Image Map Image Map
Results 1 to 6 of 6

Thread: Dim composite video on Tandy 1000 HX.

  1. #1

    Default Dim composite video on Tandy 1000 HX.

    Tandy 1000 HX is plugged into a C= 1084-D via composite. Video is very dim, to the point where I have to turn the Brightness and Contrast all the way up to see much of anything (and then it's really washed out of course).

    Haven't tried it on DB-9. Waiting on connectors so I can make a cable.

    I hooked up another 1000 HX and it was working great, so I am pretty sure it's not the monitor (phew). I don't see anything obviously wrong on disassembly. Any clues on how to fix this? I assume I need to recap... Something... Somewhere....... :P Unfortunately I am self-taught at electronics (read: dumber than a sack of hammers), so I am not sure what to do other than start randomly recapping stuff that looks to have something to do with video until it works right.... :3
    May Be Interested In Acquiring: Mac SE-30, IIsi, Quadra 840AV, Commodore PC(286+)/64C/128D, PC-era Tandy stuff.

  2. #2
    Join Date
    May 2011
    Location
    Outer Mongolia
    Posts
    1,482

    Default

    Quote Originally Posted by bladamson View Post
    Haven't tried it on DB-9. Waiting on connectors so I can make a cable.
    I know this is jumping ahead, and maybe you know this already, but I'm going to throw it out there because I was bit by it recently: when you make your RGB cable make sure pin 7 isn't connected. That pin is "NC" on an original CGA card, but the 1000 EX at least cross-connects it to the "green" pin to support some not-at-all-documented-in-the-manual "mono" mode. When I connected my EX to my 1084D using a cable with all pins wired straight through the 1084 grounded out that wire and in the process killed the green output.

    (Maybe this doesn't happen with all 1084s, I know Commodore made a million subtle variants. Luckily it didn't actually damage the output, cutting the pin fixed it.)

    As for the dim composite I'm not sure what the deal there might be. The schematic seems to show a really simple resistor ladder is used on the RGBI outputs to generate the intensity input upstream of an output transistor. Maybe that transistor is blown?

  3. #3

    Default

    Aha, thank you for the heads up. I was not aware of that. =:O

    Maybe I have a cold solder joint somewhere. I didn't try to bench-thump it very hard. Perhaps the next step is to try it on DB-9, and then I'll know whether it's upstream or downstream of where the composite circuitry starts.
    Last edited by bladamson; October 9th, 2019 at 05:34 PM.
    May Be Interested In Acquiring: Mac SE-30, IIsi, Quadra 840AV, Commodore PC(286+)/64C/128D, PC-era Tandy stuff.

  4. #4
    Join Date
    May 2011
    Location
    Outer Mongolia
    Posts
    1,482

    Default

    Quote Originally Posted by bladamson View Post
    Perhaps the next step is to try it on DB-9, and then I'll know whether it's upstream or downstream of where the composite circuitry starts.
    That's not a bad plan. The next thing upstream is a '244 buffer, if you're not getting good RGBI out it could be that.

  5. #5

    Default

    I don't know , but it sounds similar to the way IBM combined the RGBI signals with a resistor mixer at the base of a video output transistor. The transistor acts in a emitter follower mode. More than 90% of the current that the transistor can source comes via its collector-emitter circuit , the base currents are low. If the transistor fails, with an open circuit collector, you video output then becomes sourced from the resistor matrix alone, so the composite video voltage would drop substantially when the signal was terminated by the usual 75R resistor at the monitor end. However the sync pulses would also expected to be low level in that case.
    It could be that the G signal has been shorted/failed as suggested as it provides the bulk of the luminance signal level.

  6. #6

    Default

    .....normally for turning the GCA R, G and B signal into a composite level at the resistor matrix in the base circuit of the video output transistor, IBM at least mixed the green to be about 55% the blue around 17% and the red 28%, not quite the same as the NTSC mix, but close. Also, the composite sync is mixed into the resistor matrix at the base. Probably other computer makers did similar.

    So a quick way to tell if it's the output transistor or perhaps the G channel that has dropped out:

    If you look on the scope, at the video output not terminated into 75 Ohms( not connected to the monitor), for an all white screen the video peak too peak voltage should be about 2V, and the sync portion should be about 0.6V of that. When terminated into 75 R, the PP voltage should only drop in half to about 1V pp for an all white screen (if the video output transistor is normal). And the sync voltage be about 0.3v if that total. For a black screen, you should just see sync voltage of about 0.3V peak terminated and 0.6V unterminated.

    If the G channel is out, the syncs will have the right level, but the pp video voltage will be low.
    If the transistor if defective, the video level and the syncs will crash to a low value when it's terminated, but possibly look OK unterminated.

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
  •