14 Aug
2019
14 Aug
'19
5:25 a.m.
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 :-(
Regards,
Peter Coghlan.