Both the card edge and pin header connectors are IDC - Insulation Displacement Connector - that describes how they attach to the ribbon cable

Card edge ribbon connectors are somewhat difficult to find though, especially when you're looking for connectors other than the standard 34-pin for floppy drives. And the prices of loose ribbon cables on eBay are just absurd.

Yes I see ebay has MFM Hard Drive cable sets for about $20 and up. I have made my own before from two universal floppy cables....only because I had a lot at the time....
make the 34 pin control cable from universal floppy cable by cutting off just past the first edge connector..
make the 20 pin data cable from a universal floppy cable by cutting off just past first edge connector then cut after #20 like in the picture, is not easy but can be done....

I'll be happy to send the ebay link to the connectors via DM, but site rules prevent me from posting it openly.

Search for "5 Pcs 20 Pin Card Edge Female IDC Connector for Flat Ribbon Cable" on ebay to see the ones I purchased to make my own MFM data cables two years ago.

Nice Max ! those with the "10-Pack 20-Pin (2x10) Female IDC 2.54mm Pitch Connectors for Flat Ribbon Cable" and you could easy make lots 😀

I've managed to find some time to test this stuff and the st-251 is working (at least for now). The bearing does sound a little loud, but having connect with two 34pin floppy cables (the 20 pin uses only part of the cable) I'm able to at least start formatting the drive.

I was very interested in finding out what was on the drive, but this wasn't possible without the original controller. The drive was making very loud banging noises and it wasn't detected.

Unfortunately the format progressed well until the last cylinder. The drive has 820 cylinders. But when formatting 819 it started with the banging noises. Then it tried to validate, but every track would be a bad track...

So I wonder. Is the disk done for? Or perhaps the head is misaligned and it is trying to go to cylinder >820?

I'll try formatting it by telling it the drive has 819 cylinders and see if that helps in any way.

I woukd be grateful for any advice anyone may have.

Edit: Also I didn't know what interleave to set for this drive (there is nothing in the manual) so I set 3 which is recommended as default. Could this be the source of this issue?

Edit2: Setting 819 cylinders doesn't help. Once it completes it starts banging again and displays this:

It seems the other drive behaves very similarly. There is no banging noise, but there is the same message about bad tracks....

This is the built in controller's format utility. Is there any other utility I can use to do a low-er level format perhaps?

Or is the drive dead?

If verification fails on every track on both drives, it is likely that the connection 20-pin cable doesn't make the intended connection. The 20-pin is used for the track data only. So when you format a track, the data to be written is on the 20-pin cable, and when verifying, the data read from the head is on the 20-pin cable. Anything else is performed using the 34-pin cable. So if seeking seems to succeed, but formatting/verifying fails, you might have a problem with the 20-pin cable, possibly it is misaligned or turned by 180 degrees. You should double-check that, especially as you are using an ad-hoc cable, so accidental mistakes are more likely.

mkarcher wrote on 2022-11-27, 16:13:

If verification fails on every track on both drives, it is likely that the connection 20-pin cable doesn't make the intended connection. The 20-pin is used for the track data only. So when you format a track, the data to be written is on the 20-pin cable, and when verifying, the data read from the head is on the 20-pin cable. Anything else is performed using the 34-pin cable. So if seeking seems to succeed, but formatting/verifying fails, you might have a problem with the 20-pin cable, possibly it is misaligned or turned by 180 degrees. You should double-check that, especially as you are using an ad-hoc cable, so accidental mistakes are more likely.

I switched the nec drive to radial, just in case.

I've sprayed all connectors with contact cleaner. I also made sure the signal connection cable is straight (not coiled). This time it did show errors on about 18 first cylinders on the nec drive but verified the rest fine. It then successfully created a partition with fdisk. Format displayed lots of "recovering file allocation table" messages in the beginning of the drive, but paste first few percent seemed to finish fine.

So the NEC disk is at least partially alive.

The st-251 however, every time it tries to read it seems to bang its head repeatedly. I know it can position the heads on all tracks, because this banging noise happens only when reading. When formatting i can hear it clicking from track to track fine.

So I'm leaning towards st-251 being faulty and/or the 20 pin (34 in my case) cable maybe causing some interference? If it sends analog data of few tens of mhz perhaps the extra length is causing trouble? I woukd rather not have to cut one of my last 5 5.25 in floppy cables, but I might, just to test it.

I also heard in st-251 sometimes the plastic stop wears out. What are the symptoms of this?

