Multi Elmac AF-67 Transmitter & Power Supply

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The Elmac AF-67 is often overlooked for an AM transmitter. It has two 6L6s for high level modulation that is superior to low level modulation as in the Heathkit DX-60 and others. The modulator tubes require 22.5 volts bias, I use two 12 volt batteries about 1" long and wire in series for 24 volts bias. A 5 pack batteries is about $5. Lasts many years.

Many AF-67 are listed on eBay for about $75 and the light weight does not have an excessive shipping cost. I would replace the two 10uF 50 volt and 10uF 450 electrolytic caps with new capacitors. Two paper caps at .03uF and .1uF both at 400 volts finishes the capacitor replacements.

Spray all the rotary switches and tube sockets with De-Oxit. The issue is most are sold without a power supply. An inexpensive power supply can be built using an Antek transformer model AS-2T200 transformer for $52. Wire the two 200 volt windings in series and use a full-wave bridge with four 1N4007 diodes. Use two 470uF 450 volt capacitors (C1 and C2) in series for HV and use one 270uF 450 volt cap (C3) for the low voltage at the HV winding center tap positive voltage and to C2 capacitor ground side. 400 volt caps are also acceptable and all three caps in 400 volt can be bought for about $10 on eBay from one supplier with one shipping cost.

A three pole 'ice cube size' (not smaller) relay is needed to remove high voltages when not transmitting. Use a single 1N4007 into a 220uF 50 volt capacitor from the 6.3 volt AC filament ungrounded side of winding. Wire the 6 volt DC to the 6 volt DC relay coil. The AF-67 has a microphone jack keying circuit that provides a ground from the microphone NO contact at the connector. Wire that to other side of the relay. The third relay contact is for the Dow Key antenna relay. Looking at the relay use the contact side with the one wire for the 520 volts. (The other side of relay has two contact wires). The other side of relay that has two wires, use that for low B+ voltage and Dow Key antenna relay. A 300-volt rated relay does not arc over when wired as stated. Finally use a two watt (one each) 300 ohm resistors and .1uF 1KV ceramic cap and wire in series for two arc suppressors. Then, place the arc suppressors across the low voltage and high voltage contacts to remove contact arcing. This is important and especially so on the 520 volt contact. eBay has a 9-lug terminal solder lug strip for $6.60 shipped.

The HV windings center tap for low B+ provides about 260 volts. The high B+ produces about 520 volts with 7 volts AC ripple under peak current that is more than low enough. Wire the two 6.3 volt windings in parallel. An inexpensive Bud chassis can be found on eBay. Cost is about $100 (less shipping). Schematic below, add cap (C3) at center tap (not shown). Center tap is not connected to C1 and C2 junction.

Note- The AF-67 needs -24 volt bias. This is accomplished using a two battery holder with #23A 12 volt batteries. -24 volts is better with the 260 volts from the power supply above on the modulator screens vs the stock 220 volts from the Elmac power supply with originally used -22 volts. Batteries lasts at least 5 years.