My ongoing Nova 4/X saga...

From: Tom Jennings <>
Date: Sat Feb 19 22:20:12 2005

Turned the nova on to try to run diags again, and AGAIN another
tape drive vacuum column switch died! Really. Tape won't load, so
I've learned to check each switch by sucking on the hose before
anything else -- big leak.

So I decided to fix the damned things. I disassembled six grey
switches; in two the membranes were misshapen, probably a fine
crack. Two of them were not (yet!) leaking, but it seems
inevitable so I disassembled all four.

I took some photos of the repair process; I'll put 'em on my
website later.

Turns out there are two different membrane designs. Most are a
simple thin plastic (not latex) membrane; two of the still
not-leaking ones had an additional black reinforcement. In this
model (no external or marking difference, seems like a revision
issue) the membranes were fine; I reassembled them without change.

The others I replaced with kitchen saran wrap. Three of them
worked just fine; I was able to adjust for 10" and 20" of water,
they seem quite stable. Time will tell if the new membranes will
hole, saran might "creep" with time. If they pop I'll find
another material.

One leaked after repair; again after 2nd repair. I'll check it for
a leak in other areas of the assembly.

Disassembly involved drilling out rivets; reassembly was simply
four #4-40 x 1/2" screws, nut, lockwasher.

The black vacuum switches are made by FAIRCHILD and are a terrible
design; two membranes, on each side of the vacuum chamber! These
are latex; it had practically dissolved and adhered to the copper
switchleafs. Cleaned up OK. Harder to replace the membrane, sued
saran again. The black ones are NOT adjustable, they are fixed at
20" water. The one I repaired checked out at 19" - 21", pretty
damn close. The others are repairable, I'll do them later.

So... the tape loads perfectly! All five switches are repaired
ones. Looks like this problem is solved, unless the saran
stretches under pressure, and with time. I'll check this later
with a scratch tape, eg. leave the tape LOADed for a few days then
re-check the calibrations (PITA).

I boot the diag tape.... nothing happens! Reset, I, 100022L... no
tape motion. Weird. After many checks I power off the CPU, turn
on... fails power-on test! Aaargh.

Took out the BBU MEMORY, cleaned the card edge with alcohol got a
lot of crud off, reinsert card... boots fine! Sheesh.

It's running N3MORT S. (There's no N3MORT L on the tape).

What an afternoon! For all the stupid problems I think I'm getting
to the root of problems.
Received on Sat Feb 19 2005 - 22:20:12 GMT

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