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TRS-80 Model 3 - Repair

Clint

Clint

Experienced Member
Joined
Aug 3, 2013
Messages
180
Location
Sevenoaks, Kent, United Kingdom
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.

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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.

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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
 
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
 
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.
 
Dustym said:
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
Click to expand...

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.
 
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.

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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.

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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.

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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:

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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. :p
 
Clint said:
Can you check that capacitor in the post above please Jon.
Click to expand...

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.
 
I opened her up and found I have a different power supply... sorry.

Plus, I just managed to fling tea all over my desk, trousers, seat and (plastic) laminated floor (which is rewarding me by swelling at the joints).

Now I'm cross...! :mad1: I seem to have contracted "clumsy old git disease"!
 
Epic fail indeed!

Ermm, well. Yes. I posted something about it. What I did was desolder each broken key, dismantle and clean the contacts. Then reassemble.

Which keys are broken?
 
Thanks Jon, but I am going to have to wait till tomorrow now, I went to check which keys did not work and the machine started to prompt then went to a blank screen.

I tried powering off etc and got a screen of rubbish.

Know I knew one of my next steps would be to go over the mainboard but I am actually quite glad I had to do it now.

First thing I noticed that the screws that was holding the expansion board in were loose and a big nut was about to fall off right above the PSU. There were no pillars on the bolt so there was actually no way to ever do these up.

No every having a Tandy before I though the expansion board was standard but it turns out to be a Cumana Floppy Disk Interface. So I wonder if I can fit a cumana drive like one from a BBC ????

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I also found a stray wire, one end had come off so I removed it completely, anyone any idea what this might have been for ?

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I took every socketed IC off the board and cleaned them with vinegar and a scotchbrite before washing them off with soap and warm water, tedious task especially with the RAM, only the RAM and the Z80 were tarnished quite badly I guess they get the hottest.

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Anyway the good news is with that done I am back to where I was yesterday, a working computer except for a few keys :D


So my questions are:

Has anyone any idea what the stray wire could have been for ?

Can I connect the easy to get BBC Cumana drives to my Cumana controller ?
 
The wire looks like a post production fix, but I can't be sure.

The BBC disk units use a standard Shugart interface, so should work. But I would advise you get a HxC Floppy Emulator and try that first. They're not cheap, but very, very useful.
 
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