my Data General Nova 4/X, disk woes continue

[Joe R.]

At 04:55 PM 4/24/05 -0700, you wrote:
>I've been working on my car project
>(http://wps.com/AMC/1970-AMC-Hornet/index.html) because car season
>is coming up soon enough, but I peeled time out for the Nova here
>and there.
>
>I originally repaired a popped +5V logic bypass cap on the
>Read/Write Board, (some monolithic ceramic job), and changed all
>the funky old caps for new ceramic disks. That worked for 20 - 40
>hours of operation without a single error. Disk had formatted with
>zero media defects.  Then the disk started throwing read errors
>that moved around (bad blocks later good; more and more bad
>blocks...).  Diagnostics never indicated any error other than read
>errors; seek, format, diags all run 100% perfect except read
>errors.
>
>I suspect the Read/Write Board. So I ordered modern monolithic
>replacements for the remaining old mono chips from Digikey, and
>shotgunned all the old electrolytics etc for good measure, in two
>places electrolytics replaced with tantalums (didn't have
>.82uF's).
>
>Install board with all new chips and alcohol wash, format -- hard
>100% read failure! D'OH! Long story short, I found internal +6V
>was not; turns out I installed a tant backwards. doh. New tant,
>+6V fine.
>
>Formats, reads OK, then errors again! Sheesh! System has been on
>for two hours (I'm getting paranoid, so temp stabilize the whole
>thing); halted diags, loaded disk heads, installed drive cover,
>pushed into the rack, left for an hour for the drive temp to
>stabilize (this is a religious rite, might as well kill a chicken)
>then format and test again.
>
>After warm-up, it's worse. Signals all through the path look
>OK, hot and cold. I can see no difference, but this is 10 MHz
>NRZ data, in a two-state amp with AGC. I think I'm screwed.
>
>
>The R/W board is a piece of work too. I suspect it's an early
>revision, as the board is a hack job, covered in cut traces,
>components and jumpers on the bottom, and there's a !#$%!! trimpot
>crammed in somewhere as well as kludgey RC (lopass) networks in
>some digital logic. Plus, the PC board was milled out for access
>to a middle layer. Not Good. Plus, it doesn't match the schematic
>(though it's close enough to poke around in with a scope).
>
>Photos of the nasty board here,
>
>http://wps.com/NOVA4/pitchas.html

   How come it says "forbidden" when I try to get a close up view of the
picture?


>
>About halfway down.
>
>
>Anyone got a spare board?!


  Maybe! There's a pair of Nova 4s in a local scrap yard. One looks
complete and the other gutted.  Exactly where is the board located in the
unit?  Where are you located?

    Joe


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