Friend N2CQR posted my KWM-2 Power Switch Fix on an Antique Radio Forum and of course that prompted a posting (possibly from the resident blog police person) that in essence said that is not the way to do it. The blog poster also mentioned something about using relays in the 516F-2 power supply.
In truth, the relay in the power supply is a better solution and now I will show you how that is done. But what the blog poster police person forgot is that to enable the relay in the power supply you still need a working switch in the KWM2 or 2A. So, step #1 fixes the switch and step #2 installs the relay.
The theory of the relay has as its roots in what is called the Motor Start Circuit. If you want to start a 50HP motor, you do not use a toggle switch in line with the motor. But what you do is to use the toggle switch to control the field coil of a 50HP rated contactor (fancy name for a really big relay) where our field coil draws hundreds of milliamps while the contactor pulls in the current for the 50HP motor (read 187 amps single phase at 230VAC). A fancier form is to use two momentary push buttons where one starts the motor and the other stops the motor.
But we will just stick with our simple snap switch. Using the relay dramatically reduces the current draw through the snap switch. Now, as long as you are using the 516F-2 power supply with the relay you could get by with a one-amp snap switch. But if you use the Collins supply without the relay then you risk smoking the 1-amp snap switch.
So, what comprises the Motor Start Circuit?
Essentially the KWM-2 Power Switch is in series with the field coil of the 120VAC relay and the switch and relay coil are connected across the Hot (Black) and Neutral (White) wires in the power supply. The Neutral is subsequently connected to the Power Transformer. With the Relay, the Hot side is further connected to a set of N.O. (Normally Open) contacts on the relay. Engage the KWM-2 power switch and the relay closes feeding 120VAC to the primary of the transformer. Our newly installed Relay should have a contact rating of 10Amps AC at 125 Volts.
Internally the 516F-2 Collins supply must be modified, and this entails a four-lug terminal strip, a place to locate the relay and a bit of internal re-wiring.
START BY DISCONNECTING THE 516F-2 FROM THE POWER MAINS AND ENSURING THE ELECTROLYTIC CAPS ARE FULLY DISCHARGED! SAFETY FIRST!!!!
Any internal wiring must be downstream of the fuse block. Thus, begin the rewire by carefully observing the wire colors on the schematic to the as built actual supply. Next find the two wires (Green White and Gray White) that come from the KWM-2 Power Switch (pins 5 & 7 on the plug) leave the Green White wire intact but disconnect the Gray White which was connected to the power transformer but now goes to a lug (non-ground) on the new Terminal Strip.
Our Gray White wire on the new terminal strip will now connect to one of the pins of the relay field coil. You will need a short piece of wire to make the connection depending on where you locate the relay. Ensure the wire is insulated and can carry 5 amps. The other end from the relay coil is now connected to the White (Neutral) lead. You may need to create an interim tie point to connect from the other side of the relay to the White (Neutral) lead.
A new wire from the Fuse block where the Green White lead is connected is now connected to one of the NO contacts and the other NO contact is now connected to the primary side of the transformer (Black wire on the Transformer) where the Gray White wire was formerly connected.
Check and double check the wiring and read these instructions several hundred times. If you have never done this kind of wiring, you best not undertake the project as you will likely smoke your radio. The alternative for those lacking homebrew genes is to use a surge protector with an On Off switch and use that to turn on the KWM-2.
The operation is simple. Turn the KWM2, to OFF. Plug in the 516F-2 Power Supply to the radio and the main power cable to the 120VAC mains. Turn ON the KWM-2 and you should hear the relay click and the KWM-2 lights will come on.
Them That Know, Can Make It Go!
73's
Pete N6QW
Hopefully Brad Pitt's 30 something girlfriend has something exciting planned to celebrate his special day. As for me I plan on getting myself a cupcake at Albertson's and have some candles left over from birthday's past. Just another day on the hamster wheel.