may be bad not alowing the power supply to run at full power.
    Last edited by R_J; 08-22-2023, 05:36 PM.

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      #3
      Re: Faulty CRT AKF12/Philips CM8500: bad flyback?

      Thank you!

      I should have posted the schematics, it took me a while to find them as it's the 240V version and not 110V. They're attached.

      is it C461 on T401?

      I'll check the horizontal transistor even though from preliminary tests it looks fine. Damper diode is D461?



      Good point about the SMPS not being able to keep up the voltage under load. I'll test with some dummy loads for sure.

      Back on the basics, I see the horizontal transistor is maybe driving the flyback? I'll research but any pointers would be great thanks!
      Attached Files

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        #4
        Re: Faulty CRT AKF12/Philips CM8500: bad flyback?

        TS461 is the horz drive transistor, it drives T401 which is the horz. drive transformer, TS467 is the horz. o/p and the damper diode is D467,D468
        C462 can cause problems if it is bad/open the horz pulse will be distorted and not drive the circuit properly.
        in the power supply I would first replace C122 1µf/50v and maybe C116 47µf/16v
        Last edited by R_J; 08-23-2023, 04:12 PM.

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          #5
          Re: Faulty CRT AKF12/Philips CM8500: bad flyback?

          Thank you.

          How can I diagnose a bad flyback? Would it read as a short? I guess it would normally read a very low resistance?

          Comment

            #6
            Re: Faulty CRT AKF12/Philips CM8500: bad flyback?

            you need a "ringer" to test a flyback.
            the problem can even be a short after the flyback on the seconary rails.

            if you desolder the flyback you can meter the output pads for shorted diodes etc.
            you can also see if the overload has gone away that is drawing the B+ voltage down.
            on that subject, there should be a smoothing cap on the B+ rated anywhere between 160v and 250v - check it.

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              #7
              Re: Faulty CRT AKF12/Philips CM8500: bad flyback?

              Thanks, I read up online and educated myself about how the flyback works and how it's connected.

              I'll poke around and indeed I'm thinking of removing the flyback to see whether the short goes away when it's de-soldered.

              I'll take a look at those caps.

              Thanks again as usual, I'll report back!

              Comment

                #8
                Re: Faulty CRT AKF12/Philips CM8500: bad flyback?

                The diagnosis seems to be pointing to the flyback unfortunately.

                - The Horizontal transistor tests fine
                - The capacitors you mentioned test fine
                - all diodes seem to be fine
                - No shorts I can detect on the board
                - the PSU works fine with the main PCB disconnected and driving some dummy loads
                - I removed the flyback, no shorts on secondary windings and the PSU powers up the board fine with the flyback removed.

                So it's either something on the secondary circuits or it's the flyback itself. I'd say it's likely the flyback.

                Thanks for all your help so far!
                Attached Files

                Comment

                  #9
                  Re: Faulty CRT AKF12/Philips CM8500: bad flyback?

                  little update: the new flyback (and new HOT transistor just in case) fixed the monitor.

                  https://youtu.be/3-LrkMFsfw8

                  But a week later it failed again
                  I haven't had a chance to take a look yet but I can see 15Khz coming out of it so it seems that part of the circuit is working. But no HV and no picture.

                  Comment

                    #10
                    Re: Faulty CRT AKF12/Philips CM8500: bad flyback?

                    at time of CRT Philips & Grundig ... we never replace Flyback Transformer unless its shorter or burned , we repair them air it as any other transformer repair usually there will be cutting line on one pins leg.. we make hole to free the legs from foam then extend the wire to the pins ..LOL

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