Reviving DEC RX01 power supply

J Blaser oldcpu2 at
Tue Sep 4 04:12:22 CDT 2007

> IIRC, the 'raw DC' is unregulated, just rectified and smoothed from the
> transformer. As is the 24V line. Both are about 1.7* what they should be.

That's the strange thing to me.  The two secondaries from the transformer (one 
for the 24V line, and the other for the 10V line [which sources the regulated
5V output]) are 'proper' at 27.8V and 11.2V, respectively.  What I don't understand is how a 24V line can suddenly produce 42V!  It's like something 
is 'pumping' the circuit, and I admit I have never studied how such things work.

The funny thing is that the 5V regulation is working, even with 17V input
instead of 10V.

> It's gettign late, so I'll not find the prints tonight. But IIRC, the
> transformer in this supply is a ferroresonant one, and that's what
> stabilises the 'raw' and 24V lines. What happens if the capacitor hung
> off that is defective?

Fair point, but can a faulty capacitor 'pump' up the voltage like I'm seeing?

> I suspect the last part is very ture. Actually replacing the faulty part
> is the easy bit :-)

Hey, thanks for your confidence!  :-)  I can use a little outside support!  ;-)

Let me add that this RX01 is the second of two RX01 units in this system.
When I went through the first one a week ago, it checked out fine...that is
I saw 25V/10.2v on the outputs, without a load.

And you're not going to like this :-), but for an electronics novice, tonight I
followed my instinct and swapped the regulator PCBs between units.
Yup, the 'good' system's PCB now is putting out 42V and 17V!  So, it's not
the regulation circuit, I guess, or any component on the two system's PCBs.

But after swapping the PS regulator PCBs, all that's left are the transformer (which appears to be putting out 'expected' voltages), a 660V AC capacitor thingy (which I obviously don't it part of the ferrroresonance?) that only connects back to the transformer itself, and the two smoothing caps (one for the 24V line and one for the 10V line).  Is it possible that one/both
caps are 'pumping' the circuit?  <shrug>  I'm just too novice to know.

Another suggestion I received privately hints that putting a dummy load 
on the PS might bring it into line, and that I will try.  I have access to an
oscilloscope, and will put it on the thing, too, to see if I can discover
anything strange.


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