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Military
Collector Group Post
Backmail #55
(13 pages) Index: UNIVERSAL INVERTER POWER SUPPLY, PART I; by Dennis
Starks UNIVERSAL INVERTER POWER SUPPLY, PART II; by Dennis Starks UNIVERSAL
INVERTER POWER SUPPLY, PART III; Applications (PRC-6 & CPRC-26), by Dennis
Starks BC-611, AND INVERTER POWER SUPPLY SWITCHING PROBLEMS; From Ed Guzick,
and Dennis Starks *********************************************** UNIVERSAL
INVERTER POWER SUPPLY, PART I; by Dennis Starks FORWARD: The below series
will be essentially the same material that is to be sent out with the
copies of the PRC-6 manuals that Ralph Hogan is making available. If you
have ordered these, you will be getting it again, along with schematics,
and block diagrams. If you are able to receive graphics via email, Ralph
has the ability to send you copies of the schematics, and block diagrams.
The material should be in his hands shortly. If you have not ordered PRC-6
manuals, and are not able to receive graphics via email, you can still
contact Ralph at for hard copies. GENERAL: This
power supply is capable of operating almost any vintage tube type radio,
either receiver, or low powered transceiver that had originally been intended
for battery powered operation. Only minimal modification will needed to
this basic circuit to serve your needs. Input voltage requirements varie
depending on the radio in question, type of transformer used and your
choice of power source. Some particular radios that this supply has been
used with include; GRC-9 or BC-1306 receiver section(90-105volts), PRC-6(45
& 90volts), CPRC-26(45 & 90volts), GRR-5 receiver(90-105volts), PRC- 8,9,10(67.5
& 135volts), BC-1000(135volts), BC-611(90-105volts), many old Motorola
"Handie Talkies", and a host of others. The completed inverter, can be
built on to a board that measures as small as 1.5" wide x 1.75" long x
.75" tall, making it small enough to fit inside nearly any radio leaving
ample room for rechargeable batteries. NOTES(refer to schematic) #1)Transformer
considerations(T-1). Any 88mh, or even 44mh toriod type transformer can
be used and your added primary, and feed back windings will remain the
same. However there will be some types better suited to your particular
application, and with the dwindling availability of these transformers,
it's best to use only the one you actually need and save any others for
later. Unless you have some rather sophisticated test equipment, you will
not be able to tell these transformers apart. So if you should go to all
the trouble of winding one, and placing it in an operational circuit only
to find it's the wrong one, fear not! Just set it aside, and wind another
one, the first will still be useful on a future project. Several types
of 88mh toriods can be found, they include dual 44mh(two windings), single
winding 88mh with a center tap, and single winding types that do not have
a center tap. The dual 44mh, and those with a center tap are the most
versatile as they will provide dual output voltages without the need for
outboard voltage dividers, or regulators(either of which would eat up
efficiency). The dual output voltage is obtained by connecting the two
44hm windings together so that they are in series, thus producing a center
tap that will provide 1/2 the total output voltage. If yours has a single
winding with a center tap, this job is already done. As can be seen, this
is of major advantage when working with radios like the PRC-6,7,8,9,10,26
etc. All of which require two B+ voltages one being 1/2 the other. So
do not waist these type transformers on say a BC-611, or GRC-9 receiver
which only require a single B+ voltage. The second type 88mh toriod you
might find is the single winding type. These are not well suited to radios
that require a dual B+ voltage because to obtain the second(lower) B+
voltage, a resistive voltage divider, or regulator must be used. Neither
of these alternatives are acceptable due to the extreme efficiency loss
they will produce. And when we consider the battery supplies that we may
often be restricted to, this efficiency must be kept high at all cost.
These type transformers will work where a single, relatively high output
voltage is needed but a minimal voltage source is available. One case
in point is the BC-611, here we need only a single operating voltage 90-105
volts to operate the radio, but we have at best enough room inside the
radio for 4.8vdc if 4ea "D" cell nicads(5amp hour) are used. This will
be just enough! The third toroid type you might encounter will be the
44mh variety. The output voltage they produce will be about 1/2 that of
the 88mh types for same input voltage. They will usually not have dual
windings, or a center tap. Their best application will be with radios
who's operation will not be tightly regulated by the type primary power
source available, and those requiring only a singe B+ voltage. In most
these cases an external supply, either battery pack or 12vdc automotive
source will be used, with very little if any need for space conservation.
