my Data General Nova 4/X, disk woes continue

[Allison]

>
>Subject: my Data General Nova 4/X, disk woes continue
>   From: Tom Jennings <tomj at wps.com>
>   Date: Sun, 24 Apr 2005 16:55:35 -0700 (PDT)
>     To: "General Discussion: On-Topic Posts Only" <cctech at classiccmp.org>
>
>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).

I hope they are not in any timing circuits.  Another way to 
really shoot ones foot is change all logic and any analog
stuff or worse digital used as analog.  Newer or older parts 
often will behave different and at 10mhz that was high speed 
then.  There be serpents and devils lurking there.

>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.

This tells me you've not looked at the cause only apparent problems 
that may not be relevent. 

>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.
>

OK,  the drive uses the same CART as a DEC RL01/02 and the base design 
is a CDC creation if memory serves.  DEC nor DG designed the beast.

There are two things to pay attention to.  Servo amps for the 
positioner they are analog and can drift.  They must be set up 
at TEMP.  If I remember that varient of the drive also had 
embedded servo information for the head position controls.

The second is the read/write electronics have a PLL for read 
clock recovery.  If thats not set up right it will drift 
outside the lock in range and start tossing errors. It also 
must be setup at working TEMP. The PLL is to track drive 
speed errors and irregularities in position and bit shifting.

Did you mean "two state amp" or "two stage amp"?  Likely 
it's not an amp problem, a quick look with any old scope 
could check that as it warms up.  


>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).

Early disk systems like that seem to have their fair share of green,
blue or white and red wires.  Grean being factory changes, white or 
blue were often depot repair and red field implemented.  I'd not be
too concerned if it worked before it should again.



Allison