24 Nov
2013
24 Nov
'13
9:09 p.m.
On Mon, 25 Nov 2013, Mark J. Blair wrote:
Yeah. I'm not quite sure if it's worth the effort...the PSU needs a VERY
good cleaning...which will prove difficult. Transformers don't look too
good...but they don't look riskily bad, either.
I might also do it merely for the sake of repair and restoration practise.
--
Cory Smelosky
http://gewt.net Personal stuff
http://gimme-sympathy.org Projects
On Nov 24, 2013, at 19:55 , Cory Smelosky <b4 at gewt.net> wrote:
Yup.
On Mon, 25 Nov 2013, Ethan Dicks wrote:
Okay. I grabbed some pictures and I think I've found the culprit(s).
http://uploads.gewt.net/me/psu-goo.jpg
The white glorp might be hot melt glue, or it might be some leaked innards of something.
The yellowish block to the right of the glorp appears to be one of those ^#%!$$#%
metallized-paper dielectric EMI filtering caps that I mentioned. The TRS-80 Model 12 power
supply of mine which blew out one of those caps was also made by Astec. I would replace it
with a class "X" safety-rated EMI filtering cap with the same voltage and
capacitance ratings, but NOT a metallized paper type. Identical Rifa caps (actually
Kemet-Evox-Rifa due to acquisitions) are still available from Digi-Key, but I would NOT
use those blasted things.
I theorize that the failure mechanism is that after a
few decades, the plastic cap shell cracks, allowing moisture to get it. That gets absorbed
by the paper dielectric, and eventually the dielectric breaks down under power.
Polypropylene film dielectric is supposed to have very low moisture absorption, so I
figure they should last longer, even after their seals degrade.
Notice the cracking of the plastic on that cap in your picture. If it hasn't already
blown, it will.
Yeah.
http://uploads.gewt.net/me/deformed-component.jpg
I have NO idea what that...deformed...thing was once, but it does not look
quite right. (directly behind the...transformer?)
The one right above where it says "C102"? That looks like another one of those
&#%!*&$* metallized-paper dielectric EMI caps which has blown its top. If you
can't find a schematic diagram to get its ratings, then maybe somebody else here with
a BA23 can try to read the markings off that cap in their supply. 
I found that safety-rated EMI caps other than the (#&%*^$ Rifa ones have different
lead spacings. At least in my Model 12 supply, there were extra holes to allow caps with
different lead spacings to be substituted, and the Panasonic caps I bought dropped right
in. Their lead spacings weren't quite right for the Rifa caps in my Model 4 supply
(also made by Astec). Pick out caps with long wire leads instead of short pins in case the
lead spacing isn't the same as any of the holes on your PCB.
Yeah. I'll always go with long-lead ones incase I make a mistake anyway.
;)
If you can get the numbers off the top and sides of those caps, then I can help you pick
out some replacement parts if you don't know how to. I did this just last week for a
couple of my TRS-80 systems, and I expect I'll be doing the same thing again once I
get started on my PDP-11/44 project soon.
I have an H7140 with a present 300VDC rail that does nothing else. (It
measures 320VDC but I can't see anything obviously blown). I need to get
the replacement for it, though. I just need to pick it up...not sure if
it'll work.
No DC OK light comes on.
I recommend replacing every one of those metallized-paper caps that you encounter,
whether it's already blown or not.
I'd agree with that. I need to do the same on some wax-paper cap gear
that still works fine.