Trying to fix/troubleshoot a D357B floppy drive

By keith56

Master (149)

keith56's picture

09-06-2018, 10:45

I'm trying to fix a D357B floppy drive, the drive is showing no useful signs of life, and does not respond when a disk is inserted.

I've noticed a high pitched squeal from the transistor on the drive belt motor, it seems to be an AN6651... I've found a datasheet on this part, and it seems it's operational minimal is 3.5V - but it seems it's only receiving 2V ... the motor is not faulty, as powering it with a 1.5v aa battery gets it to spin fine.

There's some kind of transistor marked 'BA RK' next to it, which I'm assuming may be a power regulator or something - I've been unable to find any clear info on it, the top pin of the chip (far left in my photo) is connected to the power line of the transistor, but, in fact, ALL the other pins on the BA RK chip seem to be at 2V (I had expected OUT voltage to be higher than the one coming from the big square CPU)

I've tried powering the drive via the MSX, and a stand alone 5v psu, and the 'squeal' and voltages are unchanged, so I don't think it's the MSX at fault

Does anyone have any advice where to proceed, or access to any documentation on this drive? I've been unable to find any repair docs on the drive itself, and I'm unskilled at this kind of thing - anyone with any advice would be really appreciated!

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By RetroTechie

Paragon (1563)

RetroTechie's picture

09-06-2018, 15:28

First things first: drive belt seems missing? Question (or just removed for disassembly / checking the pcb?)

Have you tried hooking up the drive to a PC, with known-working floppy controller, using a known-good cable? (and perhaps check that other floppy drives work OK in that setup)

Any jumpers / solder pads to (dis)connect signals on the drive? Otherwise might be a later model where everything is hardwired rather than selectable through jumper settings... Sad I've seen some info for Mitsumi D359, but seems to have a different pcb. Also have a look (with eyeballs v1.0 Wink ) at the read/write heads - just in case something's bent / ripped off or so.

By jltursan

Prophet (2129)

jltursan's picture

10-06-2018, 11:03

Maybe I'm wrong; but IIRC these Mitsumi drives doesn't have belt. I have a pile of them, and all of them faulty, I've never been able to repair a single one.
From the usual cleaning and lubing to a complete capacitor replacement; so I've always suspected: a) a header misplacement through time or b) a header degradation.
The second possibility has no solution at all and to solve the first, you need some idea of how the header can be adjusted.

About your problem seems indeed power related, not very common. You definitely need the schematics or maybe a known working drive to check it replacing PCB.

By keith56

Master (149)

keith56's picture

10-06-2018, 14:33

The drive motor most certainly should have a belt, but it's missing... not that it matters - as I said, the motor never tries to spin up, I have other MSX computers I could put the drive in, but I'm not willing to risk messing with (and possibly damaging) a working MSX to try to fix this one, I only bought it to practice my repair skills.
I managed to find a schematic of the drive head motor controller chip, and can confirm that it is getting 5 volts as expected, and I've seen some suggestion that the BA RK chip may be an amplifier, but as I said before, why would it be getting 2v in and outputting 2v ... surely it should be outputting 3.5v or more as I believe the drive belt motor controller requires, or nothing?

I suppose the main controller chip could be faulty - in which case the thing is dead, but I'll do some more research, and have more of a think, I don't want to just give up yet, because I'll never learn anything that way.

By RetroTechie

Paragon (1563)

RetroTechie's picture

10-06-2018, 19:41

Judging how it's connected / used, that "BA RK" looks like a PNP power transistor to me. I checked in an SMD codes data book, and found a likely candidate: 2SB1132.

Left pin (lower pin in photo) is the base.
Middle (+ tab, they'll be connected) is the collector, this should deliver the motor current.
And right pin (top pin in photo) pulls that current from 5V supply.

I see a pull-up resistor on the base, and a series resistor going to main controller? Read: pull-up 'closes' the transistor, and the main controller can 'open' it by pulling the base low. Motor current should start flowing with base below 4.4V or so. Interesting to know would be the current flowing through the transistor base pin, when motor should activate. This base current might be measured across that series resistor (in photo, above the round hole in pcb).

Note there may be some intelligence in the controller, that knows motor gets powered, sees no rotation (Hall sensor!), and then decides motor is stuck / faulty / not connected, or belt broken. Possibly leading to repeated on/off cycling of the motor current. Not sure if that's on/off or some analog (or PWM?) type of motor control btw...

How are you controlling Drive Select and Motor On signals? With a floppy inserted, have you seen "in use" LED flashing? That would be a precondition before expecting anything from the drive, I think. And probably wise to add something as drive belt, even if just for testing.