Wow, thanks for your fast response. I won't be able to work on it much for
a day or two, but this will definitely help. One thing I'll have to do is
run and find an article or two on switching power supply organization. I'm
sure I have some around. I have a rough idea, but if it isn't just the
capacitor, I'll want a better idea of how these guys work.
This is really helpful -- it is one thing to look at a schematic and think
you have a fair idea, but another to get that confirmed from one who
obviously knows what they are talking about.
I really appreciate the help and guidance.
At 11:52 PM 11/11/2001 +0000, you wrote:
> >
> > 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 (!).
Yeah, I had noticed that. Ironic, a bit. I have an old HP disk drive I
got a few years back. It had a completely blown out bias supply (as in
charred). Yanked the whole thing and bought a little +24V sealed unit that
I use now.
>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).
Yup, sheet 3 is where I have been focusing so far (which you could probably
tell from what I had measured).
> > 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.
Yes, I had noticed that. The +13V has as its ground the 300V Return. The
+12V has chassis as ground, unless I missed something (If I did, then then
the +7V measurement is wrong). (Measured against the 300V return, the +12V
is some other obscene voltage -- around 150V if I remember -- I measured it
just for grins and to make sure I was on the right track).
>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.
Yeah, an electrolytic would be a good place to start -- I'll try looking at
him first. In the collection, those are high on the list of dead things
(along with lamps and vacuum sensors and bridge rectifiers killed by
shorted or underformed electrolytics).
> > 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.
Yeah, there is a 10V Zener on the M/B (D2) from what I can see. That
voltage is running at 9.2V -- I am guessing that that is probably OK,
though I think I'll scope it (that is where my test lead is right now,
anyway). It could always be a bad bridge on the +300V doubler, so it is
worth a quick check with the scope.
>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).
Thanks -- I suspected as much, but without a book I wasn't sure.
> > 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!.
Yeah, no kidding. The red label on the top of the supply got my attention,
and 2 minutes with the schematic was enough to convince me to be verrrry
careful. At least it's DC.
>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.
Yeah -- I have been checking it with a voltmeter before I proceed each time
to yank the bias card to put on a test point. Slow, but safe. I also kick
off all the breakers AND unplug the machine. Last think I want is to
accidentally close a breaker. =8^/
>But it's not my favourite supply to work on by any means...
>
>-tony
For me, any switching supply is a challenge. Not one of the things they
taught us about when I went to school originally as an EE.
I'll keep folks posted on progress (which is likely to be slow -- as they
say, work sure cuts into your free time. 8^)
Jay Jaeger
---
Jay R. Jaeger The Computer Collection
cube1_at_home.com visit
http://members.home.net/thecomputercollection
Received on Sun Nov 11 2001 - 22:20:37 GMT