M7891-DC 2a fuse on +12? fixed!

From: CRC <technobug_at_comcast.net>
Date: Thu Feb 5 15:35:25 2004

On Thu, 05 Feb 2004 12:11:19 -0500 Brad Parker <brad_at_heeltoe.com>
pounded out:
[............]

> Do old electrolytic's short out? The MS11 parts list just say "AL EL"
> which I assume is aluminum electrolytic. For some reason I was
> thinking
> it would be a tantalum (based on it's silvery look) but now I'm
> thinking
> not. I think I'll replace it with a nice axial lead tantalum.

If you are going to use the equipment only occasionally, a tantalum cap
is an excellent choice. However, with the current reliability of
electrolytic caps, the added expense is something that has to be
considered.

> I know sometimes badly mfg'd electrolytics will leak, but I've never
> seen a dead short - but my expeience is mostly with *new* products, no
> ones that are so "experienced" as this one :-)

With old equipment, especially those items that have been in storage a
long time, an electrolytic will depolarize. When you hit the ON button,
the dielectric layer is missing and the conduction can cause a short,
or occasionally, a small bang as the cap and its contents scatter
around the chassis. Leakage is generally due to badly formed dielectric
layers. In a pinch, you can cure a leaky cap by applying voltage of
reverse polarity of about 10% of the rating for several seconds and
then bringing the cap slowly up to its rated value to reform the
dielectric.

To safely re-energize a piece of equipment that is long in the tooth
and has been sitting around for some time, you have to bring the
voltage up relatively slowly to allow the electrolytic caps to
re-polarize. Everyone who is collecting should own a good-sized Variac.
You power the box up by bringing the voltage from naught to the
operational level with the Variac over a period of about a second or
so. This should allow the input caps (biggest and most expensive) to
re-polarize without taking out the fuze, or worse, the rectifier. Even
with switchers, this will also have a ripple effect throughout the
system unless the unit has power management. I have some 50 y/o stuff
around here that continues to work without repair by taking the above
precautions.

In your case, I would replace all the bypass caps on the board if you
have had a failure in one - although good, they are probably marginal
at this point in time. When replacing electrolytics, I typically go for
the 105 degree items if they will fit (the current versions are
substantially smaller than their predecessors), or for higher voltage
beasties with the same temp rating. I also add heat protectors to caps
that have been sited next to power resistors or large, silicon embers
(monitors are notorious in this regard). Heat will take out an
electrolytic faster than anything else.

> Normally I wouldn't post this sort of thing but I figured someone else
> might find it interesting/amusing.
>

Only if it doesn't happen to me...

> -brad
Received on Thu Feb 05 2004 - 15:35:25 GMT