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Mitsubishi MF355F-250MG HD Floppy - only reliable with DD disks

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First post, by snufkin

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I've been having fun with a 1993 Mitsubishi drive. This used to work a year ago, but has since developed a, or several, problems.

I've with some suggestions I've got it to the point where it now seems to reliably read DD disks (Norton disk doctor happily breezes through a surface scan), but not HD disks. It can eventually read the disks, but with lots of 'data error, Abort, Retry, Fail'. If I keep hitting R then it eventually copies ok. It'll pass a surface scan in Norton, but only with lots of seeking back to track 0 and it takes a long time. I've tried with multiple disks that are fine in another drive.

I thought maybe it was an rpm problem. That looks very slightly fast, but what I thought was the speed adjuster turns out not to be, so I assume the speed is fixed. I've had a go at measuring the output of the /RDATA pin whilst running the Alignment test in IMD on track 70 of one of the HD disks I've been testing with and it looks like this:

RDATA_HD.png
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RDATA_HD.png
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I've also got one for DD:

RDATA_DD.png
Filename
RDATA_DD.png
File size
4.9 KiB
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293 views
File license
Public domain

They both look sort of ok to me. 0-5V, slight overshoot on Hi-Lo, a bit slow to come back up, but within what look like the 2us window for HD disks. Does anyone know what the waveform should look like? I've had a quick look on Google for examples of the waveform but can't just spot anything, and getting the other floppy drive out so I can use it to compare with is quite a lot of hassle, so I thought I'd check here first to see if anyone knows if these look ok.

Thanks for any advice.

Reply 1 of 3, by waterbeesje

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Can you check if the heads are properly aligned? If the alignment is just minimal out of track it may still get the data but three next track could interfere, giving you these errors. Not sure why that would involve 144 disks only...
Of maybe there are bad caps on the drive? Those may also cause read errors.
Last: did you verify the disks working ok on other machines?

About the wave form I don't have the slightest idea tbh

Stuck at 10MHz...

Reply 3 of 3, by snufkin

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Thanks for that, it sounds right. I've been poking and prodding at it on and off, and just found that there's 3 pin device marked 362, which I think is a 3.6V Zener diode connected to +5V, with the other side going through some resistors and off to what I think is the read/write amplifier. I've measured the voltage across it in place, and it's only about 2V, which seems odd. Out of circuit my cheapo component tester says it's 3.3V, which is (just) outside the spec in the datasheet for it (3.55V).

Debating whether to get a 3.6v zener to test with. May wait until next time I'm placing an RS/Farnell/Mouser order.

Hmm, haven't thought this through yet, but could I test by removing the zener and attaching a NiMH cell (~1.4V) ? 5V supply, minus the 3.6V zener should leave 1.4V?

There's always a possibility that something got damaged when the two electrolytic caps leaked. It looks like the power design tries to isolate the motors from the control, but without the smoothing caps (they measured down in the pF range when I took them out) then maybe some nasty spike got through to the control side. There's an inductor separating the two, but that may have been eaten by corrosion (the resistance is measuring ~4ohm, which seems high for an inductor to me).

Very frustrating. If it'd just properly break then at this point I'd kind of not mind too much. But it keeps feeling as though it's close to working, and I just need to find the one thing that's stopping it. If nothing else, it's got a bunch of jumpers on it to make converting it to an Amiga drive easy. But it's not like 3.5" drives are rare yet.

There's a question... At what point do you finally accept that something's broken?

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