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Thread: Model 4P Screen brightness

  1. #1

    Default Model 4P Screen brightness

    Hi folks,

    Apropos of a conversation I was having with a friend after seeing someone post pics of their 'new' 4P on FBook I got my own 4P on the bench to see if I could revive it. I got it maybe 16 years ago and it didn't work, or so I thought.

    After checking the PSU (and replacing the RIFA 'explodacap' before it could explode) and reseating all the thankfully socketed chips on the motherboard I powered up and was surprised to see a drive activity light. Turning the brightness right up shows that the machine is actually running but the contrast on the screen is pretty much non-existent, see picture.

    I get nothing from the keyboard yet but I need to sort the screen first. You can just make out the Tandy logo in the pic and I know the machine is running because RESET will clear the screen. None of the pots on the video board are concerned with contrast so where do I look next?



    SaveSave - UK home computer history
    Where RIFA capacitors come to die

  2. #2


    You could try popping the controls off the left (they have little connectors on the back) and test the contrast pot. If that is fine, you may have a problem with the video board itself and since we know the video is driving its really a small area of it. Do you have the Model 4P GA service manual? Its easily found by searching, and it should show you "some" of the video boards, as well as the SAMS manual. Could be a transistor or capacitor if that pot isnt bad.

    Good news is you have an image and plenty of brightness, so video amplification is where I would start poking around. I think there are a few transistors that do it, but each board numbers them differently. Drives me absolutely mad that most of them are the version you cant find the schematic for.

  3. #3


    Do you have good test equipment? Q101 (it could be TR) is the video control, Q102 is drive, and Q103 is out. Thats for the 613 board.

    I'd first test out Q101 and if you can get a quick replacement its an easy change on that board once you figure out how to get the four screws out (You already have the bezel off, so popping out the tube makes that much easier.

    I dont think those two ICs would affect this.

  4. #4


    Oh yeah, there is a transistor on the yoke board, and I think the final part of the circuit 103, but this is from memory.

  5. #5


    Hi Tibs,

    Wow, that was quick! I was just about to pull the contrast pot and give it a squirt of contact cleaner. I've been searching through boxes for my tube discharge screwdriver too, I don't want any surprises when I remove the CRT

    Whoever decided that 4 screws holding the bezel on wasn't enough and there should be another two in a really difficult location to get to with anything other than a Really Long Screwdriver should have a word with themselves.

    *edit* got the service manual and that shows the same video board, excellent.

    Last edited by Witchy; November 22nd, 2017 at 03:42 PM. - UK home computer history
    Where RIFA capacitors come to die

  6. #6


    I've been able to remove the board with the bezel on and a CRT in place, just need a good stubby for that bottom and top screw in the front.

    I agree that bezel is a pain to remove, need a really long screwdriver and patience.

  7. #7


    True, I can see that and have the right size stubby, but I'd need to get the board to my desoldering bench I'll make another discharge tool and pop the anode cap, it should be pretty well discharged after 24h of no power but I'm not taking any chances.

    Cheers - UK home computer history
    Where RIFA capacitors come to die

  8. #8


    Smart move, but I've done it after 5-10 minutes and had no discharge. Something good with those boards does a nice drain.

    I've been zapped once, and it didn't even do anything like a nice house line zap does, surprised me is all and was more like getting static electricity zap.

    But be smart....above all respect that power.

  9. #9


    Testing so far: C101, TR101, TR102, TR103 (on the yoke) all test OK. I've just tried running it with C101 and TR101 removed and the picture is the same. Both brightness and contrast controls work as they should with the correct-ish resistance showing. Voltages look OK on the yoke board. Given the video output of the machine is TTL video+H+V sync I'm going to make up an extension cable to run to the monitor of a PET for curiosity's sake...

    W - UK home computer history
    Where RIFA capacitors come to die

  10. #10


    Where is the schematic you are using? Can you turn down the green background to where it's black and still see the characters?

    Larry G


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