charlesmorris800 at centurytel.net
Wed Aug 14 09:24:47 CDT 2019
Thanks. I believe you are right also :)
The expensive ceramic packages have hermetic seals, not so the plastic
(epoxy) packages used in commercial grade parts.
There are some kind of failures that can be fixed by baking - but I don't
know if this is one of them (if the bond wire is soldered to the die it
might work). If it detached from the weld at the lead frame, no go. Anyway
there are over 100 chips on the ADM-3A board and I would be more worried
about damaging the others with heat.
I just paid 71 cents for another LS193 ;)
From: Dave Wade
Sent: Wednesday, August 14, 2019 5:02 AM
To: 'Charles' ; 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: ADM-3A question
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).
> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Charles via
> 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
> 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
> column terminal :)
> The monitor was occasionally intermittent (no display at all, no HV, +15
> 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
> PCB and it seemed to be fixed - but then the VERTICAL deflection collapsed
> tweaking the height adjustment caused increasing loss. The 100 ohm pot to
> 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
> of run time.
> This ADM-3A could have been unpowered (and in a storage area without
> control) for a very long time. I wonder if that contributed to the
> seeing... hope there aren't any more until I get to use it for a while on
> 8/A (or 11/23+).
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