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Thread: Model 4GA Woes: Catastropic Failure of the Smoke Test, Visible Victim U81

  1. #1

    Default Model 4GA Woes: Catastropic Failure of the Smoke Test, Visible Victim U81

    U81.jpg

    Well as for being a trusting soul, the M4GA arrived last night and I decided to give it the Smoke Test and then, of course, I let the smoke out.

    Shut it down quickly and dug out the trusty digital meter. The power supply 12v was all over the place and then it settled to 13.8v, well that seemed reasonable, it was within 1%. Plugged everything back in turned it on and more smoke.

    So I have pulled the PS (gotta replace the Rifas anyway) and pulled the cover off the back and started to examine the MB, and found U81 (see picture) U81 (a mc1488P) with a good sized bubble (from where the smoke got out from). U81 is part of the rs232 module.

    With U81 going out like that, could it been caused by the flaky power supply and when the 1488 went out, what else would the 1488 take out???

    Any and all assistance would be greatly appreciated.

    Cheers

    Ross
    Last edited by ac7kh; September 14th, 2018 at 06:27 PM.
    Collection: TRS-80 Model 3 w/autoboot FreHD, TRS-80 Model 4 (X2), TI-99/4a (X2) w/PEB, Timex Sinclair ZX-81 w 16K module, Raspberry PI (X5), 1u eServer running Linux, Intel Brick Computer running Linux, Dell Optiplex 160 (X2) 1 running Win7, 1 running Linux, Dell Optiplex 755 running Win10, Mini MAC (Apple)(X2), a hacked together AMD A6-5400K running Win10, and several laptops running everything from XP to Win10 (X5+)

  2. #2
    Join Date
    Sep 2011
    Location
    Melbourne, Australia
    Posts
    948

    Default

    You're going to have to remove that chip (computer will run OK without it) and really test your power supply thoroughly. don't connect it to your motherboard, but connect it to something which doesn't matter if it gets destroyed (it's why I hang onto old hard drives with bad blocks or the click of death) and monitor the 5V and 12V over a decent amount of time.

    I had a PSU in a Model 4 once (a Tandy PSU) which started out at 12.6V but over time that would increase at about 0.1V / 5min so I came back a few hours later and it was reading over 16V which is waaay to much.

    Once you are certain the PSU is stable then connect it to the rest of the computer.

    TIP: Model 4 PSUs are designed to run with a load so if its not plugged into anything then the voltages you get from it are meaningless... and it's not too great for the PSU either.
    *There is never a charge for Tech Support even if you don't purchase from me - We are Enthusiasts Helping Other Enthusiast and that is just the way it is

  3. #3

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    So that should render the rs232 as useless correct ??? and if so, would I be able to expand the 232 on a FreHD to use in it's place ??? Also do you have any other "bad" HDs laying around so when I do my next order (as you know I will, you and Jay keep me happy) it could be added on ????

    Ross
    Collection: TRS-80 Model 3 w/autoboot FreHD, TRS-80 Model 4 (X2), TI-99/4a (X2) w/PEB, Timex Sinclair ZX-81 w 16K module, Raspberry PI (X5), 1u eServer running Linux, Intel Brick Computer running Linux, Dell Optiplex 160 (X2) 1 running Win7, 1 running Linux, Dell Optiplex 755 running Win10, Mini MAC (Apple)(X2), a hacked together AMD A6-5400K running Win10, and several laptops running everything from XP to Win10 (X5+)

  4. #4

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    +1 on TRS-Ian's advice. 13.8V on the +12 is out of spec to begin with so there's an issue there.

  5. #5

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    Roger that Pete, have already PMed Jay about a replacement (I know Ian has some but I am trying to save on shipping). Will keep the old one for spare parts.

    I have pulled the chip and I am guessing I should pull the UART but I am waiting to see what Ian says about the FreHD before doing more.

    Ross
    Last edited by ac7kh; September 15th, 2018 at 02:56 PM.
    Collection: TRS-80 Model 3 w/autoboot FreHD, TRS-80 Model 4 (X2), TI-99/4a (X2) w/PEB, Timex Sinclair ZX-81 w 16K module, Raspberry PI (X5), 1u eServer running Linux, Intel Brick Computer running Linux, Dell Optiplex 160 (X2) 1 running Win7, 1 running Linux, Dell Optiplex 755 running Win10, Mini MAC (Apple)(X2), a hacked together AMD A6-5400K running Win10, and several laptops running everything from XP to Win10 (X5+)

  6. #6
    Join Date
    Mar 2016
    Location
    Pikesville, Maryland
    Posts
    100

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    I have plenty of HD's hanging around ....

  7. #7
    Join Date
    Sep 2011
    Location
    Melbourne, Australia
    Posts
    948

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    Hi Ross, the RS232 on the FreHD is not implemented for TRS-80 use. I know that may sound odd but it was original put there as a diagnostic aid when debugging FreHD during the alpha and beta test stages.

    Best thing for you would be to fix the power problem and get your system running reliably, then fix the RS232 chips.
    *There is never a charge for Tech Support even if you don't purchase from me - We are Enthusiasts Helping Other Enthusiast and that is just the way it is

  8. #8

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    OK got a power supply from Jay, installed it and installed the motherboard. Powered up still garbage on the screen and U82 (UART) is hotter than h$$l, but the drives did spin, begrudgingly. After letting the UART cool off, I pulled the drives and powered up long enough to verify no cass?? prompt and shot a picture. Any ideas ???

    Garbage Screen.jpg

    Ross

    Forgot to note that this is the same video display that I got when the machine arrived. Last night, after I first posted this. I also changed out the 64K ram and again no change, so I think it only leaves the CPU or the video card, but I would think that if it was just the video card itself it would a least in reference to the video display try to boot (either to disk or cass)
    Last edited by ac7kh; Yesterday at 12:08 PM. Reason: Additional Info
    Collection: TRS-80 Model 3 w/autoboot FreHD, TRS-80 Model 4 (X2), TI-99/4a (X2) w/PEB, Timex Sinclair ZX-81 w 16K module, Raspberry PI (X5), 1u eServer running Linux, Intel Brick Computer running Linux, Dell Optiplex 160 (X2) 1 running Win7, 1 running Linux, Dell Optiplex 755 running Win10, Mini MAC (Apple)(X2), a hacked together AMD A6-5400K running Win10, and several laptops running everything from XP to Win10 (X5+)

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