Re: CBM 8250LP update (hints welcome)

You can run the diagnostic to test this and adjust in real time.    If
you're using a B Series, not sure if there is a drive speed program for the
PET you're using.
BIll

On Fri, Jan 6, 2023 at 4:35 PM Francesco Messineo <
francesco.messineo_at_gmail.com> wrote:

> I mentioned I set the speed for 200ms index interval on the 8250LP
> drives, but forgot to mention that the owner of the 8250 also set the
> speed on the other two drives using the strobo patterns.
>
> On Fri, Jan 6, 2023 at 10:25 PM Bill Degnan <billdegnan_at_gmail.com> wrote:
> >
> > Is the drive speed correct?  There are utilities to test and adjust that
> too.  I may have missed a reference to drive speed in your post but that
> could explain the problems you've reported. THere is a pot to adjust drive
> speed on the drive control board IIRC
> > BIll
> >
> > On Fri, Jan 6, 2023 at 4:10 PM Francesco Messineo <
> francesco.messineo_at_gmail.com> 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
> >>
>
>