[Coco] FD-50X series power supplies

[Gene Heskett]

On Thursday 29 October 2009, Chad H wrote:
>I was just wondering if anyone has ever heard of the FD-500, 501, or 502
>floppy systems' power supplies ever going bad.  I have a FD-502 unit and
>it's working properly but I just came across a possible PCB part that might
>substitute as a replacement for the existing one should it ever go bad.
>But I don't want to go buying up too many extra parts if these things are
> as rock solid as they seem.  Seems the older my system gets though the
> more paranoid I get about something going wrong with it.  Don't need to
> turn my computer room into a parts warehouse or 'radio shack' either if I
> can help it.   (maybe too late on that one)
>
>
>
>-          Chad

Filter capacitors are a ticking clock, usually runs faster when not powered 
up because the capacitor 'de-forms', losing capacitance slowly.  Or, because 
they are at heart aluminum foil, and its hard to make  a gas tight joint in 
foils only microns thick, they will develop high resistance connections, a 
defect known as poor power factor, or excessive ESR (Equivalent Series 
Resistance).  But for those, keeping them on the shelf isn't great, so one 
should go buy fresh ones, they don't keep for decades on the shelf. Any 
capacitor whose top is bulged up from flat, or which has signs of leakage on 
the bottom s/b replaced forthwith, its already bad.

Semiconductors are next, but generally speaking that's power stuff, and newer 
is generally better.  I rarely try for the exact same part, just use whatever 
the circuit needs.  I had to replace the hexfet power stage of the variable 
speed spindle motor control of my Chinese milling machine a year ago.  I 
looked up the original one on google, then picked up a failed PC psu carcass 
and checked what was in it, it was a way better part, so its in the mills VSD 
now, running cooler than the original ever did.  I could probably double the 
size of the fuse but haven't, cheap plastic drive gears.

In those supplies, there was a resistor or two, one of which was a 1 watt, or 
maybe a 2, carbon based at the time of manufacture.  _Any_ of those with a 
heat discoloration in the middle should be replaced by the same value in the 
modern metal film 'fireproof' variety, and that is usually a 1 time job, the 
new ones will still be good when the universe runs down.

Transformers are of course subject to the laws of inadequate cooling, and in 
some low wattage cases, (wall warts) failure includes the solder eating away 
at the joint where that 40+ gauge primary wire is soldered to the plugin lug 
at some indeterminate time, sometimes in less than 3 years.  For the coco's 
disk drive supplies, an annual trip to the air hose to blow the dust bunnies 
out should reduce those failures to the vanishing point.
>
>
>
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-- 
Cheers, Gene
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