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Thread: TRS-80 III and 4 part interchangability?

  1. #1
    Join Date
    Oct 2016
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    Default TRS-80 III and 4 part interchangability?

    So, I've got an ailing TRS-80 Model 4 I'm troubleshooting and a TRS-80 Model III in more certain shape. Could I swap the PSU and floppy drives from the III into the 4 for testing purposes?

    Also, with regards to the 4 power supply... is J3 on the PSU supposed to be an absolute bear to get soldered down? The thing was barely held in place at intake time and it's taken multiple tries to get it back in place. Also, how long of a screw should I be using for the ground connection? I'm suspecting some sort of ground fault stemming from me not taking close enough notes on what screw types go where, but i'm not sure. As for that RIFA at C33, is it one of the filter caps that's safe to just pull and omit entirely?



    Bonus question: With regards to TRS-80 III keyboard contacts, is Deoxit safe to use to clean them? I know straight up isopropyl can apparently remove graphite that ought not be removed.
    Last edited by MrArgent; September 17th, 2019 at 09:39 AM.

  2. #2

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    (1) the model 3 with disk drives uses 2x 35W power supplies, if your model 4 has dual power supplies then they are interchangeable as they are the same, later on and the model 4 uses a single power supply of 65w (not compatible because different connections/wires)

    (2) The 2 pin issue is simple, push the pins down the plastic and use a nice blob of solder that is all there is to that. (see image)


    (3) How long a screw to ground connections? which connection are you speaking off?
    (4) That RIFA is already cracking and you can unsolder it safely to use without for a time. (its a voltage filter I would replace it)
    (5) the alcohol used in keyboards are used on the metal contact at bottom of the key contact. I see no issue using detoxit in the same manner.
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  3. #3
    Join Date
    Mar 2013
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    Default

    If you clean those pins good, then insert them in the PCB, they should solder properly. You will need to flux
    the parts and use a temperature controlled Iron along with some good Rosin Core Solder. I use the Kester
    63/37 and it just works.

    If you use too large Solder gun, or apply too much heat, you will lift the PCB pad.

    Larry

  4. #4
    Join Date
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    Hold you head high, Mr Argent

    Yes you need to change C33. J3 will take a bunch of heat and solder since the pins are so thick. I new connector may help, as your current J3 connector looks a bit corroded.

    Any screw will do for the ground connection, the GND pad just presses against the chassis.

    You can use the Model III drives, towers and PSUs in a Model 4, no problems there.

    Hope this helps,

    Ian.
    *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

  5. #5
    Join Date
    Oct 2016
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    Got 'em down! just took a couple of tries and a lot of solder. Machine's basically operational, though as of yet I can't format a clean disk on :0 or :1 (Sectors 36, 37, 38, and 39 get marked bad) and i'm having trouble getting the flow of imaging diskettes down since I don't have any machines suited for SD work and i'm having a hard time converting .DMKs to double density. The Model 3's next for service, but I know already it'll probably just need a drive cleaning and contact cleaning on the keyboard.

    p.s. For any Model 100 buffs that see this, is there anywhere I might still procure a REX kit sometime down the line?

  6. #6
    Join Date
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    MrArgent,
    I sure hope you checked the Solder Joints on the J2 Connector too. Those are prone to
    getting a thin ring around them where the solder is broken fee from thr PCB. If you didn't
    look at each one with a Magnifying Glass, I'd suggest doing that to make 100% sure.

    I've seen several with bad solder joints on J2. I'm sure Ian has also.

    Thanks.

    Larry

  7. #7
    Join Date
    Oct 2016
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    Yeah, I gave those a one-over in my mission to get J3 to stay put. Couple of pins had the ring o' death forming, but luckily that one held up better. I need to get a better soldering station lol

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