More Commodore PET drives - more help?

From: Dave Dunfield <>
Date: Sat Sep 25 06:16:42 2004

Hi Tony,

>> It looks like the on-board CPU is not running at all.
>I would guess that's likely to be the case (although I've never worked on
>this unit).
>In which case I'd start by checking the +5V power rail, then check the
>CPU clock and reset pins (make sure the thing isn't held in the reset
>state), then see what the aaddres and data lines are doing.

All good advise - assuming that it's running (clocked), and the address/data
bus looks ok (being driven, no signs of contention etc.), data being fetched
from the ROMs, I would probably next check the ROM content against the image
on funet (I've seen a number of these ROMs go bad).

Opened it up this morning - unfortunately, it is very badly corroded inside,
and the drive unit itself shows a LOT of rust.

Some of this equipment originally was rescued from a shed which we were told
flooded with water every year - one casualty was a 2040 disk unit which has
**VERY** badly corroded drives and plenty of rust all around the lower section
of the case - we also have a B128 in which the edge connectors and any exposed
sections of copper have gone green and fuzzy - not to mention all lables/stickers
etc. have disintegrated - amazingly, after an initial cleaning of the worst of
it, the machine still works! - one of my upcoming tasks will be to completely
clean it up and restore it.

I'm guessing this drive must have also been near the bottom of the pile.

Board looks not too bad - power supplies check out, after cleaning and reseating
all of the socketed chips and connections it still appears completely dead, so
now its on to more serious debugging...

Replaced ANOTHER 4116 DRAM in a CBM 2001-N last night - this makes 7 or 8 I've
changed so far - symptoms are always the same - amount of memory "found" by BASIC
differs, usually higher when the machine is cold, and lower if the machine is
reset after it warms up (meaning the RAM gets worse after it has been on a while).
Fortunately most of them have been able to run BASIC or at least the diagnostic
monitor, and a little pokeing and peeking identifies the bit which is affected,
allowing me to locate the exact chip without to much trouble.

Have also replaced 4-5 2114s - I'm amazed that so many RAMs have gone bad in the
Commodore equipment...

dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools:
com             Vintage computing equipment collector.
Received on Sat Sep 25 2004 - 06:16:42 BST

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