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Tandon TM848-2 fuse

C

crock

Experienced Member
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Jan 12, 2012
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Zurich, Switzerland
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
 
crock said:
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
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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
 
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, which would take a few more components, but eliminate that hot spot.
 
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Chuck(G) said:
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.
Click to expand...
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.
 
I do not know which model of TM848 that you have, but the photo at [here] 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) said:
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.
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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.

DSCN0126.jpg
DSCN0128.jpg
 
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.
 
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:

tm848b.JPG

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
 
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Chuck(G) said:
Well, Mike, you know my opinion of Tandon in general... :)
Click to expand...

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...
 
Chuck(G) said:
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.
Click to expand...
Nope, that was as I received it. From the look of the back side of the board, it was factory installed though.
 
Thanks for the cap tips! I just got my hands on an 848-2. Doesn't look to thrashed. Gently removed front bezel and the hub clamp assembly and cleaned out with windex and q-tips. Not very much dust and gunk. Whew. Interesting is that a region on the pcb at the front showed corroded traces near several of the mentioned 'typically' kaput ones. Hmmmmm... Got those replaced. No shorts on C2 and the like. One cap apparently was apparently struck and one lead was pulled out. Replaced it too. Good to know about the 12-volt down regulator on the 24v input. I have some 4A adjustable switcher modules, so might as well only feed in like 14-16 volts to reduce the stress on that LM 7812/340-12! A drop of silicone oil on the head assembly travel rod -slides nicely.
 
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