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Color Coded Crystals In The Drake R-4A & B Series
By: Garey Barrell, K4OAH
The R-4 / T-4X series of receivers and transmitters were able to operate either as separates or to transceive under control of either unit. The "Transceive Align" procedure described in the T-4X manual is required to ensure that the receiver and transmitter are on exactly the same frequency when in transceive. The transceive system used in the 4, A and B lines relies on the relative stability of the carrier oscillators in the receiver and transmitter. This was less than successful in the R-4 and R-4A versions, especially when the equipment was turned on and off between uses and/or room temperatures varied more than a little. Somewhere late in the life of the A series, Drake began color coding the carrier oscillator crystals in an attempt to match their frequency vs. temperature characteristics. There were at least three color coded varieties of crystals. Red, Green and Blue. There may have been others, but I have owned all of the above. The serial numbers marked on the rear of the coded receiver and transmitter each end in a letter, R, G or B. There is also a color stripe of paint on the crystal under the chassis. For best temperature tracking, you need to have the same "color" crystal in both the receiver and transmitter. I am not absolutely certain about the following, but to the best of my recollection the Red crystals were a positive TC, (Temperature Coefficient), increase in frequency with increase in temperature, the Green were approximately zero TC, and the Blue were negative TC. I think this information came from Drake some years ago. IF this is true, the worst "mixed" case would be a Red/Blue pair, with a Red/Green or Green/Blue better.
Obviously, a matched pair is the optimum, but we don't always have that choice these days. Many years ago, when I first ran into this situation, I was able to exchange a crystal for the receiver in my pair with Drake. I had a Blue/Green set, and exchanged the Green for a Blue at no charge with Drake. SO.... Don't rely on the letter on the serial number. You may get the receiver I modified, or one of the many others that were probably similarly done. Look at the color stripe on the crystal to be sure.
Today, if you have a pair of Drake's that are "keepers" and you find that the "Transceive Align" adjustment doesn't hold over time, temperature matched crystal pairs may be ordered from International Crystal Manufacturing, Inc. The C Line solved this problem by adding another control cable between the receiver and transmitter. The CAR OSC cable patches the two crystal oscillators together, enabling a sort of "phase lock" between them, thereby keeping them on exactly the same frequency.
Garey Barrell, K4OAH