5 Feb
2004
5 Feb
'04
3:11 a.m.
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