On Sun, Feb 12, 2023 at 3:52 PM <afachat_at_gmx.de> wrote: > > 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. no, that was happening on some 1571s I serviced a while ago, restoring the proper spring action would restore a perfect read/write capability. Although it was more of head angled and not perfectly parallel to the disk surface. > > 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. pressure and head inclination was the first thing I thought of and I tried to increase the pressure and slightly vary the head inclination, nothing changed, or better, nothing improved, it turns out that anything I tried actually made the signal even worse. If you watch the read/write waveform video, the one with the scope vertical gain low enough to show the actual write signal, you can see it looks too "spike-y" when writing to the top head, write interval always happen before a read interval on the same head, so it's easy to tell what head is writing/reading to/from. I think the only explanation for distorted voltage waveform is a change in the inductance properties. Of course I may be wrong, that's why I asked for hints in the first place. Frank > 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 > > > > >Received on 2023-02-12 21:00:03
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