I recently restored a Viking Ranger that I picked up at a local auction. The unit already had the keying mod with the 12AU7 and 6AL5 bias rectifier installed. I added the PTT to talk mod, solid state diodes (1N4007) to the power supply and increased the capacitance in the HV circuit. For the PTT, I used a 12VDC relay and for the coil relay voltage I constructed a voltage doubler circuit taken from the six volt filament supply. I am always a bit nervous about the power transformers in this older equipment. The manufacturers often spec'd components to operate at pretty much close to their limits, and since these transformers have been around for quite a few years they often are getting a little tired, so any help they can get is usually appreciated. With solid state rectifiers in place of the rectifier tubes, the load on the transformer is reduced by approximately 17 watts because of not having to heat the rectifier filaments, however with SS rectifiers that means at turn on, there is an immediate surge to charge up the filter capacitors. I thought I would like to reduce that surge a bit by installing a soft start circuit which would limit the input current for a few seconds before allowing full voltage to be applied. I chose to use a solid state relay, mainly because I had one in the junk box. It is an Opto-22 model 120D10, capable of switching up to 20 amps. The turn on voltage can be as little as three volts.
http://radioremembered.org/images/RangerSoftStart/ssr-1.jpgThe relay contacts (the "contacts" are two Silicon Controlled Rectifiers back-to-back) are in series with the power transformer primary and a 90 ohm 10 watt resistor is connected across the contacts. This puts the 90 ohm resistor in series with the primary until the contacts close, which then shorts out the resistor.
The relay turn on time delay consists of a 3.9K resistor in series with a 4700 mfd capacitor. The voltage is taken from the 12vdc relay supply. When power is turned on, DC voltage from the PTT voltage doubler circuit is applied across the resistor and capacitor and it takes approximately six seconds for the charge on the capacitor to reach around 3 volts, at which time the relay kicks in to short the 90 ohm resistor. This not only gives the filter capacitors time to partially charge, but also is less shock on the tube filaments. See the circuite below. Added components are in red.
http://radioremembered.org/images/RangerSoftStart/softstart.jpgThe SS relay was mounted on the side of the chassis directly behind where the line cord enters. I had to remove the input filter coils and capacitors in order to have room for the relay (TVI is not the worry it was back then). I left the two terminal strips in place and the relay fit nicely between them. Picture below.
http://radioremembered.org/images/RangerSoftStart/ssr-2.jpgThe values for the time constant (3.9K and 4700 mfd) can be changed to give longer or shorter relay engage times.
I plan on adding a bucking transformer to reduce the line voltage from the 120+ volts I have here to around 115 which should reduce stress on the components even more.
Bill - K5MIL
http://radioremembered.org
