13 Sep
2014
13 Sep
'14
10:46 p.m.
On Sat, 13 Sep 2014, Noel Chiappa wrote:
OK, next! As I mentioned, I have two systems I'm
working on, and I'm
starting with the 11/23. It uses a Sigma BA11NL-1 chassis - very similar
to the DEC chassis of the similar id.
(Speaking of which... the chassis was in a box with a DEC "11L03-HA"
label on it - I can't work out if whoever owned it upgraded it with the
Sigma [which is Q22] or if DEC supplied the unit with the Sigma. I can't
work out why DEC would have used an outside supplier for that - unless
either i) Sigma could build a chassis much cheaper than DEC could, or
ii) DEC was having supply issues, and turned to Sigma to fill the gap.)
Anyway, so I've had it apart, and it's generally in very good
condition... with this exception in the power supply:
http://ana-3.lcs.mit.edu/~jnc/jpg/PDP11s/SigmaBurnedConnector.jpg
This spade lug appears to be the +5V common, although I'm too lazy to
check the prints to absolutely confirm that (there are two other similar
wires coming out of the transformer, but without the white stripe of
this one, and they both go to a honking great full-wave rectifier, the
output of which goes to a heat sink (!) which appears to be doing double
duty as a bus bar).
Clearly, there was probably excessive resistance there somewhere,
although it's not clear if it was across the spade connector itself, or
in the wire-connector junction (note the brown spot on the insulating
shield - I first thought that indicated that the wire-connector joint
was the problem, but iff the spade connector got hot enough, the heat
could have gotten into the wire, and caused that spot).
So here's my question: what do I do? Do I:
- Pull the spade connector off, clean everything well (e.g. with a
Dremel wire brush), and put it back together, and hope?
- Replace the connector on the wire side only (slightly tricky, as
there's not much spare length in the harness, so I can't clip it off,
I'd have to cut the old one off with a Dremel wheel)?
- Try and replace the lug on the board too? (It looks like it got hot
enough to melt the solder - the solder has the surface crazing to it.)
That might be tricky...
- Do nothing and hope (on the basis that it didn't melt down yet :-)?
Whatever I do, of course, I should monitor it, and I'd like to avoid a
lot of grief replacing things unless I really need to, but I'm genuinely
uncertain what the best course is here. Thoughts?
The first question that comes to mind for me, is the terminal even
original to the chassis or did someone replace it at some point in the
field? I would have expected to see a nylon insulated commercial type
terminal in a chassis, and not a vinyl insulated terminal. Is that
terminal a dual crimped type with a separate insulation crimp? It looks
like it might be, and I suppose someone may have used a vinyl insulated
commercial type terminal originally, but those are not as common as the
nylon insulated variety in commercial applications.
The .250 tab type terminal on the board is likely made by either AMP or
Molex and would probably be pretty easy to source as a replacement part.
They aren't expensive, either, and both Digi-Key and Mouser stock them.
What caused the excessive current draw though? Is there a marginal part on
the pc board? You shouldn't be seeing that much current draw though a pc
board.
