Re: CBM 8250LP update (hints welcome)

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?

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