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Thread: TRS-80 Model 3 - Repair

  1. #1
    Join Date
    Aug 2013
    Location
    Sevenoaks, Kent, United Kingdom
    Posts
    174

    Default TRS-80 Model 3 - Repair

    In my hunt for a Model 2, I managed to buy 2 other computers, so today I collected this diskless 48k TRS-80 Model 3, sold with a suspected PSU and Keyboard fault.

    I think the computer case and keyboard cosmetics are in great condition, the insides also look very clean.

    It came with no screws holding the top to the bottom so need to work out what size of screws to use, if someone can tell me that would be great.

    IMG_1446.jpg

    IMG_1447.jpg

    It was soon obvious that the Astec PSU had reached the end of it's MTBF - luckily the board looks in great condition, so resurrecting this should be quite easy.

    IMG_1449.jpg

    IMG_1452.jpg

    IMG_1453.jpg

    I didn have any of the higher voltage capacitors in stock, and had to order 5 of each, so ended up spending 1/4 of the price I paid for the whole computer just on fixing the PSU..... ho hum

  2. #2
    Join Date
    May 2013
    Location
    Williamstown Victoria Australia
    Posts
    374

    Default

    @clint

    Just unsolder the 3 RIFA Capacitors (The blown ones) and power the machine up, if it works just get some replacement " NON Rifa caps ".
    Good luck Ray
    If it ain't Broke then PLEASE DONT fix it.
    Procrastinators Unite.
    I keep telling you I am Not Pedantic.

  3. #3

    Default

    Clint -
    Bill Deg has some NOS M3 disk controllers for sale on Ebay (USA)
    I went down the quinterface/FreHD pathway on my diskless M3 though! It works very well

  4. #4

    Default

    Nice one Clint

    I have a Model IV here which I expect has a similar PSU. Let me know if you want me to check anything out for you.

  5. #5
    Join Date
    Aug 2013
    Location
    Sevenoaks, Kent, United Kingdom
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    Default

    Quote Originally Posted by Dustym View Post
    Clint -
    Bill Deg has some NOS M3 disk controllers for sale on Ebay (USA)
    I went down the quinterface/FreHD pathway on my diskless M3 though! It works very well
    I saw them at 24 they I would take one but for some reason ebay is calculating the freight to be 40 to me which is crazy. that totals as much as I paid for the machine.

    Is this all I need: http://www.vecoven.com/trs80/trs80.html

    My capacitors came this morning great service from RS so will venture out and install them in a bit.

  6. #6
    Join Date
    Aug 2013
    Location
    Sevenoaks, Kent, United Kingdom
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    Default

    I spent an hour this morning working on this, I removed ever capacitor checking polarity was correctly marked on the PCB as I took them out then I checked through the PSU with a multimeter.

    IMG_1459.jpg

    Replacement was easy however I did notice that this capacitor was marked as a Ceramic on the board and not an X type. Can someone confirm what should be here as I can find a schematic for this PSU. The value of the X Rated Capacitor was 10n

    wrong cap.jpg

    I replaced them all with high grade Panasonic electrolytics and KEMET/RIFA PME271M 275v X2 Capacitors.

    If anyone is interested I have 2 full sets left which I can supply at cost.

    IMG_1457.jpg

    Anyway after replacing all the capacitors, I wound up my variac with the PSU disconnected and checked the PSU with the thermal camera, I then tried again with the Motherboard plugged in, and checked with the thermal camera again that there was power getting to the board, you can clearly see the Ram getting warm. I checked all the voltages with a meter and they were spot on.

    FLIR0220.jpg

    FLIR0223.jpg

    I then refitted the screen and got the Cass? then Memory? prompt, the keyboard does have some issues the 'S' key wont work and some of the other keys need a good push. - Can someone point me in the right direction of what I need to fix the keyboard, someone mentioned foam ?


    But very happy to have it up and running:

    IMG_1458.jpg

  7. #7
    Join Date
    Aug 2013
    Location
    Sevenoaks, Kent, United Kingdom
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    174

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    Quote Originally Posted by JonB View Post
    Nice one Clint

    I have a Model IV here which I expect has a similar PSU. Let me know if you want me to check anything out for you.
    Can you check that capacitor in the post above please Jon.

  8. #8

    Default

    I would have used poly film caps in place of the Kemet/Rifa exact replacements. You're going to need to replace them again in another 20-30 years.

  9. #9

    Default

    Quote Originally Posted by Clint View Post
    Can you check that capacitor in the post above please Jon.
    Which one? There are about 9 of them.. give me the designation on the board.

    They all look OK to me. The RIFAs are used for noise suppression, so as long as they have the right voltage rating and values they should be OK. You look to have replaced like for like anyway. Can't see a problem. Do you want me to check the values on my board? Presumably for those RIFAs as they have burst and can't be read?

    Meanwhile, congratulations on getting it going! Mighty satisfying.

  10. #10
    Join Date
    Aug 2013
    Location
    Sevenoaks, Kent, United Kingdom
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    174

    Default

    Jon, sorry I meant the one arrowed in the above pictures.

    wrong cap.jpg

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