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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
DC-to-AC Inverters From The Valve Era
There have always been odd little offshoots
from the mainstream technology of radio and
DC-to-AC inverters are one such offshoot.
This month, we’ll take a look at two of those
early inverter circuits and describe their
restoration.
D
URING THE VALVE era, many
radio manufacturers also made
DC-to-AC inverters to power items
such as electric shavers, TV sets and
other 240VAC items from 6, 12 or
32V DC. However, with the exception
of radiograms, 240VAC radios were
rarely powered from inverters, as the
inverters were not very efficient.
90 Silicon Chip
Getting some of those early inverters
working again can be quite a challenge.
So let’s take a look at a couple of the
more common units.
Bland’s shaver inverter
Bland Radio Ltd of Adelaide were
well-known for their Operatic series
of good performance radios. They also
made various other devices, one of
which was an electric shaver inverter.
This ran off 6V DC and produced 240V
AC with a near square-wave output
waveform. Its current drain was approximately 4A for a power output of
up to 15W.
My unit was obtained when a friend
decided to reduce his radio collection.
When I subsequently pulled the cover
off the unit to see what was inside I
found an Oak V5211 vibrator, an ironcored transformer and a 0.5mF 600V
buffer capacitor. These buffer capacitors can be unreliable so I immediately
replaced it with a new polyester type
with a slightly higher voltage rating
than the original unit.
There being nothing else to check,
I then attached the inverter to a 6V
power supply and absolutely nothing happened. Further examination
of the device then revealed that the
inverter had to have a shaver or some
other similar device connected to it
to work.
Basically, the earth pin on the appliance’s 240V plug is used as the
switch to turn the device on – see Fig.1.
As shown, the earth terminal on the
inverter’s 240V socket was modified
so that it had two separate sections.
Plugging in the appliance connected
these two sections (via the earth pin
on the plug), thus allowing the inverter
to operate from the 6V supply.
In practice, this means that the
inverter will not operate until a 3-pin
plug is inserted. It then turns off automatically when you remove the plug.
It’s quite a neat scheme but I wonder
how many men complained that the
unit didn’t work, not knowing that
their shaver needed to have a 3-pin
plug and not a 2-pin plug!
No voltage
Having solved that problem, the visiliconchip.com.au
brator started but I still couldn’t get any
voltage out of the unit. An ohmmeter
soon showed that the transformer was
still OK so that left the vibrator itself,
although it appeared to be working.
When the vibrator was removed
from its case it initially appeared to be
OK. However, the ohmmeter showed
that all the contacts except for the reed
drive had oxidised. So although they
were making physical contact with
each other, there was no conductivity
across the contacts.
This is not a common problem but
I’ve seen it before and the solution is
quite simple. It’s just a matter of cleaning the contacts using some very fine
wet and dry paper.
The procedure is as follows. First,
tear off a small amount of wet and dry
paper about 20mm square. That done,
fold it in half with the abrasive side
out and insert the paper between each
set of points. Finally, press the points
lightly together and rub the paper back
and forth between the points until they
are clean.
In practice, several pieces of paper
are usually needed to get the points
thoroughly clean and conducting
again. In this case, once cleaning
had been completed, an output voltage of over 400V peak-to-peak was
obtained with no load. I then put a
5.6kW wirewound resistor across the
output and briefly obtained an output
of about 260V before the unit suddenly
stopped.
For such a simple device, it was certainly causing more than its fair share
of trouble. I checked the circuit around
the vibrator and the voltages around
it were normal. I then re-checked the
points and this time found that the
reed drive had fouled up.
As a result, the points were all given
a further clean-up, after which the unit
worked well. I then checked the waveform on the oscilloscope and found
what was nominally a square wave
but with some slight resemblance to
a sinewave. There was no significant
overshoot on the waveform.
I don’t have a true RMS meter to
measure the output but according to
the oscilloscope, it appeared to be
producing roughly 240V AC.
