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crock
February 11th, 2017, 12:06 PM
I have a Tandon TM848-2 8" floppy drive where the fuse next to the power connector has blown. No idea yet where the cause is, but I can find no info on the rating of the fuse, and it doesn't appear on the schematics, at least that I can find. Does anyone know what a suitable replacement would be? I read in the specs that there can be as much as 2.5 amps drawn on the 24v line as the spindle motor starts, so in the absence of any further info, I've found some 4A slo-blow axial fuses. Does this sound reasonable?

cheers, Rob

MikeS
February 11th, 2017, 12:21 PM
I have a Tandon TM848-2 8" floppy drive where the fuse next to the power connector has blown. No idea yet where the cause is, but I can find no info on the rating of the fuse, and it doesn't appear on the schematics, at least that I can find. Does anyone know what a suitable replacement would be? I read in the specs that there can be as much as 2.5 amps drawn on the 24v line as the spindle motor starts, so in the absence of any further info, I've found some 4A slo-blow axial fuses. Does this sound reasonable?

cheers, Rob
Hi Rob,

Only two out of my four TM848s actually have that fuse installed and it certainly looks like an afterthought; FWIW it is a BEL 3A, no indication to suggest that it's an SB type.

m

Chuck(G)
February 11th, 2017, 12:48 PM
One of the reasons that the fuse on the 848 can blow is that the 4.7uF cap C6 can short or the 7812 regulator that follows it has failed. You might also check out the 4.7 uF cap at C2. I've had drives fail twice because of the 7812 being run too hot. It's a lousy design and probably an afterthought. If you have a separate 12V supply, you may want to consider bypassing the regulator entirely.

Consider, at a draw of 2A, that regulator has to dissipate 24 watts in heat!

A more pragmatic approach would be to use a "simple switcher" to drop the 24V. Such as the LM2576 (http://www.ti.com/lit/ds/symlink/lm2576.pdf), which would take a few more components, but eliminate that hot spot.

crock
February 11th, 2017, 03:32 PM
One of the reasons that the fuse on the 848 can blow is that the 4.7uF cap C6 can short or the 7812 regulator that follows it has failed. You might also check out the 4.7 uF cap at C2. I've had drives fail twice because of the 7812 being run too hot. It's a lousy design and probably an afterthought. If you have a separate 12V supply, you may want to consider bypassing the regulator entirely.
You were bang on the money Chuck, the C2 capacitor was shorted. That's been replaced and the fuse bypassed and the drive at least runs now. It's still not able to format a disk yet, but I suspect that might be the controller board.

Chuck(G)
February 11th, 2017, 04:33 PM
Just curious--did the bad capacitor have a yellow jacket? Mine did--and I'm wondering if that's one of the first things to replace.

modem7
February 11th, 2017, 05:03 PM
I do not know which model of TM848 that you have, but the photo at [here (http://minuszerodegrees.net/images4/Tandon%20TM848%20-%20replaced%20caps.jpg)] shows a particular make/value cap that I often found faulty when diagnosing my newly acquired TM848's. On careful inspection, some were visibly leaking. I ended up just replacing them all.

Chuck(G)
February 11th, 2017, 05:13 PM
Got photos of C2 and C6? Those are the ones most prone to failure, it seems.

modem7
February 11th, 2017, 06:28 PM
Got photos of C2 and C6? Those are the ones most prone to failure, it seems.
[Here (http://minuszerodegrees.net/temp/3/temp_89fhhvrwy357hf.jpg)].
C1 was what went short circuit on one of my drives.

Chuck(G)
February 11th, 2017, 08:34 PM
I guess it's safe to posit that if you've got a problem with an 848, the capacitors might be the first place to check...

crock
February 12th, 2017, 03:36 AM
Just curious--did the bad capacitor have a yellow jacket? Mine did--and I'm wondering if that's one of the first things to replace.
Nope, black bullet shaped ones. Here's photos of the failed caps and fuse, plus my ugly repair until I receive some axial 4.7 uf caps.

36090
36091

Chuck(G)
February 12th, 2017, 07:52 AM
Is that "dead bug" C7 your work or is that original? If original, it looks as if the factory ran out of axial caps and so used a radial one. Weird.

MikeS
February 12th, 2017, 08:23 AM
FWIW, I have two drives Rev M that have the bullet-shaped black caps and the fuse, a Rev P with the same caps but no fuse, and a Rev R with wrapped aluminum types in C1 and C5 (but still the black bullets elsewhere, incl. C2 and C3); they look like factory vs. field-installed:

36092

Looks like they were a problem but Jugi was too cheap to replace them all ;-).

The -E versions TM848-E are substantially different and probably irrelevant to this discussion.

m

Chuck(G)
February 12th, 2017, 09:15 AM
Well, Mike, you know my opinion of Tandon in general... :)

MikeS
February 12th, 2017, 09:48 AM
Well, Mike, you know my opinion of Tandon in general... :)

Indeed; that crack about Jugi was meant for you ;-)

I must say though that my experience with their floppies hasn't been that bad, especially the 8" drives. Their hard disks though, now that's another matter...

crock
February 12th, 2017, 10:03 AM
Is that "dead bug" C7 your work or is that original? If original, it looks as if the factory ran out of axial caps and so used a radial one. Weird.
Nope, that was as I received it. From the look of the back side of the board, it was factory installed though.