ADM-3A question

Peter Coghlan cctalk at
Wed Aug 14 07:25:13 CDT 2019

Charles wrote:
> 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+). 

I had a similar cascade of failures on an ADM-5.  Each time I fixed a fault,
another one arose.  At one point, the faults were happening quicker than I
could put them right!  Initially, there was some logic fault which required a
74LS125 to be replaced.  Luckily I ordered several because a short time after
that, another 74LS125 failed.  Before I could fit the new chips, the raster
collapsed to a vertical line and slowly faded out.  I initially thought this
was due to a bad connection in the wiring harness to the monitor board but I
eventually tracked it down to a bad joint on the line driver transformer.
This probably arose when I accidentally dropped the lid rather hard.
I think a few other faults which I don't recall now also arose during the
faultfinding session.

Perhaps the ADM-3 and ADM-5 both suffer from these sort of multiple failures?

I dug out my ADM-5 just now to see what chips I had replaced and while I was
at it, powered it up.  It seemed to work ok initially, in that the cursor
appeared (after I wiggled the noisy brightness control) but before I could 
find a loopback connector to make more complete tests, I heard a soft whoosh
from the insides and when I lifted the lid, was greeted by a cloud of foul
smelling smoke.  It seemed like the sort of smoke that mains filter capacitors
make but disappointingly, it does not seem to have such a capacitor :-(
It is currently sitting outside waiting for the smoke to dissipate before
further investigation.

I have a second ADM-5 which has a logic fault that I have not been able
to trace.  While there is an ADM-3 maintenance manual available, I have not
been able to find one for an ADM-5.  There is some commonality but my problem
seems to be in an area where they differ :-(

Peter Coghlan.

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