Re: CBM 8250LP update (hints welcome)

On Sonntag, 12. Februar 2023 15:27:57 CET Francesco Messineo wrote:
> On Sun, Feb 12, 2023 at 3:18 PM <afachat_at_gmx.de> wrote:
> > I'll certainly have to have a look at your videos.
> > 
> > Out of my 5 low profile disk drives (2x8250LP - one standalone and one in
> > a
> > 8296-D) and the 1001 IIRC last time I checked 3 or four had this head 1
> > problem.
> > 
> > Do you have any ideas how to improve the head 1 situation? Maybe increase
> > the gain on the head 1 RW analog circuitry or so?
> 
> no idea, the 4 heads are switched by diodes, so the gain is fixed at
> the preamplifier for all heads.
> Since also the write signal was different, I think that the head
> somehow has developed some crack on the ferrite core, or otherwise
> another mechanical fault that doesn't alter the resistance but does
> alter the inductive properties.
> Of course you might have a slightly misaligned top head, but I don't
> recommend fiddling with that alignment if you don't have the right
> tools (programs, reference disk) and patience.
> It is scary that the low profile 100 tpi drives seem to have this
> tendency, they're extremely rare. However I think one could
> try to transplant an HD 96 tpi head (if a mechanically compatible HD
> head does exist in the first place).

It is indeed scary. One other possible cause I heard someone mentioned was 
that possibly the pressure of the upper head is somehow reduced due to 
mechanical fatigue in the springs that push the head down. This would (could) 
result in less magnetic flux. This could have been caused by storing the drives 
with the lid open.

What do you think? Could this be a reason?
(maybe a test would be to add a small weight on the upper head and re-run the 
tests with the scope attached.

André

> HTH
> Frank IZ8DWF
> 
> > André
> > 
> > On Freitag, 6. Januar 2023 22:06:42 CET Francesco Messineo wrote:
> > > Hi all,
> > > just a little update on the 8250LP drive I started some weeks ago.
> > > Basically, when I started, it seemed like the 6502 "FDC" would not
> > > execute any meaningful code. I found it had a dead PHI1 output, but
> > > would work fine into a PET. Also I've verified that PHI1 isn't
> > > connected on both the 6502s on these drives. In the end, by swapping
> > > the two 6502, the 8250LP started to have some life.
> > > It would however not read the disks (2) formatted with another 8250
> > > (non LP) that I received together with the unit to be serviced (I
> > > don't have any 100tpi drive myself).
> > > I've changed the capacitors on the Panasonic mechanics and verified
> > > that the nearby traces weren't damaged, I've removed the darkned
> > > solder mask around some leaking caps and re-tinned the traces. I've
> > > then verified and slightly tuned the spindle speed on both drives
> > > using the index sensor that I wired to 5V with a pullup resistor and
> > > ground, the sensors on both drives are not used by the Commodore
> > > digital board but are wired to pins 21 and 22 of each drive's
> > > connector. I've found both drives to have 201ms interval between index
> > > pulses, so I slightly tuned both to have 200ms nominal pulse interval.
> > > I believe -/+ 3ms is ok however.
> > > I've then tried formatting new floppies and drive 1 could complete the
> > > format on just about any floppy I tried (more than 20 of them, never
> > > failed one). However drive 0 would never finish formatting even on
> > > de-gaussed media.
> > > I've identified the problem of drive 0 to be a marginal or defective
> > > r/w coil on head 1, see these short videos:
> > > https://www.youtube.com/watch?v=_IWwMZAJjcI
> > > https://www.youtube.com/watch?v=BsykFp8qZw4
> > > https://www.youtube.com/watch?v=fn6r5pwQEqo
> > > 
> > > The first one is read signal on drive 1, the format routine formats
> > > first the lower side (head 0), then the upper side (head 1) then step
> > > to the next track, You see there's no difference between the read
> > > signal on head0/head1 on the first video, this is drive 1. The
> > > interval when the signal disappears is a write phase, the signal is
> > > too big to be seen with the vertical scale I was using.
> > > Second video is drive 0 attempting to format, there's a noticeable
> > > difference between head 0 (normal) signal and head 1. Of course, as
> > > the drive progresses to inner traces, the radial speed of the media
> > > decreases and the signal gets eventually too low.
> > > Third video uses a lower vertical scale on the scope so also the write
> > > waveform is showed. The write waveform is also slightly different when
> > > it's switched to head 1.
> > > Of course I've tried to swap the drives to exclude head select diode
> > > issues on the analog board.
> > > Out of desperation I've also tried to see if I could re-align the
> > > upper head on drive 0. This isn't something to be attempted without a
> > > reference disk and the right programs on 100 tpi drives, but after a
> > > few hours tweaking the head position (I've got a macro picture of the
> > > mounting position before starting), I haven't at least made it worse.
> > > Drive 0 is still able to read all tracks on a disk formatted by drive
> > > 1, albeit with some occasional head bumps on media having a not
> > > brilliant S/N (and only when reading tracks on head 1).
> > > 
> > > Now, the really puzzling part is: why I can't read the disks formatted
> > > on the other 8250? All I get is 20 READ ERROR 39 0
> > > Things that I've checked:
> > > DS0/1 start at 1/1 then change at tracks 40, 54, 65
> > > The GCR rom has the correct dump.
> > > I've written a program that leaves the motor on, and can step in or
> > > out by pressing a key on the PET, it seems the disks formatted on the
> > > other 8250 have lower signal (as if alignment isn't really spot-on)
> > > but the analog signal isn't too bad to justify a read error anyway.
> > > Sync is clearly recognized or I would get a 21 and not a 20.
> > > Is there something else I'm missing? The likely explanation would be
> > > that two drives in the 8250LP have exact track alignment and two
> > > drives in an 8250 also have exact alignment between them but one pair
> > > is badly out of alignment?
> > > This seems really unlikely to me.
> > > Any hint is welcome.
> > > Frank IZ8DWF