[ARC5] BC-455 adventure

[Mike Hanz]

On 6/16/2012 2:03 PM, David Stinson wrote:

>> Speaking of which, is there a reasonable way to run them with the
>> original
>> dynamotor? I seem to recall something about reduced voltage/RPM not
>> being a
>> good thing? I've probably asked this before, just don't remember the
>> answer. Old age 'n all that....
> I've had sets in which the fils had been rewired for 12 volts.
> I put a 28 volt dyno on them, run it at 12 volts and it worked
> just fine.  Ran for hours.

Yup, they will certainly run, Dave.  Unfortunately, they aren't very 
stable - at least in the samples I tested a few years ago.  They "hunt" 
all over the place in speed, at least on the tachometer I tested them 
with, and the voltage regulation goes to pot compared to what they 
traditionally have at 24-28 volts.  Turn the BFO switch on and the B+ 
drops.  Go from low to high volume and the B+ drops again.  Let them run 
for a while to warm up and watch the output voltage - it wanders all 
over the place. :-(   It made any SSB listening with the command sets a 
bit difficult - at least with mine.  That's the main reason I gave up on 
simply lowering the input voltage as a simple expedient.

Your discovery and sharing of the small PWM devices a while back seems 
to me to have the best promise for transplantation, though you do lose 
the wonderful sound that the dynos produce.  FWIW and armed with your 
research, I added the following random thoughts to the end of my 
eternally expanding dynamotor article - the one that was initially 
published in 1997:

******************************

Closing thoughts...

There is a related subject that probably fits here, and that is the 
aging and deterioration of WWII avionics equipment. While this factor is 
not affecting the dynamotors in a big way yet (except occasionally for 
the small hash filter capacitors that look like micas on each end), the 
growing age of capacitors and other components is causing a lot of 
people to look at lowering B+ voltages in an effort to provide at least 
a palliative measure, for radio receivers if nothing else. It isn't a 
cure...only replacement with new components can do that...but it 
provides a "quality of life" approach that extends the usefulness of the 
sets without more serious surgery. After much testing, it turns out that 
there is no serious downside to lowering plate voltages in most of these 
receivers. There is one major reason to keep the B+ near its original 
design center for a receiver, and that is the audio amplifier stage. 
Without enough overhead to B+, the audio power available from the set 
goes down quite rapidly with lower plate voltage. Happily, this loss of 
power can easily be made up with an outboard amplifier of one form or 
another. My favorite solution to that is to use one of the interphone 
amplifiers of the period and wire it in so that the receiver output can 
be fed into the microphone input.

There are five basic techniques for lowering a dynamotor's output 
voltage without altering external appearance, none of which could be 
exactly termed as non-intrusive. Be aware that simply using a 24v 
dynamotor at 12v isn't a good solution. Reducing the input voltage, 
while it does reduce the output, generally introduces a significant 
amount of instability in the tests I've run, and the excellent voltage 
regulation for which dynamotors are well known goes rapidly south at 
voltages below 75% to 80% of design center.

I've done some work with series regulators, but there is no miracle here 
- Ohm's law requires the dissipation of some 5 watts or more of power, 
not to mention the problem of finding a device that will drop 100-150 
volts, will fit into the dynamotor base, and use the baseplate as a heat 
sink. The baseplate gets pretty warm with 5 watts pumping into it, and 
it in turn heats the entire dynamotor to uncomfortable levels in the 
test rig I put together. One approach would be a switching regulator, 
but I haven't had enough time to delve into that approach. A preliminary 
sizing suggests it won't fit into the base of the dynamotor, but if I 
can find the right devices, it may be possible. It is certainly possible 
to do the job with an outboard regulator, but the only thing that buys 
you over a complete outboard power supply is the privilege of hearing 
the dynamotor whine, and hiding such a construct isn't easy.

