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-boun...@classiccmp.org> On Behalf Of Charles via cctalk > Sent: 14 August 2019 00:20 > To: cctalk digest <cctalk@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+). > > > --- > This email has been checked for viruses by Avast antivirus software. > https://www.avast.com/antivirus
