Reviving DEC RX01 power supply

Roy J. Tellason rtellason at
Tue Sep 4 21:34:22 CDT 2007

On Tuesday 04 September 2007 05:12, J Blaser wrote:
> > 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.

What pops into my head on reading this is that you might possibly be seeing 
some sort of an artifact,  if you're using a digital meter!

> 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?

A faulty cap can have all sorts of noise,  hum,  hash,  and other nonsense 
riding on top of the DC level,  and that might (falsely) end up giving you a 
higher reading than what you'd expect.

> > 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.

One point about ferroresonant transformers -- that capacitor you mention is 
typically "selected" to go with that particular transformer.  If you look,  
you'll probably find a small dot of the same color paint on both of them.

> 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.

One more point about ferroresonant transformers -- their output is often NOT 
sinewave,  but a rather distorted waveform,  that would tend to upset some 
meters,  I'd think.  Your scope should show you more of what's actually going 
on there.

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