The Collins 20V Series:
Some Conversion Tips
Phil Galasso, K2PG
If your transmitter is already on the high end of the AM broadcast band, you won't have to do much to get on 160. Before you get started, be sure to test the transmitter on its original frequency by running it into a dummy load. This can save you a lot of aggravation during the conversion. Plug in a 160 meter crystal. The crystal goes to pins 3 and 7 on the octal crystal socket. Crystal ovens are not used in this transmitter. You will have to make an adapter for your crystals, as the pins of an FT-243 holder will not reach pins 3 and 7. Solder some bare copper wire to the lugs of a crystal socket, insert them into pins 3 and 7 of an octal plug, and solder them into place. If you want to be a purist, International Crystals sells direct replacement crystals for both the Collins and Gates transmitters. They cost about $50 apiece. For the initial tune-up, remove the high voltage rectifiers and the modulator tubes. Turn the transmitter on and press the PLATE ON button when the green "ready" light comes on. Open the two front doors on either side of the picture window. Adjust the screwdriver adjustments (butterfly capacitors that resonate the oscillator, buffer, and driver tank coils) for maximum drive to the buffer grid, the 807 grid, and the PA grid. If you cannot get adequate drive, you will have to open each coil can and bypass one winding. You should get approximately 15 mA of grid drive to the PA stage. Turn the transmitter off and shut off all primary power.
Reinstall the high voltage rectifiers and the modulator tubes. Open the doors in the back of the transmitter. Open the aluminum PA tank compartment. On the vertical coil to the right, select the fourth tap from the top and the first tap from the bottom. If there are only three taps on the top part of this coil, select the third tap and omit the tap on the bottom. On the large horizontal coil running from front to back, select the 13th. full turn from the back of the transmitter. The transmitter should tune up normally on 1885 and 1945 kHz. The PA tank is a Pi-L network that will match a wide variety of loads while suppressing harmonics.
Some people have put the 20V series on other bands. Collins did make an HF version of this transmitter that could operate up to 30 MHz. You would have to rewind the plug-in coils in order to use any band other than 160 meters and you would have to change a fixed capacitor in the PA tank compartment. One configuration converts the 6SJ7 buffer stage into a frequency doubler. I have never tried it.
The indicator lamps on this transmitter are Type 10S6, 10 watts, 230 volts.
They are available from:
Griffith Electric Supply Co.
5 Second, Trenton, NJ 08540
The meter panel lamps are old-fashioned GE Lumiline bulbs, 40 watts at 115 volts. Two of them are connected in series in the transmitter. They are available from:
1436 East Cliff Road, Burnsville, MN 55337
Toll free 888-990-9933 (USA) or 952-707-8600 (Canada & outside USA)
To key the plate voltage on and off, you may use the remote control terminal strip (E-105) at the lower left of the transmitter, as you face the rear of the transmitter. For turning the plate on, connect a set of normally open contacts to terminals 6 and 8. For turning the plate off, remove the jumper between terminals 7 and 12 and connect a set of normally closed contacts to those terminals. Relays should be used here, as the control voltage is a VERY DANGEROUS 230 VAC.
This transmitter does not have provisions for remote control, as the remote control of AM broadcast stations was illegal in 1952, when Collins released this model. You will need to use a latching relay or a regular relay connected as a latching relay in order to switch the plate contactor on and off. The contactor will normally pull in when the time delay relay has cycled, although the plate voltage will not come on until the PLATE ON circuit breaker has been turned on. Just connect the contacts of the latching relay in series with the winding of the plate contactor. You are switching 230 VAC here.