These applications come to mind; GRC-9 or BC-1306 receiver section, RS-1/GRC-109
receiver, GRR-5 receiver, RS-6 receiver, RT-70 transceiver,etc. So as
can be seen, if you wind one of these, then find it will not suite your
needs, fear not, it will someday. Transformer construction; Transformer
winding is not at all critical. Suiteable wire can be salvaged from old
audio transformers, coils, etc. Due to the very small typical size of
these transformers, complete insulation between each of the windings will
not be possible. Start with a short length of masking tape(or whatever
you prefer) that has been cut to the same width as the transformer. Wrap
it around the outside edge of the original existing winding(secondary)
one full turn. Insure you don't cover up it's leads. All the following
coil windings should be done in the same direction as the existing coil(secondary).
Next wind the Primary(transistor's Collector winding), it's 48 turns of
No.24 enamel wire center tapped at 24 turns. Finish by again wrapping
a single turn of insulating tape around the outside edge. Last is the
feed back coil(transistor's Base winding), it's 18 turns of No.28 wire
center tapped at 9 turns. Then wrap a couple layers of tape around the
finished transformer. Again, only around the outside edge. If you screwed
up and wound the feed back winding first, no big deal, it will still work
anyway. #2) Transistor Considerations; About any pair of transistors that
are the approximate equivalent to an ECG-104 can be used. Due their physical
size, and the minimal power dissipation in this application, no heat sinks
will be required. When using these type transistors, R-2[between the primary
& feadback wingings center taps] will typically start out at 1000 ohms,
but when using some hamfest, or junkbox transistors, this value may need
to be reduced to around 500 ohms. In applications where there are no internal
space limitations, the ECG-104 type devices will do just fine. Some radios
however, like the CPRC-26 and BC-611, will have minimal internal space
to house both the inverter, and it's batteries. In such cases I prefer
to use TIP-41 type transistors due to their much smaller size, even when
the required heat sink is taken into account. The same basic circuit is
used, but the polarity of the primary power source is reversed, and the
value of R-2 is reduced to about 100ohms, or until the circuit starts
to oscillate. VOLTAGE REGULATORS; Two voltage regulators are needed for
each one of our inverter power supplies with few exceptions. The first
is needed to supply our heater voltage needs. As the heaters in all types
of battery powered tube type radios are very fragile, their voltages need
to be regulated to insure their protection and proper operation. Resistive
voltage dividers are a bad choice for several reasons, #1 is the extreme
heat that will be dissipated by the large power resistors needed, and
along with that the associated efficiency loss. #2 the growing rarity
and expense the size resistors needed. #3 Their large size will waist
precious internal space. #4 And most important is that there is no regulation
available. The later is VERY important! Tube heaters do not present a
uniform load to a power source. This is because when they are cold, their
resistance is very low which will produce a correspondingly high current
drain. As they heat up, this resistance raises, and with that, the current
drain is reduced. When a simple resister is used to drop the heater supply
voltage, it will do no voltage dropping until a load is presented, I/E
the voltage(be it 12vdc) on one end of the resistor, will be the same(or
12vdc) as that on the other end until a load is connected. So essentially,
the very low cold heater resistance will absorb this 12vdc(surge current)
until they have a chance to heat up. In the case of the very small heaters
in these radios, they will have no chance to heat up. It's easy to see
what will happen to a 1.5 volt tube when it is exposed to 12vdc even if
for a fraction of a second. They'll pop like a flash bulb! The second
regulator is connected to the input of the inverter so as to allow some
sort of output voltage adjustment and regulation. This and the heater
regulator also provide for a much wider variety of power sources. I/E,
a basic power supply for a PRC-6 only needs 4.8 volts, but with voltage
regulators on both the inverter supply and the heaters, either the radios
internal 4.8 volt batteries can be used, or any external power supply
of 6-30vdc can also be used without modification or adjustment. A further
advantage is in that only one battery will need be fabricated, but it
might then be usable in several different radio types. I/E if you make
up a 10vdc battery for your PRC-6, this same battery could be useable
in your PRC-10. A simple variable voltage regulator circuit is presented
here using the very common, cheep, and versatile LM-317 regulator IC.