Having got the unit working, I decided to trace out the circuit and it
turned out to be a little different to
most vibrator circuits. In this unit,
the synchronous split-reed V5211
vibrator is wired so that the whole
siliconchip.com.au
This simple 6V DC to 24VAC inverter was made by Bland Radio Ltd of Adelaide
and was designed to power electric shavers.
Fig.1: the Bland Radio inverter circuit. Its mains socket used a 2-piece
earth terminal which functioned as a switch for the 6V DC input (a scheme
that would now be illegal). This meant that the mains plug fitted to the
appliance had to have an earth pin in order for the inverter to work.
primary winding is used but the current through the winding is reversed
at the end of each half cycle.
Many vibrator inverters did not
work well on inductive loads and
shavers usually are inductive. How
ever, there was no sign of excessive
pitting on the vibrator contacts so it
would appear that it did a satisfactory
job, despite the nature of the devices
likely to have been connected to it.
In fact, Bland Radio’s vibrator power
supplies were well designed and rarely
required vibrator replacement.
Van Ruyten model VR58TV
Up until the late 1950s and even
into the 1960s, 240V mains power
was still not available to some farms
and other remote areas. Instead, they
mostly relied on 32V DC power plants
for lighting but only some household
equipment was designed to operate
from this supply voltage.
June 2007 91
There’s not much inside the Bland Radio inverter’s case – just a standard
Oak vibrator unit, a transformer, a capacitor and the mains socket.
Items like washing machines, electric irons, food mixers and vacuum
cleaners were available but 32V refrigerators were not (kerosene refrigerators
were used instead). A 32V 2-bar radiator was just not practical (an open fire
or a kerosene heater were used instead)
and the pleasure of watching TV was
largely denied to these rural citizens
as early TV sets were only designed
for 240V mains operation and used
upwards of 200W of power.
Now 200W of power consumption
was not in itself too much for a 32V
system but the fact that people like
to watch TV for many hours per day
meant that the battery bank would
have been flattened quite quickly. In
addition, the cost of home generated
power was about a dollar per kilowatt
hour or more, which is a lot more than
we now pay for electricity. Converting 32V DC to 240V AC is inherently
inefficient and when the efficiency is
taken into account, the total power
consumption climbs to nearly 300W.
This photo shows the Van Ruyten power
vibrator (top) alongside a standard Oak
vibrator.
During that era, the only manufacturer to produce DC-powered TV
sets was Ferris. These purpose-built
set incorporated their own vibrator
supply and were more efficient than
mains sets operating from an inverter.
However, despite the inefficiency and
the cost, there was still some demand
for inverters to run mains-operated
TV receivers.
One well-known DC-AC inverter
manufacturer was Liebmann Clarke
Pty Ltd of Richmond in Victoria.
The company manufactured several
different models, designed to power
240V AC equipment from 6V, 12V or
32V DC.
Their highest power unit was the
Van Ruyten model VR58TV. This 32Vto-240V inverter had an output power
of 200W and weighed in at 10kg. It was
specifically designed to power black
and white TV sets from a 32V bank of
batteries on a farm or station.
In fact, it would appear that the
model number indicates the design
year and that its prime purpose was
to power TV sets.
Cleaning up
When I obtained the inverter, it
looked pretty shabby, with rust showing through the paint work, the voltage
adjustment knob missing and the front
panel hanging loose. Unfortunately,
I didn’t have any knobs that exactly
matched the type used so I used one
that suited the era.
On the other hand, the inside of
the inverter was quite clean and only
a quick clean-up with a small paint
brush was required. That done, I
separated the unit from its case and
removed the front panel. The case and
its panel were then washed with soapy
water and left to dry.
Once they had dried, I set about
removing the rust and old paint from
these items using an angle grinder. The
two parts were then sprayed with grey
hammertone paint and the unit now
looks almost like new. It’s certainly
vastly better than the rusty unit it was
before restoration.
