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Floppy drive repair info needed - bad R/W head?

WimWalther

Experienced Member
Joined
Jan 12, 2019
Messages
449
Location
St. Paul, MN
Howdy,

I have here a Chinon-made Enhancer 2000 floppy drive, which is a CBM 1541 compatible designed for use with C-64 & VIC-20. It worked just dandy until.. suddenly, it didn't. There was no warning - just powered it up one day and it wouldn't read or write discs. The logic board seems largely ok, as it communicates via the serial bus, operates the motors, etc.

My suspicions fell on the R/W head, here's what I've found: The head connects via 5-wire shielded cable with red, yellow, black, white and shield contacts.

Red-yellow reads 26 ohms. I can get no reading between white/black and/or any other color. Shield connects to gnd on PCB end of cable.

Shouldn't there be TWO coils in that head - one each for R/W and erase? Or maybe separate R&W coils? Sound like the head has an open coil?


Thanks in advance.
 
Should also mention that the analog board, the one that's part of the drive mechanism, is also working - at least in that it's able to operate the spindle & head drive motors under control of the digital board (which is itself an SBC, with RAM, ROM, 6502 CPU & 6522 VIA chips - just as in the 1541 drives.)
 
There may be three, actually, if the R/W head is center tapped.

Do you see a MC3470 series pre-amplifier located close to the terminal block for the R/W heads? Prior to makers consolidating the read / write / erase circuitry into a single LSI, the the R/W head would be connected to a pair of FETs ( or switching diodes ) that electrically isolated the head from the pre-amplifier during write. If a FET failed, it could allow the head to be driven continuously.

Anyway, you should be seeing more than one coil. Test at the head itself to exclude the wiring. Also, when testing use a current limiting resistor in series with your meter. A 10 K resistor should be sufficient to limit the current through the windings. This prevents you from frying the windings with your meter - which can happen.
 
Hi Bob, thanks for stepping up.

The MC3470 IC is present, and there are 4-5 transistors in the same area. I don't recognize the case style, but they're not the typical TO-92 - they're smaller and Iack the semi-cylinder shape.

I was not aware that the head coils were fine enough to be fried by a DMM.. Hopefully I've not caused further damage.

Suppose the next step is to remove the head and verify the wiring? There's a shield over the solder pads where the cable joins it that needs to come off.
 
Also, when testing use a current limiting resistor in series with your meter. A 10 K resistor should be sufficient to limit the current through the windings. This prevents you from frying the windings with your meter - which can happen.
I was not aware that the head coils were fine enough to be fried by a DMM.. Hopefully I've not caused further damage.
My memory is that this sort of thing can happen with analog meters.
Compared to digital meters, analog meters use a higher measuring current (in resistance mode) because that current is what deflects the needle.
And that up to the battery voltage is applied to the component under measurement; a possible problem for some components.
 
Yes, head coil death is very common, at least on the Mitsumi/Newtronics drives.

Chinon not so much that I've seen, but it can happen.

26ohms should be across the whole RW coil. You should have a center tap of about half that, typically ~11ohms. I think they are all around the same design. The erase coil or tap will have a diode, so it only measures one-way.

 
My memory is that this sort of thing can happen with analog meters.
Compared to digital meters, analog meters use a higher measuring current (in resistance mode) because that current is what deflects the needle.
And that up to the battery voltage is applied to the component under measurement; a possible problem for some components.
You never know what type of meter someone is using. I'll admit its being over cautious for an 1980's floppy head - they were designed for a few milliwatts of dissipation. But of all the components in these drives, the least easy to source is the head, and possibly more subject to damage.
 
Yes, head coil death is very common, at least on the Mitsumi/Newtronics drives.

Chinon not so much that I've seen, but it can happen.

26ohms should be across the whole RW coil. You should have a center tap of about half that, typically ~11ohms. I think they are all around the same design. The erase coil or tap will have a diode, so it only ...
A diode in series with the erase head might explain my inability to obtain a reading - but it doesn't explain the lack of a CT connection, unless it too has a series diode.

The meter I'm using is my kept-handy RS 22-811. I like these meters - fairly rugged, cheap, really good feature set (for the period), inexpensive and accurate. Settling time is longish, typical for basic auto-rangers of the time. Long batt life, too.. auto shutoff helps a lot.
 
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