Otrona Attache: The adventure continues

Tony Duell ard at p850ug1.demon.co.uk
Mon Oct 6 12:56:18 CDT 2008

> 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?

I've not looked at the schematics, but in general it's easier to 
troubleshoot a 'write' fault than a 'read' one (if only because less 
analogue circuitry tends to be involved) and the fact that the machine 
doesn't format correctly indicates a write fault as well.

Now, the 765 is certainly doing something, in that the heads move, the 
machine _thinks_ it's formatting and so on. I would start by looking at 
thw Write Clock signal (is it present, is it the right frequency). Then 
look at the write data and write gate outputs from the 765, trace them 
through any precompensation circuit to the disk drive connector.

And then I'd look at the read clock. Is it running, is it on-frequency 
when you try to read a disk?


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