After replacing the RAM, the display is now back to normal (there's also a
test switch on the motherboard (S6) that switches the display from blank
spaces to all zeroes.
The one that failed was a National Semi 2102, whereas the others are all
>from another manufacturer. No sign of previous replacement. Interesting.
(I once fixed up a PDP-8/L, chased down several bad chips. Nearly all were
Signetics 7440's).
I may have made a tactical error though - turns out that wet/damp PVA is
electrically conductive! Enough to overcome the 5k pullup resistors. An
unpleasant surprise. So now the keyboard thinks multiple keys are being
pressed and won't work at all. Removing the keyboard to clean underneath it
would be very tedious since every key has two soldered pins...
There was a significant buildup of goop underneath the key scan mux and
demux chips where the pins are close together (0.1"). I gave the bottom half
of the board another good
rinse and will let it dry overnight, maybe a hair dryer too. Or get some
more 91% alcohol to it and a longer low-temp bake. It all worked before my
"cleanup" so I expect it will resume normal operation once the moisture is
out of any remaining goop. I hope ;)
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The DEC VT20 terminal apparently included a PDP-11/05 with a direct mapped
character display and was intended for text editing and typesetting. It
seems to have been followed by the VT21, and then VT71/VT72, all three based
on an LSI-11 (KD11-F). There's a real lack of documentation about these
online, although the VT72 does have a print set.
Apparently the VT20 used the M792-YK as its bootstrap; the Field Guide is
silent regarding the boot device and M792 documentation stops earlier in the
series of variants.
According to the VT72 print set, it used the MRV11-VC (M9942-YC; described
in the Field Guide as a "bootstrap/diagnostic module") for its bootstrap but
is also silent regarding the boot device. In interestingly, the Field Guide
also describes a MRV11-AA (M7942-TB) as a "M7942 with VT52 emulator, VT71
bootstrap".
For async. communications the VT20 used a DL11-B (M7800 (EIA)). the VT72 a
DLV11-F (M8028).
Looking in a DEC "Options and Modules" listing I see VT20 bundles including
Typeset-11 and DECset-11, and it appears that the VT20 could be configured
with two displays & serial lines in a single 11/05.
So . is the boot device in these systems the remote host via the serial
line? What protocol would that have been? Something native to Typeset-11
and DECset-11?
paul
Thanks Bill, I hadn?t seen that particular page. As I mentioned already, the
formerly clear ?stuff? was so deteriorated I could just pull the glass plate
off with gentle fingertip pressure.
I ran a bead of clear silicone around the outside of the clean plate and CRT
face and bonded them back together.
Display looks great!
However, I can now see that every other line, starting with line 2, is
showing a full line of double quotes (0x22) instead of spaces (0x20). I read
the circuit description and schematic, and it appears that bit ?2? is stuck
high on the even-line RAM ? for some reason the designer decided to call the
LSB bit 1 instead of bit 0.
Typing (for example) ?abcdef123? shows the correct text on the blank odd
lines, but on the even lines it echoes as ?cbcfef323?. Confirming that stuck
bit.
Looks like the RAM at location H15 should be the bad one... we?re having a
heat wave and it?s too hot upstairs to work on it until tomorrow morning at
the earliest.
ETA: Now it's tomorrow morning and just cool enough (although 100% humidity,
at least outside) but my replies aren't showing up in the archive - filtered
somehow.
Anyway. I did a bit more Googling and discovered that plain water dissolves
the PVA goop just fine. No need to use a lot of expensive alcohol which
seems to be a less effective solvent anyway!
So I took the board out and scrubbed it in the kitchen sink with running
warm water and an old toothbrush. Rinse with distilled water, now gently
baking in the oven at around 140F to get the water out of the keyboard. Then
onto the RAM replacement.
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I have access to 3 ES45s, a DS15, and an RA8000 in a tall blue Compaq rack
in Athabasca, Alberta. All the in-service disks were removed but all the
spares are available. The box also has the fibre switches used the connect
the RA8000 to the servers and the cables, much of the paper documentation,
and assorted doodads. It would probably work if plugged in but it has been
a year since it was turned off.
Athabasca, Alberta is about 1.000km North of the US Montana border and
10,000km from nowhere but it is summer and the weather is nice so we might
be able to load this great heavy beast on a trailer and haul it up to a
day's drive away if anybody want this stuff. This is rural Alberta so
a day's drive is a l-o-n-g way (like 1,000km?).
I tried to give this away in early 2019 but the deal fell through. If
I don't get a place to send it then I will keep the DS15 and convert the
rest of it into scrap metal.
--
Richard Loken VE6BSV : "...underneath those tuques we wear,
Athabasca, Alberta Canada : our heads are naked!"
** rlloken at telus.net ** : - Arthur Black
Testing 1,2,3... my last couple of posts don't seem to be showing up?
-Charles
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> From: Paul Birkel
> Apparently the VT20 used the M792-YK as its bootstrap; the Field Guide
> is silent regarding the boot device and M792 documentation stops
> earlier in the series of variants.
An M792-YK recently sold on eBait; I didn't get it, but I did manage to get
the seller to put up good photos of the board, so was able to dump the
contents.
I didn't fully disassamble the program, but it was clearly something serial
line related. With the VT20 info, it's now clear what it was for. It should be
pretty easy to fully disassamble, and work out the protocol.
I have the dump of the contents if anyone has a use for them.
Noel
> From: Charles Morris
> my last couple of posts don't seem to be showing up?
I see several posts from you.
To check suspected failures, look in the archive:
http://www.classiccmp.org/pipermail/cctalk/
because just because you're not getting a personal email copy, doesn't mean
it didn't go out to the list.
E.g. you may have the 'Receive your own posts to the list?' option in your
CCTalk subscription disabled.
Noel
>On 8/10/19 8:33 AM, Charles via cctalk wrote:
>> I also scrubbed off the black anti-reflective coating since it was
>> significantly scratched and peeled anyway.
>
>um.. you didn't remove the aquadag from the outside of the crt, did you?
No worries Al, I got my EE in '81 back when CRTs were still in use and I
know how they work ;)
I was referring to the translucent stuff on the (removable) glass implosion
plate itself, designed to reduce light reflection, not the 'dag.
Some say that black sheer pantyhose can be used to recreate the same effect,
or a 3M Privacy Screen.
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I decided just to fire it up and see if anything blew... it issued the
expected beep as it came up.
Set the switch to half-duplex and it does actually echo bell (Ctrl-G) and I
can see the screen moving as I type!
But there is some kind of garbage every other row, although the cursor
moves and the screen will "clear" (except for the 12 rows of garbage).
Power supply is 5.11 volts. So far so good.
Unfortunately the screen rot is even worse than I thought - the PVA layer is
so opaque and bubbled it looked like cottage cheese once removed, and I
can't make out what characters are on the CRT... I took the monitor out
(very easy disassembly). I didn't even have to use a heat gun or a hot wire,
just started at one corner, applied a steady and gentle pull, and it came
off in one sheet with a giant sucking sound.
That oily mess took a while to clean up. I also scrubbed off the black
anti-reflective coating since it was significantly scratched and peeled
anyway.
Now waiting for the bead of clear silicone (around the outside of the plate
only) to cure. I'll make sure it's air tight so no black dust will get in
there by electrostatic attraction.
I can fix the RAM problem once I can see what is actually on the screen! ;)
Also the wire bracket with threaded ends that holds the flyback to the
monitor chassis is missing one end entirely, so I need to fix that (keep the
core halves firmly together).
Looks like I didn't get TOO bad a deal for $200 shipped, especially with
upper-case installed.
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