ADM-3A question

[Dave Wade]

Charles,

I believe that TTL chips suffer from failure or detachment of the bonding
wire that runs from the die to the interconnect pin, which would result in a
floating pin as described.]
Not sure if environmental storage affects this as chips should be sealed...

I have also recently seen it suggested that heating the chip up in an oven
could affect a temporary repair (sorry I can't find the reference now).

Dave
G4UGM

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Charles via
cctalk
> Sent: 14 August 2019 00:20
> To: cctalk digest <cctalk at classiccmp.org>
> Subject: Re: ADM-3A question
> 
> 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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