Connect with us

Old Panasonic CRT TV, Adjusting Picture Geometry

Discussion in 'Electronic Repair' started by AR, Jan 16, 2013.

Scroll to continue with content
  1. AR

    AR Guest

    Hi, I want to adjust the picture geometry on my 1993 Panasonic TC26-L1R
    (chassis M16M) crt tv. The picture is zoomed in and there are parts in sides
    and upper border which are not visible. I have found a service manual on the
    web, but unfortunately in Russian. I have tried to download english service
    manuals of the models having same built in chassis to acquire information
    comparing the texts. It seems there is no info about entering the service
    menu or adjusting the picture geometry in the service manual. I would be
    very grateful if anybody can tell me how to enter the service menu, how to
    ONLY adjust the picture geometry and how to eventually get out without
    damaging other settings or having an ugly CHK sign shown on screen.
  2. Phil Allison

    Phil Allison Guest


    ** The usual cause of an oversize pic is because the EHT voltage is low.

    If the pic size also changes with brightness, that indicates poor regulation
    of the EHT.

    EHT setting is normally via a trim pot on the main PCB.

    ..... Phil
  3. Guest

    Pictures don't just get bigger. The only way for that to happen is as Phil said, something happens to the HV, but that only applies to sets with separate HV and horiz. sweep. Your set DOES not have it I can be almost 100% sure. Panasonic simply did not do that any more. Maybe in a Sony or Mits. So no fingerpoken ! (or it's digital equivalent) Plus, Panasonic sevice menus are unfriendly and unforgiving.

    What is very likely is that you are watching it through a set top or cable box. Most of the modern cable boxes at least have aspect settings to accomodate newer sets with 16:9 aspect ratios. This is where the problem usually is. With a set top or dable box, go all the wasy thorugh the menus to find things like "aspect", "zoom" and the like.

    In case you are not using a set top or cable box, or even if you are, go through the user menu on the TV as well to find similar settings because quite a few of the more modern 4:3 sets also have then, to fill the screen on 16:9 material.

    For it to require adjustment in the service menu, two parameters in EPROM would have to increase. Two out of maybe 100.

    Ain't happenin.
  4. Pictures don't just get bigger. The only way for that to happen
    I have trouble imagining a set with separate horizontal sweep and HV --
    especially a set with a reasonably sized CRT.

    The "flyback" HV circuitry can certainly lose its regulation, while the sweep
    continues to work correctly.
  5. "Michael A. Terrell" wrote in message
    I said set, not monitor. And I assume you're referring to auto-sync monitors.
    In a flyback HV system, you'd have trouble maintaining a constant HV as the
    sweep frequency changed.
  6. Guest


    I have trouble imagining a set with separate horizontal sweep and HV --
    especially a set with a reasonably sized CRT."

    One comes to mind, the Sony KX 2501. It was dubbed Profeel. It had separateHV and sweep. It was a bit strange oin a way, under secrtain conditons wwhen the scene got brighter the raster shrunk slightly because of the mass ofthe electron beam. That is why three tube CRT based projectors have these seemingly useless couple hundred Kohm resistors or whatever in series with EACH CRT anode lead inside the HV splitter. This compensated the deflectionsensitivity, if not, the convergence woulg tet thrown off with changes in scene brightness.

    Quite a few monitor quality sets used disctrete HV. Proton, Sony, Mits., Pioneer. The fad didn't last long because back then component count it does not as much.

    The practice did continue with Sony in direct views into the XBR400 series at least, and then others did also. Ironically, Sony abandoned it.

    In the latest CRT based RPTVs, they use an algorythm in the jungle IC to modify the geometry to compensate for both the variation of voltage and current in the HV, or ultor voltage. They stopped regulating it. You saw nothing..

    I remember when a weak 3A3 would give you a zoom effect from the brightnesscontrol. That HV load made a loss across the HV rectifier only, and did not affect the sweep.
  7. Guest

    The old Zenith with zoom, oh yeah. However those didn't really vary the HV,at least not on purpose. The modified the H pulse to the yoke and cranked the V height.

    When you get to the HD TV sets, you will find some with separate HV becausealot of them didn't actualy upconvert. You had 1080i and 480p, a 480i signal really just went through a line doubler. RCAs dealt with the differencesin sweep using the flyback, in fact in the DTV3XX series they varied the B+ voltage to compensate. That's why there aren't too many of them around anymore, it was not a good idea. The ITC and ATC chassis were quite a bit more reliable.

    The HD Sonys have a separate flyback driven from a totem pole of two MOSFETs. The frequency is does not give a shit Hz, but it is higher than the sweep, which seems like a good idea.

    Separate HV was really popular in CRT based RPTVs. In those, it solved alotof problems all at once.

    I can't think of any Panasonic direct views with separate HV, and now that I mention it, neither in their RPTVs. There could be a model or two out there I have not seen though.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day