Overhauling the electronics
It was now time to overhaul the
works and my first step was to replace
the 0.56mF 600V paper capacitor (C11)
which was leaky as expected. This
was swapped out for two 0.27mF 630V
polyester capacitors wired in parallel.
All the other capacitors were being run
92 Silicon Chip
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well under their voltage ratings so I
left them in circuit. That proved to be
a mistake but more of that later.
The vibrators in these inverters often
had a hard life due to the uncertainty
of whether the load would be inductive or capacitive. C5-C8 and C11 are
the buffer capacitors which “tune”
the inductances so that the circuit
resonates to around 50Hz.
However, with capacitive or inductive loads, this tuning will be altered,
leading to sparking at the vibrator
points.
I don’t have any spare 32V vibrators
so I dismantled the unit that was in
the inverter. This was done by desoldering the two solder joints between
the base and the can and then sliding
the vibrator out.
A close inspection of the points
showed that one pair out of the five
sets had a “dag” on one contact which
mated with a hole in the other point.
This was fixed by releasing the adjustment screw and filing the dag away. I
then cleaned all the points with fine
wet and dry paper.
That done, I re-installed the adjustment screw and rotated it until I had
the same gap as the other parallel set
of points. A feeler gauge was then
used to make the adjustment as accurate as possible. I then connected
the vibrator to a 12V supply to check
that the reed drive worked properly.
This is the above-chassis view of the
Van Ruyten VR58TV DC-AC inverter.
Note the two large transformers that
are used in conjunction with the power
vibrator at the rear.
This checked OK and required only a
minor adjustment.
Re-assembly
The next step was to re-assemble the
inverter. First, the 32V power leads
and the grommet were fed through the
hole at the bottom of the panel, then
the switch was mounted in position,
followed by the 240V output socket.
The 32V switch was next on the list
and this proved to be difficult, as the
screws are hard to get at. Eventually,
I got them in but then found that the
switch wouldn’t work – much to my
frustration.
On inspection, it appeared to be
fouling on the switch cut out on the
Fig.2: this is the redrawn circuit for the Van Ruyten 32V DC to
240VAC inverter. The vibrator drives the two sections of the two
transformer primary windings in a series push-pull arrangement,
while the secondaries are connected in series to drive the output
socket.
siliconchip.com.au
June 2007 93
This under-chassis view of the Van Ruyten inverter shows it to be a more complicated beast than the low-power
Bland Radio unit. The red arrow points to the four new polyester capacitors that were fitted.
front panel. Fortunately, the previous
owner had left all the screws, nuts
and bolts for the inverter in a plastic
bag. Much to my delight, there were
also two ceramic spacers in the bag
and it appeared they had been used
as spacers for the switch.
Getting the nut onto the screw nearest the top of the chassis (furthest into
the chassis) was no easy task. Eventually, I resorted to an old trick. The nut
was pressed into the end of a plastic
tube, after which I was eventually
able to position it inside the chassis
correctly to take the screw. The one
towards the bottom of the chassis is
much easier but I now know why the
previous owner passed the unit on to
This is the Van Ruyten inverter’s case before restoration. The rust was
removed using a drill fitted with a wire brush, after which the unit was
repainted so that it now looks almost like new.
94 Silicon Chip
me – he couldn’t get it back together!
The knob I selected for the High/
Low switch had a white recessed indicator line down the pointer section.
However, this had largely disappeared
so I scraped out the old paint using
a scriber and cleaned it thoroughly.
I then used “White Out” to fill the
groove in the knob.
Once this was dry, the excess was
scraped off the knob using a razor
blade, leaving a neat white line down
the channel in the pointer. It now looks
like new.
It was now only a matter of sliding
the chassis back into the case and
fitting four screws. In addition, the
rubber feet had long since disappeared
from the bottom of the case so I used
four large rubber stick-on furnituretype buffers to stand the case proud
of the bench. These can be obtained
from hardware or “$2” shops. The
finished unit now looks quite attractive, especially when compared to the
grubby unit it was before restoration.
