MN3055s and other PS stories

[Matthew S. Padden]

Hi..

Chris wrote:
>
>Couple of you were curious about what happened with my big power supply
>blow ups, so here's the update....
>
>As I mentioned, I blew up two Intl. Power PS's Thursday night., These are
>bi-polar adjustable supplies using 723 regulators and MN3055 power trannys.
>[..snip..]
> I never realized that the MN3055's, that are mounted on
>the OUTSIDE of the case, use thier casings for one of their contacts (the
>collector? - I forget) - yes, yes, this is where you can all groan, because
>you know what's coming... So, it never mentions anywhere in the fact-sheet
>for the supplies; "beware: the metal trannys on the outside of this PS are
>at +26 and -15v!"... I've never worked with these sort of two pin + case
>trannys before, so I didn't know.
>

Great story! I'll bear the advice in mind, as my PSU also uses 3055s (full 
number 2N3055).

I also have a problem with my PSU though. Its output terminals are arranged as 
follows:

-15V 0V +5.5V 0V +25V 0V -15V

After re-stoking the reservoir caps (it hasn't been switched on for 15 years!) 
I gritted my teeth and fired it up; it worked first time. :) Before switch-on, 
I strapped some resistors across each rail so it had something to drive (a few 
mA each rail). All the rails work, but for some reason the -15V rail and its 
adjacent 0V rail float about 14V DC above everything else (The -15V railk 
works relative to this floating 0V line). There's also about 1.5V AC between 
these two pins and any of the others.

Switching off, I checked for continuity between the three 0V pins. The first 
two are fine, but there's no link between them and the one next to -15V. 

Is it possible that the -15V rail is intended as an isolated supply for 
something specific? Nothing on the front panel indicates this; all the 
terminals are on the same terminal strip and there are no suspicious markings.

Alternatively, is it some strange fault? It seems odd that the -15V rail is 
referenced to a separate 0V connection.

FYI, the PSU is an open-frame unit made by Belix, and looks like it could 
supply at least a couple of amps to each rail. There's a 5amp mains fuse, and 
miles of wiring....

Any advice appreciated
--
Matthew S. Padden
Computer Music Research Group
Room M2/08, Music Dept.
Huddersfield University
Queensgate
Huddersfield
England HD1 3DH
p: +44 1484 422288 x2402
f: +44 1484 472656
e: mattp at mindless.com
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