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TRS-80 Model III keyboard fix

VERAULT

Veteran Member
Joined
Jan 30, 2012
Messages
8,227
Location
Connecticut, USA
So one of my TRS-80 Model III's I got about 5 or so years ago came with a bad keyboard. It must have had trememdous weight on the keyboard which caused the pointed post below to just pierce the keyboard PCB. One of its previous owners attempted a repair with alot of wires. I have not dont a single thing to it.

I was hoping someone could show me a high reolution image of the keyboard so I can see the traces. once I get that I can remove all these large jumper wires and wire in some small wirewrap wire to fix the keyboard matrix.

Here are photos: 1.jpg
2.jpg
3.jpg
 
now all repairs are not identical but this is a common breakdown.. mind you yours looks a touch more extensive than mine was here is how I resolve my issue, looking at your the basic layout seems to not be done properly
hardest part will be to track down all the little breaks of the board that spider away from the hole to make sure any residual traces may need repairing. I will try to get you another photo tomorrow of the unbroken M3 keyboard I have,.

 
thanks Patrick but i really need a high res photo of a good bosrd to work with. part of the bosrd is missing and i cant follow the traces.

i really have no i dea why tandy corp. would have a big keyboard pcb resting on a spike for support. thsts a stupid design decision.
 
Well better late than never. Here is my repair job.
IMG_20221130_093318.jpg

I have done better work but this keyboard has a hole punched through it so what can you do. Funny, All my Model III's seem to have had lots of rework done to them Even on this keyboard, many solder pads are burnt off.

I just need to find some rigid plastic to put under the keyboard to protect against the post that punched a hole in it. TERRIBLE DESIGN!!!
 
Looks good - main point is it works!
Radio Shack's technical design intention was always to cut production costs. We all now have to deal with the results ... :oops:
 
In all fairness I should say the bodge wiring was the easy part. Desoldering all the non-working keys to open up and clean the internal rubber dome carbon contacts took 3 days. You know for those last 5 keyswitches you keep putting together but just barely register. But now they all work on both my model III keyboards, (if your going to do a torturous task might as well make it worth it by getting everything done). Unfortunately My only model III with floppy drives has some kind of fault.. Hey vintage computers. You gotta love them.

Something to note, about four of the switches has severe corrosion and green goop inside. And the vast majority of them has severe scratches on the contact plates obviously from a screwdriver.. What a barbaric way to try and fix a a switch. Seems both my model III's were quite tampered with before I got them.
 
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Well this keyboard still isnt working. all the keys work but the reset switch. as you can see i have 2 bodges going to it. With my meter it registers a reset key push. but It still wont reset. I dont get whags wrong!
 
Mick,
The Reset Switch on the Model 3 is drawn incorrectly in the Schematics. I've corrected it and
attached it. The NO and NC contacts were drawn reversed and the Wiper is tied to GND.

You should be able to OHM out the Pins 18, 19 and 20 to make sure the switch is making and
breaking when you depress the switch. You also need +5.00 VDC to the PLUS side of C54
thru R7. That PULLUP resistor is what creates the HIGH and LOW to the CPU via Pin 26.
There is also a RESET* Signal going to other hardware from U57 Pin 13 (74LS02).
Be sure to verify it too.

Even the M3-SAMS-RESET circuit has the NO and NC Contacts drawn in the incorrect place.

Larry
 

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Mick,
Double check that you do not have Pin 18 shorted to Pin 19 in between Pins 18 and 20.
It sure looks like that may be where the problem is located. Your wiring looks correct
in the Photo's, so that is the first place I'd check.

Larry
 
@ldkraemer Well I found the problem! Can you see it? Thats Pin 19 broken off from the connector on the keyboard. Not a great spot, gonna be a little more work to fix. And I still havent figured out what kind of protective barier I need to put on the post to not cause more damage to the keyboard.IMG_20230118_180505.jpg
 
So I was scrounging around and thought I DID NOT have any 20 pin IDC connectors but I was lucky enough to find 1! I peeled back the copper mesh layer and cut the ribbon and desoldered the old connector. Strangely Pin 19 is the only wire exposed under the copper mesh layer, it clearly is meant to make contact with the copper layer as there is continuity.IMG_20230118_182100.jpg

Going to put the new IDC connector on and then test. Hopefully this will do it.IMG_20230118_182051.jpg
 
ITs better but intermittant. Maybe the other model III keyboard was the one I removed all the switches too and re-carbon painted!? Memory fades... Either way when I push either keyboard connector (at board or at keyboard PCB) IT get slightly different output. Maybe the whole cable needs to be replaces... AND the keys need re-carbon. Yeesh these keyboards SUCK!
 
I usually replace the cable with a new one,
also the Model 3 has a capacitor that goes out and prevents the reset button to work at all, I would check C54 likely need to be replaced

VY0sZ4G.png


also it is located on edge of the board near fdc controller side of motherboard
C7pLH3a.png
 
The reset is working now. That broken wire was the culprit. But like I mentioned the ribbon should probably be replaced. I need to source more IDC connectors.

edit: What is the exact name for these connectors because 20 pin IDC only brings up the detacheable ribbon ends or male ends?
 
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