NOVA4 6070 disk booboo

From: Tom Jennings <tomj_at_wps.com>
Date: Wed Feb 2 12:36:43 2005

Decided to try to get the Data General 6070 disk (10 fixed 10 removable)
going last night. There's reasonably complex LOAD/READY logic,
but from the indicators it wouldn't go ito either state.

Got out the schematics and docs, found a nice startup flow chart
and checked each signal. (DG does a great job on docs; there was a
flowchart and table of signals, each with a drawing/page/zone
callout!) so I went down the list and looked at gate inputs; power
supply checks (6 supplies all threshold checked), cover in place,
a bunch of random logic for zero RPM, temp difference (fixed to
removable), heads HOME, blah blah -- turns out it was the fromt
panel LOAD/READY switch! It drives an RS flop but the grease in
the rocker switch hardened/corroded so it was a two position
OFF/OFF switch. Disassembled, cleaned, LOAD/READY worked.

Got proper LOAD state, turned to READY, spindle spun up, brush
cycled, ... no head load. ALl the logic was OK and saw the
RNENABLE or whateveritwas command, just didn't move. The linear
motor had acquired stiction from sitting. I took the dust cover
off, EXTREMELY CAREFULLY extracted the head assembly about 1/8"
'til it clicked the HOME microswitch, a few times, it loosened up.

Reassembled, READY, spinup, brush, head load.... CRASH. Top
surface of the fixed disk unmistakably crashed.

While I had the dust cover off the first time I did look at the
heads and the platter surfaces. I didn't clean the heads (I do
have the right stuff for the job; hydroentangled sterile wipes and
99.5% anhydrous isopropyl) first, and maybe I should have looked
closer, but I saw the typical "I" shaped pole in the white ceramic
substrate, assumed it was OK.

Post-crash, there was definitely a lot of FeO on the head. Cleaned
that. Ovbious grooving on the platter; 'far as I can tell all
other (3) heads/surfaces are fine. Can't see the bottom of the
fixed but thehead remained clean.

Upon closer examination of the offending head there is definitely
something caked on it on the "far" side of the head (from where I
can reach) that won't come off with the cleaning pads. It may have
been there, but if so, it would indicate a previous crash.

Knowing that the platter was trashed, I took off the dust cover,
cleaned the platter with a new pad, and felt for large bumps, and
ran the READY sequence again. Crashed two more times then I
stopped.

I'm 90% sure (...) that there was a preexisting crash, but it's
entirely possible it happened on my first power-up. Since it's sat
for so long all sorts of subtle things couldhave happened which
makes that seem likely, though I did examine it before power-up,
though probably not closely enough.

It suuuuure looks like the platters on the removable cartridge are
physically the same as the fixed platter; and since I have two
spare (three total) carts I may attempt surgery.
Received on Wed Feb 02 2005 - 12:36:43 GMT