Gee, how about another post on this thread that actually relates to the
thread... 8^) Funny how that happens on mailing lists. 8^)
After a little looking and digging, I discovered that the capacitor that
failed is a Sprague series 672D -- indeed designed for switching
use. Newark had 12 of them left, so I ordered 2 of the "real McCoy" just
for authenticity, though I am sure I could have easily found something
adequate locally for a lot less money.
In the Newark catalog illustrations and descriptions, their is NO MENTION
of that third ("extra") axial lead from the aluminum can (that goes off to
the working return -- the 300V Return).
I am guessing that either DEC welded that extra lead on the aluminum can
(or had Sprague do it as a special order) as a safety against shorting to
the heatsink of the transistor next to it. [Since the board is currently
buttoned-up inside the power supply, I cannot easily look without some
effort -- I cannot remember for sure if that heatsink was running to
chassis ground or the +300V return, but I am guessing chassis ground]. If
it shorted, it would short to the 300V return, rather than to a floating
can -- which, if the transisitor heatsink is strapped to chassis ground,
would put +150V or so between the can and the inside of the capacitor --
presumably with "spectacular" results. Then they put all of that in
heat-shrink outside the original plastic wrap on the capacitor.
How would one go about duplicating that feat? One thought that had
occurred to me was to just try (say, with heat shrink) to get a third lead
in contact with the outer can surface -- by taking off the factory plastic
wrap, taping on a wire, and then heat shinking that in a new cylinder. It
would not be a real sound connection, but it presumably would be better
than nothing.
Jay
At 07:09 PM 11/13/2001 +0000, you wrote:
> >
> > At 12:12 AM 11/13/2001 +0000, you wrote:
> >
> > > > Now, off to find a replacement (it is a 3 lead jobber, the normal
> to axial
> > > > leads, plus an extra radial (-) lead from the top of the can --
> perhaps
> > > for
> > >
> > >I don't think there's anything particularly critical about it. I'd use
> > >one of those SMPSU-rated capacitors (sold for TV repairs, etc). I'd
> > >probably fit a 105C one as well, to save having to do the job very often.
> >
> > Let's see...
> >
> > SMPSU, as in Switching Mode Power Supply Unit?
> > 105C as in temperature rating?
>
>Eight both times...
>
>The SMPSU capacitors tend to handle high frequencies better (lower
>internal inductance?). Many of the old-style electrolytics were designed
>for use in mains-frequency PSUs where they only had to work at 100Hz or
>so (well, 120Hz across the Pond).
>
>The 105C capacitors seem to last longer at high temperaturs. And the
>insides of PSUs tend to get a little warm.
>
> > >The value doesn't seem to be that critical. 100uF should be fine if
> > >that's all you can get.
> >
> > Hee hee. That is exactly what the temporarily replacement is. (But it
> > certainly isn't rated for use in switching supplies, and certainly not
> > marked as 105C).
>
>I would get a 'better' one (certainly an SMPSU one -- they're not hard to
>get or expensive now. Most TVs are stuffed with them (or should be :-))
>and thus they are sold for repairing said TVs.
<<snip>>
---
Jay R. Jaeger The Computer Collection
cube1_at_home.com visit
http://members.home.net/thecomputercollection
Received on Thu Nov 15 2001 - 21:53:51 GMT