It will serve all our needs and many others. CONSTRUCTION NOTES; The inverter,
and regulators should be constructed on separate boards using your preferred
method. PC boards have been designed, and may be made available in the
future. PC construction will be of advantage where space limitations need
be observed, or production of multiple units is desirable. Otherwise Perf
board construction will do just fine. If you design your own PC boards,
DO NOT include a common ground on the inputs, or outputs of either the
inverter or the regulator boards, nor allow any common ground connection
on the boards themselves to be incorporated in any mounting method. This
is because in many cases input or output connections must be kept above
ground to either isolate voltage polarities, or to effect proper power
on/off switching utilizing the radios original circuits without modification.
Heat sinks will be needed on the inverter transistors only if TIP- 41
type devises are used, even then very little is needed. The LM-317 voltage
regulator IC will also require heat sinking. ECG-104 type transistors
can be mounted directly to your PC, or Perf board. Dennis Starks; MILITARY
RADIO COLLECTOR/HISTORIAN military-radio-guy@juno.com Parts List:(inverter)
Q-1, Q-2, ECG-104, or TIP-41 transisitors.(see text) R-1, 15 ohm resistor.
R-2, 1000 ohm resistor.(see text) C-1, 20 mf/25vdc electrolytic capacitor.
C-2, 5 mf/200vdc electrolytic capacitor. D-1, -2, -3, -4, 1N-4001 rectifiers.
T-1, 88 mh, or 44mh, toroid coil.(see text) Parts List:(regulator) U-1,
LM-317 voltage regulator IC. D-1, -2, -3, 1N-4001 rectifiers. C-1, -2,
10 mfd/35vdc electrolytic capacitor. C-3, .01 mfd disk capacitor. R-1,
5000 ohm, PC mount potentiometer. R-2, 220 ohm 1/2 or 1/4 watt resistor.
Related Topics: MCGP Backmail #32, Alternate Power Sources; The Ultimate
Universal Inverter Transformer? & Typical Radio Power Requirements. by
Dennis Starks More Inverter Design Discussion; by Mark Gluch, & Dennis
Starks Nick's Inverter Transformer Input; by Nick Broline RE: DC-DC; Ralph
Hogan, & Dennis Starks Pete's Inverter Transformer Ideas; Filement types?
*********************************************** UNIVERSAL INVERTER POWER
SUPPLY, PART II; by Dennis Starks INVERTER START UP: Once you've completed
winding your transformer, and have a wired circuit for the inverter, you'll
next want to fire it up. Start with a power supply of about 6 volts. If
it starts right up, and you have high voltage, you might just be in business.
If for some reason the inverter did not start oscillation, first try reversing
the feed back wires on the base of each transistor with each other. If
this doesn't fix it, reduce the value of R-2 slightly until it does. If
your inverter seems to be operating properly, but will not handle the
required load current, reduce the value of R-2 by 100ohms at a time until
it does. CONNECTING TO YOUR RADIO: Once you've completed the inverter,
and the two regulators that are also associated with this supply. All
has been tested and appears to work, it's time to hook it up to the radio.
You'll first want to connect the heater supply, insure that the voltage
adjust control on the regulator is set at it's minimum output setting(test
it first without being connected to the radio!). Connect a voltmeter to
the heater supply, turn the radio on, apply power to the regulator's input,
then adjust it's control until the needed heater voltage is achieved.
Usually, the heater voltage regulator can be pre-adjusted without being
connected to the radio. In the case of 1.5 volt tubes, this adjustment
will be minimal. Once the heater circuits are established, turn the control
for the input voltage regulator to the inverter to it's minimum setting.
Connect the outputs of the inverter to the load(radio). Connect a voltmeter
to the receive B+ supply(if a multi B+ system). Apply power to the inverters
input voltage regulator, slowly adjust it's control until the needed inverter
output voltage is reached. You should by now hear noise in the radio's
speaker. If your project is a multi B+ transceiver. Connect the voltmeter
to the trans B+, insure that the near double B+ voltage is there. Now
key the radio and watch this B+ voltage, also check the rec B+ again.
