34A Power Trouble. I calmed down, here's the facts...

From: Tony Duell <ard_at_odin.phy.bris.ac.uk>
Date: Tue Nov 11 12:09:09 1997

On Tue, 11 Nov 1997, Daniel A. Seagraves wrote:
> > J5 carries the wires to the 2 primary winding on the mains transformer in
> > the PSU. On 230V units the 2 windings are linked in series, on 120V units
> > (like yours, I guess), in parallel.
> Okay, that big transformer in the top of the PSU?

That's the one, yes. It's rated at somewhere near 1000VA, and has 5
separate 20-30V secondary windings - one for each of the 4 regulators, one
for the 15V board (+15V, line clock, ACLO, DCLO signals).

> > 1) A short somewhere on the secondary side (in one of the regulators,
> > say). This will obviously cause too high a current to flow in the primary
> > circuit
> How do I check that? I've never played with voltages over 24V before.

Well, start off by unplugging all the loads from the secondary side, as I
described below. Then add them back one at a time (see the message I sent
out this morning, if it's got to you, if not I'll send you a private
copy), and see which regulator trips the breaker. Then we need to get into

Mind you, the regulators contain their own fuses, so most faults in them
only blow the internal fuse and don't trip the main breaker.

> > 3) You've reversed one of the primary windings (swapped over leads on J5).
> > I assume you've not fiddled with this
> I may have. I had to unplug the breakers to replace them.

Unless you removed individual wires from J5, I fail to see how you'd have
got them back in the wrong order.

> > 4) Did you fit a new breaker? The mains transformer takes a high surge
> > current at switch-on. The original breaker was a time-delay one - is the
> > new one of a similar type.
> The breakers came from a VAX PS, DEC model # 70-18763-00

Wait a second...

Breaker_s_??? The original 11/34 breaker is a double-pole unit designed so
that if either side trips both get turned off. Have you fitted 2
single-pole breakers in place of it? That would violate the regulations at
least (the breaker in the grounded side of the mains could trip, leaving
the entire input side to the PSU live, for example).

These breakers are made in many different ratings, and with several
time-delay characteristics. DEC used many of them - not all PSUs take the
same current from the mains, and IIRC the BA11-K PSU takes a heck of a
switch-on surge - later SMPS's have inrush current limiter circuits, and
probably use different breakers

It sounds to me as though you've fitted the wrong breaker - that is all.
Do you want me to try and find the spec of the original one? I could
certainly find out what the 240V one was.

I'm guessing now, but I think the original one was a 10A, double pole,
time delay unit. What does it say on your replacement(s)?

> Hmm. My boss used to work on big PSUs in CB radio linears, if you could
> scan the schematic or something he could troubleshoot this...

The schematic for this PSU is probably about 10-20 A3 (11"*17") sheets.
It's not that complicated, but there's a lot of it. Let's find out what
bits you need first.

The input circuitry is very straightforward, though. Mains comes in. It
goes to the breaker, then to a mains filter. The output of that goes to
the big contactor relay. That applies mains to the input of the
transformer. The output of the transformer goes to the regulator blocks.

> > What did you disconnect? If you think you may have swapped wires, let us
> > know what you fiddled with _now_ rather than later...
> I detached the 4 wires attached to the breakers.
> I detached the 2 line wires last so as to not swap them, but I may have
> anyway.

There's no way that getting any of that wrong would cause the PSU to trip
only when the transformer was connected. Just about the only thing you
could have got wrong would have been to connect the 2 input wires to the 2
terminals on one pole of the breaker, in which case the breaker would have
shorted out the mains and would have tripped instantly when you plugged it

I suspect the only problem is that you are using the wrong breaker.


Received on Tue Nov 11 1997 - 12:09:09 GMT

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