BE-5, what do the controls do?

[Jim Bromley, K7JEB]

Bob Groh,  WA2CKY, wrote:

> I got a BE-5 schematic from the British web site
> (actually a GIF file that I modified to a jpg file and
> then printed on a letter sized paper) so we have that
> in hand (a pdf file would be very nice to have also).

I'll send you one.

> I'm going to puzzle out the schematic tonight but I
> would like to know the function of the 'taps' on the
> secondary as well as the role of the rotary switch.... 

Here's my take on those:  The transformer is actually
a variac of a rather peculiar type, having the wiper
on the secondaries of a step-down transformer rather
than the usual autotransformer configuration.  By
doing the voltage control in this way, they don't waste
power and generate heat in a rheostat.

The rotary switch that selects either 6- or 12-volt
operation is pretty clever.  In 6-volt operation, the
rectifiers are connected as a full-wave bridge
across the 6-volt transformer and one of the
10,000uF at 10vdc capacitors is used as a filter.
In 12-volt operation, the rectifiers and both capacitors
are connected as a voltage doubler that allows
the 6-volt adjustable transformer to be used at both
voltage levels.

That's all there is in the BE-4.  In the BE-5, there
is an additional set of filtering components, a choke
and a 10,000 ufd at 20V capacitor to take what must
be a horrendous ripple on the output down a notch
or two.

Why such a bizarre circuit?  I suspect the original
model of this series was a 6-volt only unit, designed
back when that was the only automobile battery
voltage around.  In later models, they needed a 12-volt
output but wanted to retain that variac-like transformer
and didn't want to retool to make it dual-voltage.
The voltage doubler was an easy, clever solution.

Also, these things were originally made to power
vacuum-tube automobile radios with vibrator HV
power supplies.  Those things could tolerate massive
amounts of ripple on their primary DC supply because
it was only used to light the filaments and run the
vibrator supply, which had its own, downstream filtering
components.  I'm guessing that the additional filtering
in the BE-5 was to accommodate later, solid-state
radios.

Given this circuit, it's easy to see that any rectifier
replacement can't use a single, bridge-rectifier module.
Rather, it needs two of them, or two stacks of two,
individual diodes.  Also, any thought of using a modern,
adjustable voltage regulator as a back end runs into
the problem that there isn't any excess voltage output
to accommodate the required 2-3 volt drop across the
regulator pass transistor.  (A switching regulator
solution might be had, but then you're essentially starting
from scratch).

This has been an interesting exercise.  I always wondered
what was really inside that battery eliminator that was on
the bench in the little radio shop in which I worked as an
apprentice back in the 50's.

Jim, K7JEB

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