14 Aug
2019
14 Aug
'19
7:24 a.m.
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 ;)
Charles
WB3JOK/0
-----Original Message-----
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
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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