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

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Feb 5 14:03:32 2004

>
>
> To follow up on my own post, replacing the fuse created a *hot* 12v
> regulator. My DVM indicated that something was shorting Vdd.
>
> Figuring it was a cap, I started pulling the + lines of the big caps.
> On the second one the short went away.

You were lucky. Normally it's the last one on the board that turns out to
be shorted ;-)
 
> Turns out one of the large Sangamo 47uf caps across Vdd was a dead
> short. I removed it and now the +12 is happy and the board works.
> (I'll replace it in my next digikey order)
>
> Do old electrolytic's short out? The MS11 parts list just say "AL EL"

I've had it happen. Remember that the plates of an electrolytic cap are
the metal anode and the electrolyte. The dielectric is the oxide film on
the surface of the anode. If that disolves back into the electrolyte,
then you can get a dead short.

I had this happen in the PSU module of a Canon CX laser printer. I was
testing various bits of it (it seemed fine), I left it for a few hours,
and when I came back I found a dead short on one of the unregulated power
rails. Turned out that the 10000uF smooth cap had decided to go
short-circuit at that moment. Thank %deity I didn't power it up like that!

-tony
Received on Thu Feb 05 2004 - 14:03:32 GMT