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Thread: New additions - TRS-80 Model IIIs

  1. #1

    Default New additions - TRS-80 Model IIIs

    Just picked up two TRS-80 Model IIIs of eBay and no more than a 10 mile round trip to collect them. The seller listed just the one as "was told it was working when stored 20 years ago" which to me says it doesn't work. When I got there he had two, so I snapped the other one up hoping to get at least one working from the pair. They are both dirty but look to be in reasonable condition. I'm quite excited as it's been almost 30 years since I last used one of these.

    I'll start off by cleaning them and ensuring socketed chips are correctly seated and any fuses are working and the correct value. I'll also place a 100 watt incandescent lamp in series with the machines when first powered up. Other than that, I'll be looking around here and other sites (I've read Tezza's blog!) for hints and tips and sources for operating systems and other software.

    If it's of any interest to other TRS-80 collectors, the outer cases both show catalog number 26-1066 and the serial numbers are 0001432 and 0004509.

    Pictures to follow soon.

    Last edited by SteveH; December 1st, 2012 at 01:50 PM. Reason: Added link to Tezza's blog
    CP/M-86 Software Repository

  2. #2


    Congrats on the find! Hope it works out for ya. It's always nice to avoid the shipping price tag. I'm sure between the two you'll have some luck. Let us know how it goes!
    Looking to acquire: IBM 5100, Altair 8800

  3. #3

    Default Replacing all or just blown caps?

    I've started cleaning and checking over one machine and the first thing I've found is one of the PSUs has a blown capacitor.


    Luckily (for me) this topic is frequently discussed, even on this forum : Replacement for Caps in TRS-80 Model II Power Supply I just need to source a suitable replacement.

    Now here's a question for those more experienced in repairing this sort of thing. Should I replace just the blown capacitor or all of the paper ones?
    CP/M-86 Software Repository

  4. #4


    I'm not familiar with all the ins-and-outs of the TRS80 power supplies in particular, but just speaking generally, it would be wise to replace all of those paper/box ones. A general rule I go by is that, if one cap fails without any obvious external cause, the others of the same kind probably aren't too far behind. Those particular ones are X-type safety capacitors (designed for hot-to-neutral mains bypass, will never fail shorted), so make sure to replace 'em with the same sort. The newer ones tend to be physically smaller, but the values on those aren't super critical, so you can go up a bit in capacitance if you have trouble finding something large enough to fit the holes on the board.

  5. #5


    Thanks for the comments OT. I didn't realise about these capacitors having to be x-class. I've done some searching and have found the following which I think would be a suitable repalcement for the 0.1uF box type cap.

    0.1uF 275V, Class X2 Suppression Capacitor
    Metallised polypropylene capacitor designed for mains interference suppression
    Epoxy resin encapsulated in flame retardant plastic case
    Voltage rating: 275Vac
    Capacitance tolerance: 10%
    Test voltage: 1500Vac + 2200Vdc
    Operating temperature range: -40C to +110C
    Dimensions: L = 18mm x H = 12mm x T = 6mm, pitch 15mm

    Would this suffice to run under UK 230V mains supply?

    CP/M-86 Software Repository

  6. #6


    Yeah, notice that the X-caps are rated by AC voltage, rather than the DC rating found on most other caps. 275VAC is the most commonly found value, and is more than sufficient for use on 230-250VAC mains.

  7. #7


    Quote Originally Posted by Old Thrashbarg View Post
    The newer ones tend to be physically smaller, but the values on those aren't super critical, so you can go up a bit in capacitance if you have trouble finding something large enough to fit the holes on the board.
    OK, so how much "up a bit in capacitance" can I or should I go? Would a 0.47uf suffice in place of the 0.1uf capacitor?

    I can find 0.47uf class-x metalised polyester 275vac with 22mm lead spacing that will fit this board. Whereas all the 0.1uf caps I've found appear to be 15mm lead spacing (with the exception of the metalised paper Rifa caps which have the correct spacing, but I understand from a different thread that they should be avoided). Alternatively, I suppose I could just get the long legged 0.1uf 15mm lead spacing caps and bend the legs to fit.
    CP/M-86 Software Repository

  8. #8


    I personally see no need to up the capacitance. I like to keep things in line with the original spec. although it probably won't hurt in this case. You can buy any lead spacing you need at mouser and most Astec boards have multiple sets of holes to accommodate multiple sizes. Look at the specs on the caps and you'll find the perfect fit. Otherwise, just bend the pins, it doesn't matter or even mount the cap a little bit higher and let the pins spread a bit to accommodate the holes.

  9. #9


    There's no need to up the capacitance, but it's a reasonable option if it's inconvenient to get a particular value of cap in a particular size. It's a way better idea than just bending the leads on a wrong-sized cap... floating a bulky component like that by its leads is asking for broken solder joints or lifted traces, which would be very bad since the component is going to be on the mains line. If the board has alternate mounting holes for a different sized cap, then great, problem solved... a lot of those specialty OEM PSUs don't have such options though.

    OK, so how much "up a bit in capacitance" can I or should I go? Would a 0.47uf suffice in place of the 0.1uf capacitor?
    Yeah, 0.47uf would be fine. What those caps do is filter noise (RFI, etc.) out of the incoming power. Now, there's a good bit of theory behind the values of those caps, stuff like capacitive reactance and frequencies and whatnot to consider, but realistically, the upper limit of the value is really more a practical issue than a technical one... I've only ever seen X-caps in the range of 0.01uf - 1uf (and anything over ~0.56-0.68uf is quite uncommon, quite big, and quite pricey), so I think it's safe to say that any X-cap of similar size will work just fine.

  10. #10


    Well, if you end up going outside the original design, it'd be a good idea to note that somewhere in the machine so that someone looking at it in the future knows that it's out of spec. I usually put a sticker in the case somewhere with notes.


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