Hi
Most of the parts in the read path are easy to replace but
there is a ROM ( 3601 ) used in the read window that is
hard to find. This is part of the DDL used to create the
read window from the data. Make sure the R-Pack used to load
the input signals is working OK. It should be 150 to +5.
Both the LS74 and the LS174 are common failures in
the TTL devices.
Dwight
Date: Thu, 2 Oct 2008 00:55:26 -0700
From: derschjo at
msu.edu
To:
Subject: Otrona Attache: The adventure continues
Thanks to TonyM, I have a replacement power supply that works and I now
have the Otrona mostly running. (I do intend to repair the original
power supply -- I found what looks to be a bad transistor and I've
ordered a replacement (can't find it at the local shop.)).
The Otrona almost works properly, it just won't boot from floppy. I
need a bit of help debugging the floppy controller. Thus far I've
verified that the following are good:
- The drives themselves and the disks I'm trying to boot from (hooked
the drives up to my PC and used ImageDisk to dump the disk contents
successfully)
- The drive cable
- The NEC 765 controller (was originally bad! replaced with a known-good
765 from an old PC floppy controller and this improved things somewhat.)
Symptoms are: Drive spins up, seeks to track 0 but won't boot; running
the ROM diagnostics (thanks to Bob for a scan of the service manual!)
helps narrow down things a bit:
- The "Format" diagnostic runs, which means the heads load, the drive
seeks, and enough is working so that the Otrona thinks it's formatting.
I've tried reading the resultant diskette using ImageDisk and it chokes
on it, so I'm not sure it's formatting correctly.
- Reading or Writing a sector on a known good, CP/M formatted floppy
fails. The error code returned (4004) indicates a missing Sector ID.
There aren't a lot of chips involved in the floppy controller (about 12
or 13) so I'm hoping this shouldn't be too hard to track down. Any
suggestions on where to start? (Schematic at
http://oldcomputers.net/Attache_Schematics.pdf on page 6). Would it
make sense to start investigating the chips leading back from the /RD
DATA pin?
Thanks as always,
Josh
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