12 Nov
2001
12 Nov
'01
9:45 a.m.
Tony, you get the "superhero" rating in my book. With minimal description
on my part, you pegged the failure spot on. (Except that the designation
for the capacitor he identified as C16 is C4 on my schematic -- it was
pretty clear to me which one you pointed me to). Sure enough the 82uf
electrolytic had bit the dust. No external evidence of trauma, but when I
pulled it, it failed on my capacitance meter, and a temporary replacement
got the power supply running again. (
DC On is lit -- a very good sign -- I have a Unibus map module which is
supposed to blink the DC On LED if voltages are out of tolerance, and the
CPU card self-test LED's end up in the correct state. I won't be hooking
up the terminal, etc., until I get the correct replacement capacitor, but
there is no reason to think that it isn't just fine.
I'm curious: Other than the fact that if it were leaky it would drag down
the +13V, was there some other particular reason you were suspicious?
Incidentally, the startup drive stayed at about +9.2V.
Now, off to find a replacement (it is a 3 lead jobber, the normal to axial
leads, plus an extra radial (-) lead from the top of the can -- perhaps for
shielding. The "testing" replacement was a normal cheap electrolytic (and
15V underrated at that -- sheepish grin). That will be an adventure in
itself.
Funny thing: I think I have had 4 power supply failures in my
collection. For sure I have had 4 that were caused by bad
electrolytics. My original Altair had one die (and the ones in there
almost certainly need reforming). In a PDP-11/05 one died and took a
transistor with it. On my PDP-12, the main capacitors needed reforming and
took a bridge with them, and now this one. Capacitors won't catch up to
bad light bulbs for repairs, but they're catching up. 8^)
Reminds me of the story of the IBM 1414 I/O Synchronizer on an IBM 1410 at
the U. Wisconsin School of Business. Every day we had to wait 5 minutes
for the "sync check" light to extinguish on our 1403 printer after power
up. It was that way for years. Finally, one day a senior CE and our usual
CE had had enough, got the machine from us (probably one summer day), and
looked for the cause. It turned out that manufacturing had installed 20 or
more bypass electrolytics on the machine's backplane *backwards*. They
replaced them all, and the problem went away for good.
Thanks again, Tony!
Jay
At 11:52 PM 11/11/2001 +0000, you wrote:
---
Jay R. Jaeger The Computer Collection
cube1(a)home.com visit http://members.home.net/thecomputercollection
I have had a PDP-11/24 for some years, working just fine (last time I had
it on was about last May).
Yesterday it decided not to work. Looks like the H7140 supply. The +300V
I've come across this PSU in the 11/44....
Good luck, it's by far the most complicated SMPSU you're likely to work
on. There are 3 independant chopper circuits in there. Yes, the bias
supply is also a switch-mode device (!).
The chopper transistor for this supply is Q25 on the bias/interface
board (sheet 3 in the printset). The chopper transformer is T1 on the
H7140 motherboard. The chopper is driven by E15 (555) on the
bias/interface PCB (sheet 3 again), with regulation applied via Q15 and
Q26 (again bias/interface sheet 3).
on the voltage doubler is there, but the +13V in
the Bias/Interface board
reads about +7V. I suspect this is central to the problem. (The +12V
Bias
is also reading about +7V).
Yes, that is going to cause problems. From what I can see, the 12V line
(rectified output of the bias chopper transformer -- rectified by D6 on
the motherboard) is essentially unregulated. The regulation, such as it
is, comes from the winding linking 3-4 on the chopper transformer, which
also provides the +13V (on the 'hot' side of the PSU) via D36
(bias/interface sheet 3).
Now the fact that the +12V (on the secondary side of the PSU -- you have
realised the grounds are not common, right?) is present would seem to
indicate that the chopper is running. Just not producing the right voltages.
I would start by looking at the schematic on page 3 of the bias/interface
board prints. Check the electrolytics first -- for some reason I don't
much like the look of C16 (82uF), which is the smoothing capacitor for
the 13V line.
Any words of advice? Does anyone have a
technical description of the
H7140? I have schematics (and can read them at the component level), but
power supplies are hardly my strong point -- it would be useful to have
some description of just how this animal goes thru a power up
It appears, from a quick glance at the schematics, that the startup
voltage comes from the 150V midpoint on the voltage doubler via Q23
(bias/interface sheet 3). This gets E15 oscillating. The chopper
transformer then outputs the 13V line (which takes over from the startup
supply) and the 12V line to the rest of the PSU's electronics (the latter
being on the isolated 'secondary' side).
sequence. Plus, blind diagnosis will be slow
-- there aren't any test
points, and one needs to make sure the +300V from the doubler has been
bled off before yanking cards to solder test tails at strategic points. It
It's nasty. 380V or so on screw terminals at the top of the PSU. Lethal,
in fact!.
When I was debugging one of these (I have memory PSU problems in my
11/44), I connected a 10k high-wattage resistor across my voltmeter
probes. After pulling the mains plug I touched said probes onto the 380V
screw terminals to discharge the capacitors. Then I could start pulling
cards.
But it's not my favourite supply to work on by any means...
-tony