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SF354 repair - Chinon 14 pin drives


Tony Jay

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I have a Atari 520STFM with a dead single sided drive internally. Head will move to track zero location once, and then drive just hums. Move the head manually, power up, moves and then hums - same behaviour. Sounds a lot like the track zero sensor issue I have repaired in 1050s several times. I am going to replace with Goex drive (on order)  but wanted to have a working drive in hand for 'originality' if I ever sold. So bought 2 external SF354 drives on eBay for low price as they had the same long button access as my internal drive, no no case modifications. I also had a floppy power supply and I/O cable so took a chance on these. arrived and both are non working, which I sort of expected. I opened to find the typical gooey black remains of 40 yo belts, which I replaced but still no function even though the drive motor spins the disk. So I thought I would try these in my 520STFM, but to my frustration, these drives are the origianl '14 pin Chinon' drives, not the 22 pin floppy ribbon cable. Ouch. So back to some more diagnostics. The boards get 5V and 12V DC to the board but I have no schematics to trace. The two 100uf caps looked ok, but I replaced them with no benefit. Drive spins, stepper does not move. When I insert disk and try to format, after short time I get an error message that the drive is not connected and to 'check connections and power'. Yet I see the drive icons on boot so the ST is aware of the drive being conected. Checked the pin continuity of the one I/O cable (I can't believe how expensive these are now!) that I had from years ago, and all connections are intact.

 

The power boards on these are really minimal. Few transformers, resistors, ceramic caps, and can't see any diodes. There is only a single IC, a 7407 which I have read is a 'hex/buffer driver' that I think controls the actual 5V output (maybe 12v too?), which is the only thing left I could replace. They are not in sockets (of course they are not LOL). Any thoughts from experts here on whether this IC could be the problem. It is a bit odd that I got 2 drives with exactly the same behaviour unless the previous owner fried these somehow and then never threw them away.

 

And yes I know these are SS and will not read many disks, are beyond ancient, and should be trashed after smashing with a hammer so no one else would ever be sucked into trying repair LOL. But I hate throwing electronics away. Thanks.

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On one of my versions of the 354 I had some problems with a broken part down the front that detected whether there was a disk in the drive (it was a light detector or something like that, that determined if there was a disk in the drive), and the drive would not function without it. Mine was physically broken but I did replace it and there was a thread on Atari forum about it somewhere - it was a long time ago and can't remember much about it (unfortunately I can't find it as I made the 'mistake' of trying to change my email address there and my account is now 'inactive'). It was located near the drive opening iirc, and if yours has perhaps gone bad it might create the circumstances you mention perhaps. So could be that is worth a try?

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3 hours ago, Zogging Hell said:

On one of my versions of the 354 I had some problems with a broken part down the front that detected whether there was a disk in the drive (it was a light detector or something like that, that determined if there was a disk in the drive), and the drive would not function without it. Mine was physically broken but I did replace it and there was a thread on Atari forum about it somewhere - it was a long time ago and can't remember much about it (unfortunately I can't find it as I made the 'mistake' of trying to change my email address there and my account is now 'inactive'). It was located near the drive opening iirc, and if yours has perhaps gone bad it might create the circumstances you mention perhaps. So could be that is worth a try?

Thanks! I definitely will give that a try. I have an Indus GT drive for 8 bit Ataris where that was a problem, but always would get a bit of activity on the stepper. So will try and report any success. Interestingly these drives do not have two rails for the R/W head . Just one and the stepper shaft acts as a second. Never seen that before but I guess that can eliminate the pulley assembly.

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49 minutes ago, dhe said:

https://docs.dev-docs.org/htm/search.php?find=_f

 

This site has service manuals for both SF314 and SF354 - maybe they could provide some food for though?

The SF354 model here is a newer version with a ribbon cable (34 pin?) and not the 14 wire connector of the Chinon drive, I guess by 1985, Atari had abandoned these very early mechanisms for more common ones. There is a schematic but IC is M5101AP Disk Drive Read/Write and logic IC and not the 7407 gate. Thank you for the link, which I have bookmarked. No doubt I will need it in future repairs !

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  • 1 month later...

Replaced single (and only) IC 7407 on power board which arrived last week, and still no drive access activity. Drive motor spins, no disk seeking. Checked continuity of the cable pins and they look ok. Probably zero track optical sensor or stepper motor which would be terminal. So I hate to give up, but going to put this attempted repair on long pause LOL. On the brighter side for my 520STFM, installed a Goex floppy emulator, which came with custom long button insert. Looks great (like OEM) and works like a charm. Even makes noise with access. Not missing my floppies much LOL

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