There will be some transient voltage changes but there is nothing we can
do about it. Slight adjustments of the inverter input voltage regulator
may be needed to effect the optimum voltages for both transmit and receive.
If inverter oscillation should drop out during transmit, refer back to
"INVERTER START UP". BATTERY C0NSIDERATIONS: Only two battery types are
generally available in a sufficient amp-hour rating to suite our needs.
These are nicads and gel-cells. Dependant on the application either might
work equally well. But there are some exceptions and facts that must first
be considered. First is the input voltage versus output voltage of our
inverter. A change of only one volt on the input of our inverter can produce
as much as 15 volts difference on the output, enough to make the radio
inoperable. To a degree, the voltage regulator we use will take care of
this variance, but only if a higher voltage is available than what's needed.
So proper battery type and voltage must still be carefully chosen. Second
is the amp-hour rating, the average current drain of our completed power
supply, including heaters, is around one amp. So to have a battery pack
that will allow a usable amount of operational time, the minimum battery
we can use would be a 5ahr type. Where the amp-hour rating is 5 divided
by a current drain of 1 amp, equals 5, minus about 15% for internal losses
in the battery, we come up with about 4.2 hours of operational time. That's
practical, BUT! We must then take into account the drastically different
discharge curves inherent in the two type batteries we have available.
First is the nicad, it's output voltage will remain the same almost until
it is discharged, thus it has a very sharp discharge curve, almost a cut
off. We have the added advantage of being able to draw as much as ten
time the amp-hour rating, I/E if it's a 1 amp-hour battery, we can draw
a much as 10 amps and still maintain battery voltage(but not recommended).
Second is the gel cell, it's voltage steadily drops as the battery depletes.
Thus it has a radically different discharge curve(near linear), combined
with a sometimes drastic difference between the loaded, and no-load, battery
voltage. In the case of the latter, you may have a battery voltage of
six volts, but when a load is connected this may drop by as much as 2
volts. Resulting in an actual supply of 4vdc, which might be borderline
in some applications. So the nicads have the advantages list above, but
are limited in the amp-hour rating we might be able to obtain. They are
further plagued by the memory syndrome, and short shelf life(about one
month/charge). But are smaller than gel cells in terms of size to amp-hour
rating. Gel cells on the other hand are available cheep, in high amp-hour
ratings, do not suffer from the memory syndrome, and have a much longer
shelf life,I/E they will hold a charge for several months. Gel cells have
other problems and advantages too, as do nicads, but space does not allow
further elaboration here. What difference does all this make in our applications?
A case in point, we have a very small radio, with very little internal
space to house both our inverter power supply, and it's batteries, inter
the CPRC-26. The inverter configuration for this radio is very much like
the PRC-6, but the internal space available is very limited. We only have
room enough for four "D" cell type batteries. If we use four gel cell
types, we will not have sufficient amp-hour rating to operate the radio
for any usable amount of time(2aphr). But we will have 8 volts available.
But we only need 4 volts in this application, so two gel cells at 4 volts,
and 3-4ahr should work. Not! Because of the linear discharge curve of
gel-cell batteries, they will only produce 4 volts at the beginning of
it's use, their voltage will then drop as the battery depletes. Thus with
the borderline voltage/amp-hour rating we are able to fit in the radio
we will be limited to about 20 minutes of operational time. On the other
hand, 4ea "D" cell nicads will fit in the box, with a 5ahr capacity, giving
us about 4 hours of operational time by virtue of it's near constant 4.8
volt output(much better). The same will hold true when trying to develop
power supplies for such radios as the BC-611, MAB, DAV etc. On the other
hand, where space available is not a major concern, we have more battery
flexibility. In the case of a PRC-6 or PRC-10, there is a lot more room
than the CPRC-26. We can then use gel-cells to maximum effect, but we
must use a much higher battery voltage to compensate for the linear discharge
curve. While it's still true that only 4 volts is needed to make the PRC-6/inverter
work, we must use at least 6 volts or more to allow for the linear discharge
curve(tapering battery voltage). Here again, the voltage regulator we
installed comes into play. When designing power supplies for such radios
as the PRC-10, BC-1000, etc. Our flexibility is even further enhanced
by the greater space available for both batteries and power supply. In
