Image Map Image Map
Page 1 of 8 12345 ... LastLast
Results 1 to 10 of 71

Thread: Composite Black and White monitor dead

  1. #1

    Default Composite Black and White monitor dead

    Really it's more than just any old composite monitor...it's actually an acceptable looking TRS-80 Model 1 monitor I snagged for $3 but I figure the principles are the same regardless.

    It's dead. No raster and no glow in the tube neck. The board does heat uo though and there is that smell of old electronics being startled awake after many years. I've done no tests yet, but I have Sam's Facts for the model 1, and they provide a troubleshooting guide for the monitor and say what voltages should be on cetain components. I suspect something to do with the AC power supply of maybe horizontal sweep. Some faulty power transistor maybe?

    What I would appreciate from anyone who knows, is a link to a page or doc which explains how composite B/W monitors work. The Sam's document is great from the perspective of troubleshooting detail but it does assume you know, conceptually, just what's going on. I don't and I'd like to get some understanding before I start poking around.

    Incidently the SAM's fact PDF covers the 110V version while I have a 240V one. There are some differences, one of which is there appears to be no fuses in the AC circuits!

    Thanks

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  2. #2
    Join Date
    May 2006
    Location
    Melbourne, Australia
    Posts
    5,720

    Default

    Quote Originally Posted by tezza View Post
    It's dead. No raster and no glow in the tube neck. The board does heat uo though and there is that smell of old electronics being startled awake after many years.
    Due to the design of the IBM 5151, a good 5151 will present those same symptoms until it is connected to a video card.
    Maybe the TRS-80 Model 1 monitor is of the same basic design.

  3. #3

    Default

    Yes, good thought. That is true about the TTL IMB 5151 but I don't think it's the same with these mono composite monitors.

    The TRS-80 Model 1 monitor is actually just a modified RCA B/W television (with the tuner bits missing...there are even holes for it on the circuitboard). I've got other monitor like this that I have used as a substitute with my Model 1 keyboard/computer. It certainly shows a raster whether plugged into the computer or not, as do the other composite monitors I have around.

    From what I gather these composite monitors basically all work the same way as far as the electronics go. I'd like to know just how these circuits should work, so I can apply some intelligence to the diagnostic process rather than just blindly following steps. Given it's a 240V board and my manual is for 110V there are bound to be some differences in the specifics. If I have a general idea of what should be happening though, I might be able to connect the dots....and not electrocute myself in the process

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  4. #4
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
    Posts
    6,887

    Default

    Well, the essentials are a video amplifier and two oscillators, one around 60Hz for vertical deflection and another around 15kHz for horizontal which also generates high voltage for the CRT via the flyback or HOT (Horizontal Output Transformer); they drive the coils in the deflection yoke and magnetically move the beam around. The composite signal is split into a video signal which is amplified and modulates the electron beam, and vertical and horizontal sync signals which lock and synchronize the oscillators to the source. Of course also a power supply, although generally some of the voltages are derived from the HOT which can make it tricky to troubleshoot...

    But you probably already know all that.

    I'd probably start with finding how the CRT filament is powered and why it's not lit, and following the power supply lines.
    Last edited by MikeS; June 30th, 2012 at 02:46 PM.

  5. #5
    Join Date
    May 2006
    Location
    Melbourne, Australia
    Posts
    5,720

    Default

    Have you done a good visual inspection? Spotting a bad solder joint or spotting a bad component can save hours of work.

    What test equipment do you have?

    You have the circuit diagram. I would first examine the voltage rails produced by the power supply (as different to voltage rails produced by other circuitry).
    If those power supply generated voltage rails are good, then it's on to look at other things.
    If only one of the power supply generated rails is missing/low, then investigation as to why. Loaded down by a short, or component failure in the power supply?
    If all of the power supply generated rails are missing/low, then investigation as to why. Power supply failure, or has the power supply shut itself down due to a short elsewhere?

  6. #6
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    27,042
    Blog Entries
    20

    Default

    The heater for the CRT is sometimes powered from a winding on the horizontal output transformer; sometimes from the DC supply and sometimes from the winding on a power transformer. The situation with your inexpensive monitor is most likely the first.

    Okay, so anything in the horizontal section with a problem will cause the problem you describe. A good place to start troubleshooting is the horizontal output transistor. Does it pass a check with a tester? Is there a signal at the base?

  7. #7
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
    Posts
    6,887

    Default

    Quote Originally Posted by Chuck(G) View Post
    The heater for the CRT is sometimes powered from a winding on the horizontal output transformer; sometimes from the DC supply and sometimes from the winding on a power transformer. The situation with your inexpensive monitor is most likely the first.
    Yeah, that's what I meant by "tricky"; unlike a "normal" power supply, if the horizontal oscillator's not running there won't be power to various sections (and without power there's no horizontal oscillator ).

    Careful, there can be nasty voltages around the horizontal output section.

  8. #8

    Default

    Thanks guys, I'll take those comments on board. Good high-level overview Mike...it does help. Although I have troubleshot monitors in the past, I didn't have a full understanding of how they work. Getting there though.

    I have the usual test stuff, multimeter, scope, logic probe (not need for this probably), a healthy fear of getting zapped etc.

    One of the early things SAM's facts says if there is no life is to check that horizontal output transistor so that's what I'll be doing once I start the project. That might not be for a little while but I'll let you know of progress when I do.

    One question. With these kind of monitors, what are the likely differences going to be in the circuitry with 110V vrs 240V? Anyone know?

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  9. #9
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    27,042
    Blog Entries
    20

    Default

    No substantive difference, except for a bit in the power supply. Most of the substantive difference is in the receiver circuitry (the US is NTSC; most of the rest of the world is PAL). By the time it gets to the pure video circuitry, they're pretty much the same--only the frequencies differ.

  10. #10

    Default

    Hmm..interesting. I've got an hour or two spare so I've opened up the back. The 240V feeds straight into a 240v-->110v step-down transformer which then feeds 110V into the board. So that might be the only difference?

    One thing I'm not sure about. The SAMS sheet mentions checking for various voltages in the AC power circuit. I'm familiar with DC circuits where you have ground or earth (the ground rail, or where ever the circuit board tells me earth is). When measuring voltages in this AC circuit though, where do you put the black probe of the multimeter if you want to measure the voltage at a particular spot? Is it on the neutral line which comes into the unit? The chassis doesn't seem to be connected to either side of the AC line?

    Should I be using an isolating transformer when poking around in this part of the circuitry?

    Sorry if these are newbie questions (I am a newbie at this AC-stuff).

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


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
  •