Testing
Now it was time to test the unit. I
slipped it out of the case, connected
my 32V DC power supply to it and consiliconchip.com.au
nected a 15W 240V lamp to the output
as a load. This load was deliberately
kept small as my 32V supply is only
rated at 1.5A.
At this stage, I still had the cover
off the vibrator. I turned the power on
and the unit started up and produced
an output. Everything appeared to be
OK, so I left it running on soak test.
Unfortunately, it didn’t stay that
way for long – the next time I came
back, there were tiny bits of silver
paper and other tiny bits of powdery
material like confetti near the inverter.
The inverter was still running quite
happily so I turned it off to investigate.
When I looked under the chassis, I
was greeted by two capacitors that had
blown their insides out.
The two capacitors involved were
among the primary circuit buffers (C5C8). They had overheated so badly that
foil had been blown out of them. The
inverter had kept going despite this
catastrophic failure and none of the
foil shorted anything out.
It was easily fixed – the “confetti”
was cleaned out and the four capacitors (which were still quite hot) removed from the chassis. These original
paper capacitors were then replaced
with a batch of four polyester type
capacitors.
As shown in one of the photos, the
replacement capacitors were glued
together with contact adhesive and
then tied to the tag strip on the bottom of one of the transformers using
a plastic cable tie.
Photo Gallery: Astor Mickey Model DL
MANUFACTURED BY RADIO CORPORATION, MELBOURNE in 1947, the
“DL” was another model carrying the “Mickey” name. It was fitted with a
full-width (almost) glass dial, with the loudspeaker mounted at the side of
the cabinet. This set did not employ the reflex circuit that was later to become popular with “Mickey” models until the end of the series. Brown and
cream were probably the most common cabinet colours and this mottled
yellow example is unusual.
The valve line-up was as follows: 6A8-G frequency changer, 6B8-G reflexed
IF amplifier/1st audio amplifier/detector/AVC rectifier, 6V6-GT audio output
and 5Y3-GT rectifier. Photo: Historical Radio Society of Australia, Inc.
Why did they fail?
The original paper capacitors are
rated at 300V working, so why did
they “blow up” when only 32V was
being applied across them?
The answer is that the actual voltage across them is in fact considerably
higher than the supply voltage, as the
circuit is roughly resonant at 50Hz.
As a result, considerable voltage is
developed across the total inductance
of the primary windings as they are
completely charged and discharged
100 times a second.
I hadn’t checked for leakage across
these capacitors as I had reasoned (erroneously) that even if they did have
some leakage, it would not be serious
enough to cause much heating. How
wrong I was. The two that hadn’t
blown up showed very low insulation
resistance, so how low was the resistance in the two that did blew up?
siliconchip.com.au
In hindsight, I should have tested
these capacitors for leakage resistance
before starting the unit up and then I
should have periodically (every few
minutes) checked for any signs of
overheating.
Summary
These vibrator-powered DC-to-AC
inverters served the needs of the
public quite well before the arrival of
solid-state devices. A number of other
brands were also produced although
they were not as common as the Van
Ruyten.
Van Ruyten also produced a 100W
version of the unit described above
and it used just one transformer. The
radio frequency (RF) filtering in the Van
Ruyten unit may be sufficient so as not
to noticeably impair domestic radio
reception but the Bland Radio unit
has no such RF filtering. As a result,
the reception on any radio used with
the Bland Radio unit would have been
severely marred by interference due to
sparking at the vibrator points.
Based on my experience, these
vibrator-type inverters were only moderately reliable due to the uncertain
characteristics of the loads that they
drove. By contrast, the Davey rotary
motor alternator was a very reliable
device which produced sinewave
240V AC, compared to the roughly
square-wave output from the vibrator
inverters. They also were not affected
to any extent by the type of load that
was connected to them.
However, the Davey units were rarely seen as they were even more expensive than the vibrator inverters and
drew even more current from the 32V
SC
DC power supply.
June 2007 95
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