[Coladam] Coladam Digest, Vol 77, Issue 3

Matthew Hudson hexsane at gmail.com
Thu Dec 1 04:04:22 CET 2011


On Wed, Nov 30, 2011 at 8:42 PM, kevin laskowski <klasko54 at gmail.com> wrote:

> I have had this problem myself. And the problem was the left disk
> insertion sensor. Very easy to replace, VERY hard to find a working
> array. Cheers. Kev.
>
>
I've recently solved this. It was a combination of problems and a "logical"
error on my part. I initially tested the sensor in the drive with a logic
probe and it was indicating a HI/LOW state when a disk was inserted to
block the write protect. I "logically" assumed the sensor was good. I then
traced the signal and saw the write logic wasn't coming from the U11 floppy
controller. I removed a good controller from another drive and inserted it
into the problem drive. Same problem. I pulled out the scope and discovered
that the logic was dropping but only to around 2.2 volts (still registering
as a logical high to the floppy controller but not the logic probe) and at
the same time discovered the voltage on the drive was to low (4.75 & 11.5)
which probably wasn't helping things. A swap out of a 4700uF filter cap
corrected that but didn't correct the write problem. I then grounded the
write protect line and was able to format a disk. Viola. To sum it up:
Logic probes are good if you know the voltages are correct to begin with
and the write protect signal on the floppy controller prevents the
controller from enabling writes although it will go through all the steps
as if the write protect were not triggered. Ugh. I think thats a design
flaw but I can't call up Coleco and complain about it. ;)

-Matt


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