these applications, only about 8 volts is needed on the same inverter
supply. But we still use an input regulator to compensate for any supply
voltage that might be used. BIAS BATTERIES: Most of the radios we will
be constructing power supplies for will need some sort of negative bias
voltage. While it is often possible to modify the circuit we have presented
here to produce this voltage, doing so will often complicate the power
supply to the point that the radios original on/off switching can no,
longer be used. For this reason, and in the interest of simplicity we
will just use batteries for this voltage. For the most part, any small
type battery can be used that will fit into the available space. Such
as "N" cells, "AA", or "AAA" types. It doesn't really matter as the current
drain of these bias circuits is so low that the battery will usually last
it's normal shelf life anyway. Do not use "C", or "D" cells as you'll
just be wasting money, and room, and adding weight. Use quality batteries,
not "longer lasting", just quality made. Because these batteries will
be in your radio for a long time, and you don't want them leeking all
over the place. As always, if you have any questions, comments, or input,
please let me know. Dennis Starks; MILITARY RADIO COLLECTOR/HISTORIAN
military-radio-guy@juno.com Related Topics: MCGP, MAR.12/98, MAKING CONNECTIONS;
Power Connectors In a Pinch, by Dennis Starks MCGP, MAR.13/98, MAKING
CONNECTIONS; More Ideas MCGP Backmail #28; PC board makeing for dummies.
By Dennis Starks MCGP Backmail #32; Alternate Power Sourses. All persons
interested in this topic are strongly advised to read this file. It details
much of the information given in this series, and debate from several
of our members on the subject, thus giving an understanding of how our
specs etc have been arrived at. ***********************************************
UNIVERSAL INVERTER POWER SUPPLY, PART III; Applications (PRC-6 & CPRC-26),
by Dennis Starks As with most of the power supplies in this series, you
will need to construct 2ea voltage regulator boards, and the basic inverter
power supply. The basic difference then in any of our power supplies will
be as follows; #1)is in the way these boards are connected to the radio.
#2)the primary and bias power supplies. #3)adapting all the above to the
available space, and the radio's original power on/off switching method.
Of these, #3 is usually the most difficult. Particularly if no modifications
to the original radio are to be allowed. VOLTAGE REGULATORS; As can be
seen on the block diagram, the inputs to both regulators are connected
directly to the positive terminal of your battery pack(or alternate power
source). Referring to the universal regulator schematic, you will see
that D-1 is connected directly across the input of the regulator. It's
optional presents is to provide for reverse voltage polarity protection,
and as such should be preceded by a fuse of about 1.5-2amps. There is
no need to duplicate this circuit on both regulator boards, if indeed
you use it at all. In the event you plan to use your radio only with internal
batteries, and these batteries are either hard wired, or provided with
a polarized connection, there will be no need for D-1 on either of the
regulator boards, or a fuse. PRIMARY POWER SOURCE, With the PRC-6 we have
the unusual luxury of having ample room inside the radio for both the
power supply, and batteries. So long as you don't string your power supply
out all over the place! Just about any battery combination that will produce
5-10 volts, with a minimum of a 5 amp hour capacity will work. Even 4
to 6 "D" sized dry cells would have their advantages. If four "D" sized
batteries were used, we could have the field option of changing from nicads
to standby dry cells if needed. Where the advantage lay here is in the
limited shelf life of nicad batteries, the way they were intended to be
used, combined with the way we will actually use the radio. For instance,
I use 4ea "D" cell nicads in both my PRC-6, and CPRC-26. This relieves
logistic problems by only needing one type battery pack for two different
radios. While this works very well, a problem arises in the way these
radios are used. I/E usually at hamfest or other special event, sometimes
days, weeks, or months apart. Thus the nicads with their limited shelf
life are often dead when I go to use them. Hence dry cells can be taken
along just in case, or to be used once the nicads are depleted. Admittedly
I must do the same thing with my 2 meter HT because I don't use it enough
to keep the batteries cycled, or charged up. You should also consider
other radios that your PRC-6 might be used with like I did with the CPRC-26.
Should for instance you would like to have an operable PRC-10, it would
be nice to be able to use the same battery in it that you use in your
PRC-6. Again, relieving logistic problems. Once you've completed a power
supply for your PRC-10, it will require approximately 8 volts to operate.
