TRS-80 Model III stopped responding

rescue rescue at
Tue Nov 5 21:34:30 CST 2019

On 2019-11-05 13:50, rescue wrote:
> I haven't dug into this one yet, but I did get it booted in trsdos,
> and the letter I on the keyboard wasn't working.
> I shut it off, disconnected and removed the keyboard.  I desoldered
> and removed the APLS switch, opened it up, cleaned up the carbon pad
> and the contacts below it, reassembled it, tested it with my DVM, all
> good to go.  I soldered it back into the keyboard, put the keyboard
> back, powered on the system....
> I get CRT glow, system reset button will cause the floppy drive to
> seek, but nothing on the screen, and pressing return after inserting
> TRSDOS does not boot the drive (i.e. testing for a working core 
> system
> with no display....).
> I do need to test the power supplies and make sure I have not lost a
> power rail on one of the two supplies.  I presume the one I need to
> check is the one on the backside of the cpu board, as the one on the
> side of the drive 'cage' powers the drives and the drive controller
> board, and on power up and reset, the drives are motor on and
> tracking, so I think that supply is at least providing +5/+12V.

Well, it is something with the supplies.  I tried the floppy power 
supply in place of the system one, and got a Cass? prompt !
Then I switched back, and got a Cass? prompt !  GRRRR !   I'm still 
digging.... maybe something failed and if I mount it back up it will 
stop working again (once the ground pads are grounded on the back of the 
supply ?).

I'm still hunting for the issue, it's either intermittent or something 
flaky with the power supply that fails when it is grounded.

Now to go off and find out which.

At least the mainboard didn't go 'poof'.

-- Curt

> Seems odd that putting the keyboard back in resulted in a non working
> system.  I unplugged it, same behavior with no keyboard plugged in.  
> I
> did not connect the kb connector off by one pin or one row.... so as
> best as I can tall, Murphy has struck, and it isn't 'operator error'
> :-).
> Any tips from Model III experts welcome.
> --  Curt

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