Mits WT 46809, error code 2-3

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  • kaboom
    "Oh, Grouchy!"
    • Jan 2011
    • 2507
    • USA
    #1

    Mits WT 46809, error code 2-3

    This RPTV was going to be thrown out. "Do you want it?" "Sure," I said.

    Took the back off and tried to power it up. Standby supply brings main supply up, closes relay across 'soft start' resistor (R9A02, 2.2/5w). Drops out after a second or two. Fault code is 2/3, horiz deflection failure. During the attempt, you can -just- hear the vertical yokes. CRT filament voltage is ~6.5 volts at connector 'KF'. This is regulated by Q5A36 from ~8.5 volts derived from winding on horiz yoke TX. The FBT has its own power MOSFET and drive IC. Drive pulses for that IC are obtained from Q5A37.

    Horiz defl path:

    horiz drive from jungle ->Q5A33(drives base of horiz drive xsistor)
    Q5A33 ->Q5A32(horiz drive)
    Q5A32 ->T5A32(drives base of HOT)
    Q5A31(HOT) ->T5A31(horiz out for yoke only, NOT FBT)
    Voltage to T5A31 is regulated by Q5A01, 02, 03
    Voltage to T5A32 is regulated by Q5A34, 35

    That's horiz defl, now on to high voltage...

    An emitter follower, driven by a capacitive voltage divider off of Q5A31, drives a line. This signal is 'HPULSE.' It goes back to the jungle and back to the convergence gen of course. It also drives the input of the SIP module that drives the gate of the MOSFET that drives the FBT. IC5A00 handles everything for the HV, with exception of a quad opamp, IC5A01, for HV regulation/beam current. HV is sampled by a resistive divider in the splitter block and also by a 'proportionally derived secondary' in the FBT, which feeds D5A57. A dual opamp, IC5A02, aggregates these measurements, and if params exceed setpoints/noninverting inputs, trips xray protect.

    If I disconnect the 'KF' plug, I get audio, tuners and inputs. No raster of course and the power/timer light will then flash code 1-2 (no err). Odd- with no h or v deflection, no error. But with that board hooked up, it complains...

    Occasionally, while cycling power, I hear the tubes charge. I also hear the FBT squealing in pain. This is an HDTV- the horiz freq should be at least double, yet something is audible. It's a beat note! Ripple from the PS and the horiz freq are mixing! The sum or difference is audible in the FBT and a little less so in the power TX. Who'd think Mits would use general purpose caps in this room heater?!

    I had the chassis out to check the usual suspects for shorts. Did not check convergence amps thoroughly- only checked from +/-24 to GND, not from either supply to the sub-yokes...

    Before I pull the guts out again, I'd like to hear any tips you guys have. Maybe there are things I can check before I pull it out again and start the hunt...

    TIA,
    -Paul

    PS- no corrosion from the coolant, at least in a quick peek.
    I can't upload the schematics PDF, I found them here:
    http://www.opweb.de/model.php?id=2700
    Attached Files
    "pokemon go... to hell!"

    EOL it...
    Originally posted by shango066
    All style and no substance.
    Originally posted by smashstuff30
    guilty,guilty,guilty,guilty!
    guilty of being cheap-made!
    Tags: None
    • kaboom
      "Oh, Grouchy!"
      • Jan 2011
      • 2507
      • USA
      #2
      Re: Mits WT 46809, error code 2-3

      It's the flyback, big surprise there...

      The SIP module for the HV monitors the drain current of the HV FET by monitoring voltage at the top of the drain resistor. As the FBT started shorting out, the drain current started going up. It finally got to the point that the module would lock out on overcurrent after giving a few cycles of drive to the FET.

      The quad and dual opamps are OK.

      Here is what I believe happened. FBT starts shorting out. Eventually, module locks out when set turned on. Error ckt, one section of the quad opamp, senses low HV. None, since the FBT duty cycle is zero. Time delay fed from error ckt times out and tells controller to shut the set down.

      I have verified correct operation by studying the schematic and observing voltages on the opamps, applying what would be normal feedback signals if the HV were present. Looks good...

      That FBT primary current lockout is what saved the FET, as well as the module. If this were a conventional setup, the HV FET would go bang once the FBT shorted. It would then backfeed B+, via the gate lead, into the module, destroying it.

      If anyone can recommend a source for a 334P278A1 or 334P27801 FBT, I'd appreciate it. HR Diemen's crossref only comes up with 334P278A3. No fleebay, please.

      Thanks,
      -Paul
      Last edited by kaboom; 02-25-2011, 04:31 PM.
      "pokemon go... to hell!"

      EOL it...
      Originally posted by shango066
      All style and no substance.
      Originally posted by smashstuff30
      guilty,guilty,guilty,guilty!
      guilty of being cheap-made!

        Comment

        • kaboom
          "Oh, Grouchy!"
          • Jan 2011
          • 2507
          • USA
          #3
          Re: Mits WT 46809, error code 2-3

          Working good now!

          It was the flyback, surprise-surprise...

          Ordered one and it arrived Thursday, set works again.

          -Paul
          "pokemon go... to hell!"

          EOL it...
          Originally posted by shango066
          All style and no substance.
          Originally posted by smashstuff30
          guilty,guilty,guilty,guilty!
          guilty of being cheap-made!

            Comment

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