Another option that holds some promise is using pulse-width modulating 
DC to DC controllers. There are several available at less than $10 that 
have output voltages up to 100vdc with 28vdc input. Unfortunately, this 
pretty much involves removing the dynamotor armature to gain room for 
the parts - something that you can certainly do with a junker dynamotor, 
but then you lose the lovely whine.

  The three somewhat more practical alternatives at the moment are 
outlined below.

  1) By the time WWII rolled around, most of the dynamotor manufacturers 
had settled on some industry standards for frame size, and even went so 
far as to make some of their components essentially identical. For 
example, the common frame size for the command set receivers is 310. One 
thing I have discovered is that later dynamotors with the same frame 
size had armatures that are sometimes drop-in replacements for the WWII 
armatures, but have output voltages that range from 120 to 170 volts! 
I've had particularly good fortune with the postwar AN/AIC-10 interphone 
amplifier dynamotors, which normally go for very little money, yet their 
HV output is only 170v, much easier on the receivers. Note that this is 
luck of the draw between manufacturers, and you can't guarantee it, but 
tweaks with a lathe will usually make the necessary alterations to get 
an armature to fit. Issues to be considered include physical size, 
magnetic gap dimension between armature and stator, and stator field 
strength, but most of those factors (except physical size) contribute to 
even lower output voltages than on the original labels. It may be easier 
to simply swap the whole dynamotor to the old mounting plate, but then 
you lose the original look of nomenclature plates, etc. You'll have to 
make an assessment based on your particular case and associated barriers 
to an armature swap.

  2) This second alternative requires the skills of a metalworker. It is 
also hinges on how the HV winding on the armature is made. The idea is 
simple, and was introduced by one of the 73 magazine staff back in the 
late 1960s, but he was focusing on the primary 28v side to allow use 
with 12v car systems. Basically, you have to rotate one brush around 
from the 180 degree point to lower the voltage. The catch is the need 
for the HV winding to be in a particular commutator connection 
configuration. It requires a new brush holder to be positioned at some 
angle to the original, the angle a direct relationship to the output 
voltage desired. There is nothing easy about this alternative, and it 
requires the "Patience of Job" to get working, but work it does if the 
winding is connected in such a way that the individual loops are 
essentially in alternate segment series with each other. Rather than get 
into the technical aspects to determine the type of winding, it's easier 
to simply connect a multimeter to one brush and use a probe (preferably 
with a rounded end) to gently touch the commutator at the 90 degree 
point while the dynamotor is running. Be careful...this can produce all 
sorts of negative results if you short the output to ground or 
electrocute yourself! If you are lucky, you can vary the output voltage 
by changing the pickoff point with the probe. If it doesn't do that, 
this method won't work and you will have saved yourself a lot of work.

  3) The cleanest alternative, but by far the most time consuming and 
difficult, is to rewind the HV side of the armature to have a smaller 
number of turns. Note this requires a high degree of interest in 
experimentation, and/or the friendship of a shop that rewinds larger 
motors. Most of them aren't interested in working on such small motors, 
and will simply price any work out of reach just to get rid of you. It 
is possible to do it yourself, but requires enormous patience and 
organization (each of the multiple windings needs to be carefully 
tracked and labeled for it to work properly) and for most folks this may 
be a bridge too far. For the persistent (dare I say stubborn?) it can 
provide a number of hours of intense satisfaction for those so inclined. 
One beginning step is probably to go to a motor rewind shop with the 
dynamotor and tell them what you want to do. They may allow you to at 
least watch the process on one of their larger motor jobs. If not, a 
good text on the subject is essential in any case. I would only do this 
if I wanted to keep the receiver entirely original, and given the amount 
of work involved I would recommend it be preceded by measuring 
distortion, large signal handling capability, and overall gain (less the 
PA section) at a number of B+ settings to determine how far you could 
lower the plate voltage without significant impact. As a data point, 
following Dave's (AB5S) initial suggestion, my RAT sets work 
surprisingly well with 28 volts on the plates, but then I've long since 
established realistic expectations for their performance. For them, I 
simply jumpered the input to the output under the base and unhooked the 
B+ from the dyno.

End