ADM-3A question

Charles charlesmorris800 at
Tue Aug 13 18:19:51 CDT 2019

After hanging vertically for 36 hrs in a hot upstairs room, more goop seeped 
out from under the keyboard. It now works again. Whew.

While running on the bench for burn-in testing, a cursor problem suddenly 
appeared... it would only move every other keystroke. With the technical 
description and schematic at hand, it wasn't hard to track down a 74LS193 
up/down counter with a blown (floating) LSB output. Confirmed by manually 
toggling that bit and the cursor would move back and forth one position. 
Meanwhile I removed the bad chip and put in a DIP socket. Naturally my TTL 
collection didn't have an 'LS193 so I'm waiting on that. So I have a 24 
line, 1 column terminal :)

The monitor was occasionally intermittent (no display at all, no HV, +15 and 
drive signals OK). It seemed to change with movement of the wiring harness 
from the main board to the monitor, too. I reseated the edge connector on 
its PCB and it seemed to be fixed - but then the VERTICAL deflection 
collapsed and tweaking the height adjustment caused increasing loss. The 100 
ohm pot to the base of the vertical output transistor had picked that moment 
to go open. Changed that out and readjusted everything - so far so good 
after another hour of run time.

This ADM-3A could have been unpowered (and in a storage area without climate 
control) for a very long time. I wonder if that contributed to the failures 
I'm seeing... hope there aren't any more until I get to use it for a while 
on my PDP-8/A (or 11/23+). 

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