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

From: Brad Parker <brad_at_heeltoe.com>
Date: Thu Feb 5 11:11:19 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.

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"
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.

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 :-)

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

-brad

Brad Parker wrote:
>
>I had a spare moment yesterday so I decided to try and troubleshoot my
>bad M7891-DC (128kw) in an 11/34a.
>
>Looks like I have no +12v. The 12v regulator seems to be working as I
>have 12v up the 2A fuse. But nothing after that.
>
>I'm assuming the fuse has blown. Easy to replace, but I also suspect it
>blew for a reason. (and replacing it might just yield another blown fuse
>:-)
>
>Anyone seen this before? Do they sometimes blow due to transients?
>
>I'm tempted to apply 12v after the fuse with a bench supply that has a
>current regulator and see what it draws. It's been a while since I
>worked with "plain old DRAMs" but I assume the 12v is for them (Vdd).
>
>-brad
>
Received on Thu Feb 05 2004 - 11:11:19 GMT