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Fixing Floppy-Drives

Started by Shining, 23:45, 12 January 15

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Quote from: Audronic on 12:33, 09 January 19
The voltages at rest should be :-

IC206    74LS38.    Is it a 74LS38 ?

Using a CRO or VoltMeter

Pins 9 and 10  + 4.1 Volts.  Pin 8  + 0.2 Volts
Pins 4 and 5    + 0.2 Volts.  Pin 6  + 4.8 Volts

Monitor Pin 6 ONLY and do a Cat it should at some stage toggle LOW to + 0.2 Volts.

If not then Monitor pin 8 and repeat the test. It should go from + 0.2 Volts to +4.8 Volts.

Any of the above don't  change then :-  Change the IC206
IC206 is indeed a 7438 (quad NAND gate) and I haven't changed it yet.
But, pins 9 and 10 are tied to pin 29 in the FDD, which is US1. I put up a screenshot of that signal a few posts ago (it averages out to be around 4V since it's high with some pulses to 0).

I can confirm that pin 8 is correctly the opposite of that, and that pin 6 is similar to US1 (the negative of pin 8 again).

But when I do CAT, US1 (and any of the other pins) never change like they do on a working CPC. And because there's activity on those pins, it's also clear that they're not just shorted to 0 or 5V. That's what makes this so weird.


It could still be IC206. There's a difference between a signal going high/low on a scope and actually being able to drive a gate it's connected to. I'd change IC206 anyway before looking further.



Quote from: Bryce on 13:00, 09 January 19
It could still be IC206. There's a difference between a signal going high/low on a scope and actually being able to drive a gate it's connected to. I'd change IC206 anyway before looking further.
I'll try it because there isn't much else I can think at the moment (and I have a spare one from another CPC), but remember that the FDD IC isn't even setting the signal to a stable low to start with during a CAT operation.


I just learnt something new I didn't know. Putting it here to share with others. Remember how I was a bit puzzled by the high US0/US1 lines with periodic pulses to 0V? It turns out the FDD is polling to see if there are disks on the different drives every ms.

QuoteAfter reset has been sent to the µPD765A/µPD765, the unit select Lines US0 and US1 will automatically go into a polling mode. In between commands (and between step pulses in the seek command) the µPD765A/µPD765 polls all four FDDs looking for a change in the Ready Line from any of the drives. If the Ready line changes state (usually due to a door opening or closing) then the µPD765A/µPD765 will generate an interrupt. When Status Register 0 (ST0) is read (after Sense Interrupt Status is issued), Not Ready (NR) will be indicated. The polling of the Ready line by the µPD765A/µPD765 continues continously between commands thus notifying tthe processor which drives are on or off line. Each drive is polled every 1.024ms except during the Read/Write commands. When used with a 4MHz clock for interfacing to minifloppies, the polling rate is 2.048ms.

And this reminds me of one more thing: This CPC, unlike any others I know, when I turn it on, I hear slight noise in the floppy drive. Like it's doing one tiny step or moving the head ever so slightly. It happens every time I turn it on.


OK, this gets crazier! (but it's a new data point, so I'm hopeful).

Before I had tried Pyotr's USB floppy disk emulator on the edge connector. Now I decided to try a regular Gotek. It also worked.

But... I decided to check out drive A while I had the Gotek connected and... it worked!!!

So I started eliminating things:
- The Gotek could have the disk removed and it worked
- The Gotek could be unpowered and it worked

And this is what I came up with: As long as the Gotek cable is plugged into the edge connector, it works.  :o :o

It's a ribbon cable with a couple of switches. The one on the left sets the Gotek to be drive A or B (I imagine it commutes the NSEL1 and NSEL2 signals). This only works if it's in the "B drive" setting (meaning, the Gotek is the B drive, so supposedly not changing the signals around). The switch on the right it's an "external head drive selection switch". I don't think that one matters which setting it's on.

One more piece of data: If while I have the Gotek cable connected, I remove the 3" disk, and I do a CAT, the computer hangs there forever.

It's as if the FDD isn't detecting that the disk is in, and with the Gotek cable (somehow) it's forced to think it's in (even if it's not).

Does this make any sense to anyone?


I think I might have figured it out!!!

That cable has two connectors tied together: pins 1 and 2. That's the READY signal tied to GRND. Which gets flipped by an inverted and passed as ready to the FDD.

For drive A, the ready pin is 26, which is supposed to be connected to pin 1 on the edge connector and... it's not! I don't see a visual break (and I even desoldered the cable and cleaned up that area yesterday), but there's definitely no connectivity. I'm about to add a bridge and keep my fingers crossed.

To be continued...


That was it!!! Wow! That was an intense repair! This is actually why repairing computers is so much fun  ;D For every 10 boring RAM-burned-out repairs, you get an challenging case like this one.

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