Reversed Electrolytics redux

Jay Jaeger cube1 at
Sat Aug 22 18:06:54 CDT 2015

On 8/22/2015 2:41 PM, Jay Jaeger wrote:

> Unfortunately, bearings did not help the floppy drive, so now I am busy
> taking the floppy power supply I got with my FD 100-8, making sure it is
> up to spec to use with the Shugart (and correcting that if needs be),
> and making it safer (mains fuse, switch) and more fault tolerant (fuse
> in the 24V line, at least) and making sure all of the mains connected
> innards are not exposed.  Just about done with that.  The +24V and -7 to
> -16V are in spec, but the +5V (which, curiously for a 1A supply, does
> NOT use a simple 3 terminal regulator) is a tad high, at 5.35V.  I may
> just change it to use a three terminal unit.

Well, just found another reversed electrolytic - but this time the
original fault was not my own.  I decided to trace out this floppy
supply I have had for decades (back in my original Altair days), that I
got from the same guy from whom I purchased my Siemens FDD 100-8, and
discovered that the electrolytic on the unregulated -7 to -16 V was
reversed - installed BACKWARDS.  That capacitor is now in "reform
school".  ;)

Also, I now know why the +5 V is high.  It has a UA 7805 3 terminal
regulator serving as a voltage reference for a TIP-3055 so the supply
can provide more current.  That 7805 has a forward biased diode between
the center terminal and ground, thus raising the output voltage by right
around 0.5v.  I expect I will leave it alone.

The same design is used for the +24V, which comes out at 24.2V.

So, odds are that this supply could drive two floppy drives.



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