Edit: I don't have a head parking utility. Is there some debug command from xt days one can use to park a disk head? St-251 has autopart. The nec drive I don't think so.

For the ST-251: Have you tried: IBM AT DRIVE TYPE: 44, 40, or 3** ( **MAY REQUIRE PARTITIONING SOFTWARE) <from docs> ??
Also double check the Terminating resistor pack. If not making good contact then you will get odd errors even with a good data cable iirc.

Horun wrote on 2022-11-27, 18:08:

For the ST-251: Have you tried: IBM AT DRIVE TYPE: 44, 40, or 3** ( **MAY REQUIRE PARTITIONING SOFTWARE) <from docs> ??
Also double check the Terminating resistor pack. If not making good contact then you will get odd errors even with a good data cable iirc.

I have to set bios drive type to not installed as this is an xt controller in an at machine. If I configure drive type in bios it interferes with the controller, but I thing this part is OK, because the nec drive sort of works. Also I enabled turbo on my 386sx board. I think this is what helped me get the nec drive working.

I still suspect the cable somewhat (those first 15~17 bad tracks on the nec drive are very suspicious), but I think there is some problem with the st-251 too.

I disassembled the electronics, I disconnected all the internal connectors and cleaned them with contacts cleaner. I did the same for the termination resistor pack. I also had a look inside.

It looks perfectly normal internally(I didn't touch anything there). The banging noise is basically the head hitting a plastic stop. It is very weird. One would expect the parking area to be outside the platter, but heads actually park towards the center of the platter. (I looked up videos online of those drives operating with the cover off so I know it is not just my drive). When the head assembly moves towards the outer edge of the platter it eventually reaches a limit switch, but there is no such switch on the inside end. I imagine the only way this disk can establish its position is to count steps from the outside in. Perhaps there is supposed to be some tracking info recorded on the platter that's missing too, because when it is supposed to read from track 0 it repeatedly bangs into the plastic stop in the innermost position.

I've heard before about those plastic stops wearing out, perhaps my drive has its worn and when it bangs into it, it already moved past its correct track 0 position? If this is the case shimming it slightly could help. There is also an adjustment screw that allowed ws the stop to be moved inwards (I have no idea why). If track 0 information is recorded during low level format and surface at this place is destroyed perhaps that would help? However I decided to pause and think about it for few days before I do any physical mods. Perhaps someone will have a better idea?

I previously said the platters look good, they are, but there is like maybe a slightly worn area where track 0 is. Not really a scratch, but a slight discoloration visible only at a certain angle. Perhaps that's the reason why it doesn't read it.

Then, again, it is the parking area, so when the drive spins down one woukd expect eventually just before it stops heads would contact the platter there. If I was coming up with a drive design I would never locate the most important track in a place most likely to get damaged. So perhaps track 0 is slightly outwards from the parking area and this discoloration is irrelevant.

Another possibility is that for some reason the drive is not writing at all (perhaps electronics is faulty). Does anyone know if a controller reads anything during low level format before "verifying" phase starts? I heard of those low level formats taking hours and hours, but this controller completed the 20mb nec drive in less than 5 minutes. Perhaps it is doing some sort of "quick format"?

Unfortunately, the BIOS contains only a very simplistic format utility. One selects which drive to format, type of drive and go.

I really woukd love to save that st-251 drive. Also I'm not too keen on letting it bang its heads in that stop too much so I try limiting the duration of testing I do.

I have various electronics test equipment (oscilloscopes, logic analyzer etc) so I wonder if I should hook up a digital oscilloscope to head 0 and record what's going on when it tries to seek track 0. Does anyone know what kind of signals are to be expected from such magnetic head? (frequency, amplitude, acceptable load impedance?) Or am I better hooking up past the first amplification stage? At least with those old drives all components are neatly laid out in separate chips. This might help in troubleshooting.

There is one more thing I woukd like to ask in this context. I recently saw a video of someone fixing 5.25in floppy disk's track 0 by hitting it with a large electromagnet. Although the author talked about "remagnetising the media" he was using AC current with a handheld vhs eraser. So it was more like a degausing rather than magnetising. Did anyone try anything like that with a HDD platter?

If anyone can point me towards any documentation that might help that would be great.

Attaching few internal pics. Everything looking good. I think the electronics is shot. I can't see any track 0 sensor on this drive too.

Well at least the nec drive is working.