Power Supplies for Military ATUs
Power Supply Considerations for the Collins 180L and C/U 358 Auto-Tune Antenna Couplers
First off, you need to understand that the voltage present in this supply is much higher than usual and because of this, you must take precautions. The voltage is not high enough to kill anyone in most cases, but it is high enough to hurt and to cause you to hurt yourself getting off a bare wire. Trust me, been there, done that, got the scars to prove it. So if you undertake this project, consider this a warning. YOU CAN GET HURT IF YOU DO NOT EXERCISE CARE with this project.
In the diagram, Figure 1, transformer T-1 supplies the raw ac to the diode bridge, B-1 which then rectifies the ac, and this rectified ac goes into a choke which helps to smooth out the pulsating dc. The output from the choke goes to a capacitor C-1, which further smoothes out the voltage and removes nearly all of the remaining ripples from the dc voltage. Resistor R-1 is a bleeder resistor that serves to bleed off the voltage after the power supply is turned off. This is to keep the voltage stored in the capacitor from biting you after the power supply is turned off. This is important as a good capacitor can hold a charge for a long while and can knock you for a loop if you get across it. Another trust me. It has happened to me, after a power supply was off for 24 hours! I guarantee that it will bite you, so do not leave it out.
A parts list will be found at the end of the article. This diagram is to be used for a transformer that has a voltage output of 250 volts with a center tap which is not illustrated. A bridge rectifier will give the entire voltage out of this transformer. If you have a transformer that says 500v center tapped, then you will want to use the diagram in Figure 2. This is a conventional full wave rectifier and will give you the same 250v output as the full wave bridge. All other components are the same.
In both diagrams, the 28 volt dc supply is illustrated. The requirements are for 24-28 vdc @ 4A. This is needed for the filaments of the tubes, motor power, bias for the various stages and relay power, as there are at least 10 relays that require 24 volts. Converting to a lower voltage is out of the question. Don’t even think about it. Transformers are available that will put out that kind of voltage and current. It will involve some looking around, but you find them.
The requirement for 115vac 400 cps can’t be gotten around as this voltage is used in the coupler for various functions. This is a supply that can be built using a transformer made by Triad and they supply the plans with the transformer. The part number for this transformer is TY-462. It is used in a switching type supply that runs off of 12-13 vdc and require about 6 amps for a 60 watt load. However in this case, the load in this particular project is not that large. The output is a square wave, but the one small motor and the other components do not seem to care, at least not in the tube tuners.
Now to the power transformer for 250 volt supply. If you cannot find a suitable transformer for the 250 volts supply, then you will have to improvise. There is a way to get the voltage you need. It is by using several transformers and connecting in series. Figure 3 is one example of how to do this. By taking 3 identical filament transformers and connecting them as shown. The secondary windings of all three transformers are connected in parallel.
Now from this point on, as long as the transformers are energized, potentially lethal voltage is present on the two transformer lines. The 115vac windings are phased correctly by connecting two of the 115 vac primary wires together. Then the voltage present at the two unconnected leads is measured. If the voltage is approximately 220-250 vac, then the windings are connected correctly and can be wired into the rest of the power supply circuit. If there is no voltage present, them shut the power off to the transformers and then reconnect the primary wires of T-2 and T-3 and measure the voltage again. You should have the voltages mentioned above at this point. If not, shut down the power and recheck all of your connections.
Assuming you have all the correct voltage, then you can continue with the power supply wiring. You should use the 250 volt power supply in Figure PS-1 since you will need the total voltage from both transformers. The bridge will give you the full output from both of the transformers.
Parts List
Bridge power supply:
Diode bridge at least 400piv, .5a C-1 10-20 mfd, 350vdc, Choke can be any size you may have on hand, if you cannot find one, then use a resistor of about 500 ohms to limit the initial charging current to the filter capacitor.
Full wave power supply:
Diodes need to have about 300-400 piv at .5 A rating, and the other parts are the same as for the bridge supply.
28vdc supply:
50v piv bridge at 6A and a filter capacitor of 20 mfd at 35 vdc or so Transformer must be capable of at least 4 A.
This is the basic parts list. Fuses and power switches are going to be the same for both supplies. Use good construction practices and follow safety procedures for high voltages at all times
Power connector:
So far, no one has been able to find one of these connectors, so you will have to either solder direct to the pins, or make a connector using Molex connectors or whatever you wish to use.
Pin connections:
Some of the connectors letters of the alphabet rather than numbers. Usually pin 1 corresponds to a pin A. You should check it out though with an ohm meter before hooking up the power supply. Here are the pin connections according to the schematic I have, courtesy of Gary Reed, WB8OFU. These are the power connections required.
Pin 13 +24/28 vdc
Pin 16 +400/250 vdc
Pin 8 115vac, 400Hz
Pin 15 Ground
Pin 14 +400/250 vdc
After the power supply is finished and you have built up the 3-dB pad (if using a solid state transmitter, you will need one) that was discussed in a previous article, you should be ready to put the coupler on the air. It will be a good addition to your station.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 25th April 2024