TRS-80 Model III stopped responding

rescue rescue at
Tue Nov 5 12:50:33 CST 2019

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.

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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