So you should have a battery capable of around 10 volts to feed it. A
10 volt battery at 5 amp hours can still be fit into a PRC-6! If the "D"
cell format is adhered to, that would be 8ea nicads. True 8ea dry cells
will produce 12 volts, so what, our power supplies are REGULATED! Also
we'll need a few more volts than necessary to compensate for the discharge
rate of the dry cell. On the other hand, you could make up two, three,
or four identical battery packs, each of 5-6 volts. One pack at a time
could be used in your PRC-6, or two of the same packs in series on your
PRC-10. Our interest is in interchangeability and the advantages should
be obvious. Gel cell battery types can also be used, and they will not
exhibit the short shelf life characteristics of nicads. But we are limited
by their larger size versas amp our rating, and the fact that several
more volts will be needed to compensate for their linear discharge curve.
BIAS BATTERIES, Any type of small dry cell battery can be used for the
bias batteries. Use whatever you want, that will fit. Suitable types include
"N", "AA", or "AAA" sizes. No matter what the size, you will not be changing
them enough to even warrant using any type of a battery holder. I personally
just solder them together, and stick them down with a hot glue gun. (CPRC-26)
A special note, the power supply used with the CPRC-26 is almost identical
to the one used with the PRC-6. The only difference is in the bias voltage,
available space, and the fact that there is no chassis ground because
a separate battery box is used. Because of the latter, an aluminum frame
must be made that will just fit inside the battery box. If all is done
correctly, you will not need to make an holes in the battery box. Cut
a piece of sheet aluminum 2.75" wide x 12.75" long, bend it to fit inside
the battery box, or so that it's sides measure 4, 4.5, 4, and 4.5 inches.
Leave the sides open. You will then need to cut out the area occupied
by the radio's power connector. Acquire an spring from an old plastic
"AA" battery holder, you find a lot of them that the plastic is broken
on anyway. Screw, or pop rivet this spring to the top of your chassis.
Then grind the paint off the mating underside of the radio. This spring
will now make your needed chassis ground. The rest of the chassis can
be used to mount your power supply boards negating any need to drill holes
in the battery box. The power connector might be a problem, refer to the
already listed references for ideas on what to do. Should you be good
with your hands, and tools, you may be able to closely approximate the
original connector as I did. In this event, the connector should be mounted
to the sub frame you've constructed. The completed assembly can then be
installed, and removed exactly as the original battery was. It would be
nice if we could find a ready source for these connectors, but that's
unlikely, especially as I have never even seen an original battery before.
Some parting thoughts on the PRC-6, and the CPRC-26. I really like both
radios with a little imbalance towards the CPRC-26. Anybody can walk around
a hamfest, or other special event with a PRT-4/PRR-9, or PRC-68 hanging
from them. But it's a real trooper who go's to all the trouble to make
one of these old war horse work, then actually use it. The biggest failing
that I can see with both radios is the fact that neither government saw
fit to include a squelch circuit of any kind. If they had, battery consumption
would have been greatly reduced, not to mention user comfort enhancement.
It would be nice if someone could come up with a simple solid state squelch
circuit that could easily be added to these radios without the need of
modification, and using power supply voltages the radios already have.
Additionally, it's a simple task to add a inexpensive commercial tone
generator for compatibility with PRC-25/77 type radios. If you have any
questions or suggestions please contact; Dennis Starks Box 95 Cross Timbers
Mo. 65634 (USA) ph.1-417-998-6517 (voice or fax) Email; military-radio-guy@juno.com
*********************************************** BC-611, AND INVERTER POWER
SUPPLY SWITCHING PROBLEMS; Dennis, As you know, one problem encountered
when building an alternate power supply is how to turn it on/off. Mark's
BC-611 idea (which I never saw) using an IC that operates by rapidly shaking
it three times comes the closest as I would assume it required no hard
wired connections to the transceiver. My guess would be he used a counter
and a mercury switch? I've thought about using various switches, relays
or transistor circuits but I can't get around the hard wired connection
and that's the rub!. I heard of a uA sensing IC switch but never came
across one. However, it would probably need to be wired in. On a BC-611
there is little room for anything. I've used a small relay wired to operate
off the filament voltage and my present method uses the existing antenna
switch but requires 3 wires tacked onto the chassis. It works but there
must be a better way. All my schemes revolve around using the filament
voltage, however, I've yet to come up with a brilliant idea. How to switch
the converter without using hard wiring between the receiver and the supply?
So far I do not have a solution. Any ideas? Ed Guzick ed) Mark Gluch is
currently off line due to AOL troubles, and a dead monitor, so he can't
respond just now in regard to his BC-611 on/off switching method. But
it did use a mercury switch and an IC counter. Unlike most military radios
that use a single switch contact, & ground leg switching of all their
batteries to turn them on and off simultaneously, the BC-611 uses two
tandem switch contacts to connect the heater & B+ batteries to their respective
loads via the positive side of the batteries separately. When operation
from an inverter and rechargeable battery is desired, this presents the
peculiar problem of turning on two distinctive power supplies with a single
switch yet still keeping them isolated. To compound this problem, a primary
power source must be connected, & switched that provides both heater,
and inverter power supplies, yet this same switch must pass both the voltage
of the primary power source, and one of the secondary ones while also
keeping them isolated. Of course, this is imposible, so we must contrive
a method buy which one switch will control a second switch that will in
turn serve our purpose. I don't know what your using as a primary power
supply or it's voltage. If it is a design similar to, or the same as that
I presented you are probably using four "D" cells in series for around
5 volts which makes an output of around 90 from the inverter. Try something
along this line, I know you have a selection of tiny relays, some even
1.5 volt, use one of these with it's coil(and maybe a resistor) in series
with the heater supply, place a small 6 volt light bulb in parallel with
the seriesed relay-coil/resistor. The resultant current drain when the
radio is switched on will close the relay turning on the inverter. The
resistor will compensate for relay coil resistance, and the combination
relay coil/resistor will provide a voltage drop for the heaters to operate
from. The light bulb acts like a ballast tube to protect the tube heaters,
and when full-on contributes to the voltage drop needed. When the radio
is first turned on, the light bulb will glow brilliantly because of the
low heater resistance, but as the radios heaters warm up, this glow will
fade to near nothing as heater resistance goes up. Some experimentation
will of coarse be required due to the various relay/light bulb combinations/availability,
so it would be a good idea to calculate the radio's full-on heater resistance
so that a substitute load can be used preventing radio damage. Another
approach would be to use a two transistor "flip-flop" circuit or even
the current sensing methods you mention, but either system would still
require the use of a small relay(dependant on current drain, and transistor
size), and a ballast type heater voltage control circuit. So in the interest
of minimum parts count(the relay coil is an integral part of the heater
voltage control circuit thus serves a dual purpose), and simplicity the
above might be the best way. Once you've hit upon the right relay-coil
type, resistor, and light bulb combination, you might let us know about
it. A possible commercial source for miniature low voltage relays might
be those little DIP relays once available from (eeek) "Radio Shack", in
5, 6 and 12vdc versions. One final note, while the relay coil, dependant
on it's resistance and current requirement, may be placed in parallel
with a large wattage voltage dropping resistor(typically 5 watts or more),
their sum resistance will be higher than that needed for the operation
of the radio's heaters. It's the variable resistance of the light bulb
placed in parallel with these that brings the total circuit resistance
within the operational range of the radio's heaters. A light bulb must
be selected as a companion to the power-resistor/relay-coil, that draws
sufficient current when cold to allow the radio's heaters to slowly light
up. If you are lucky enough to have a supply of 1.5 volt relays, the coil
itself need not be a part of the heater/ballast circuit, it can simply
be connected between the output of the circuit and ground, in this case
only the power resistor, and light bulb form the ballast regulator circuit
for the radio's heaters. All your ideas and input are eagerly sought on
this subject. Dennis Starks; MILITARY RADIO COLLECTOR/HISTORIAN military-radio-guy@juno.com
*********************************************** (The preceding was a product
of the"Military Collector Group Post", an international email magazine
dedicated to the preservation of history and the equipment that made it.
Unlimited circulation of this material is authorized so long as the proper
credits to the original authors, and publisher or this group are included.
For more information conserning this group contact Dennis Starks at, military-radio-guy@juno.com)
***********************************************
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