Image Map Image Map
Results 1 to 4 of 4

Thread: Repairing a Zenith/Heathkit Z19/H19 terminal

  1. #1

    Default Repairing a Zenith/Heathkit Z19/H19 terminal

    I have come into possession of a Z19 terminal recently. Yesterday I was able to get it talking to my linux server but started encountering a problem. After being on for about five minutes or so the CRT would start to fade to black over the course of 10-15 seconds. I lifted the cover to watch for anything odd inside while it was fading and I noticed that the heater on the CRT would fade along with the picture. With no picture on the screen the "bell" would still sound when I pressed the correct key command. So the part of the logic board that processes keyboard input seems to work.

    After looking at the manual and some circuit schematics there are a lot of things that can seemingly cause this. The logic board not sending the proper video signals, bad hsync signal, problem with the flyback transformer filament winding, bad solder joint, etc. I am not sure where to start on this. I have measured the resistance of the heater filament on the CRT and it comes to 2.2 ohms. So that doesn't seem to be an open circuit in the tube. Although I am not sure what the resistance should be on a working unit.

    Where do I start troubleshooting this?

  2. #2
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,474

    Default

    Quote Originally Posted by spock345 View Post
    Where do I start troubleshooting this?
    The filament on the CRT should run from a low voltage DC power source. Make sure that the regulator output or whatever the source of the supply is staying steady. The 2 Ohms seems a little low. Is your Ohmmeter accurate?

  3. #3
    Join Date
    Jan 2013
    Location
    Marietta, GA
    Posts
    2,652

    Default

    Working fine and then stopping after a few minutes sounds like either a bad solder joint or an electrolytic capacitor going bad (stops working when it heats up). If whacking it on the side makes it come back on then it might be a bad solder joint

  4. #4

    Default

    Figured out it was two bad solder joints on both the connectors for the flyback transformer and the logic board. A connection would only be made if they were both in the exact right spot. It is working now. I just sat there wiggling connectors while watching a voltmeter hooked up to the filament lines.

    My ohmmeter reads resistors properly so I can't find any issue with it.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •