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The “RAC” Digital-Dial for Your Old Boatanchor
Written by: Jeff Covelli / WA8SAJ
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The thought of installing a new digital anything to an old boatanchor, some folks might say what is going on here. I first saw this neat little gadget at the Dayton Hamfest about 10 years ago. Lee WA3FIY was showing some QRP gear and one of them happened to have a “Digital-Dial” installed to readout the frequency of the rig. I asked where did you get that, and he replied I make them, wow I was really interested now. As we talked, he brought out a Model A-2W, which is the factory wired version of the Digital-Dial. I set out to install it into a Drake TR-4CW, and it worked great, so I thought maybe I could install it in some of my other old rigs I had laying around, the Hammarlund SP-600JX-17, Swan-700CX, Hallicrafters SX-115, and finally the Drake R-4B. I will talk about the Drake R-4B, since it was the easiest to install and the lightest one to move on the bench for this article.
The Digital-Dial is a general frequency counter up to around 50 MHZ. The six-digit readout for the frequency is bright red LED’s and they sure look nice against the blue dials on the Drake gear! plus it has the capabilities of direct readout being displayed with an I.F. offset. Now with that in mind, it also has “five” memories that can be programmed to have the readout count forward or backward, depending on the mixer stages. You can have the readout to 100 HZ plus have the LED’s blank “off” when not moving frequency to save on current draw. When you go to move frequency again, the LED’s come back on. There is a built-in feature for up to 16 different memories vs. the 5 that come with the unit, but you have to make the changes yourself. Most folks only need the five common ham-bands, so that works well with the stock unit. The readout does not jitter like most frequency counters, but has an “anti-jitter” code built into the RAC C-5 chip. The C-5 chip has all the crystal time base, counter logic, drivers, and memory control to do it all, not bad compared to just a few years ago, when you would need a pile of chips to do that. The Digital-Dial needs from 75 mv to 1 volt of drive at 50 ohms to make it count, and it should be free of mixer products, or else it will jump around. The unit is nice and small, measuring 2”H x 4.5”W x 3.75”D, in a very attractive aluminum cabinet. I use RG-174 50-ohm coax coming out of the rig to feed the counter and it works great. Most rigs have enough drive to make it work well. For the rigs that have low drive, RAC makes a Buffer/Clipper (BK-174) which I recommend. The Buffer/Clipper is used to keep the Digital-Dial from pulling down the RF drive from the circuit it is counting. On some of the rigs, there is so much drive, the Dial has no trouble at all, and the circuit is not affected. The voltage required to operate the Dial is between 9 – 13 volts DC @ 100ma. The Buffer/Clipper only needs 6.3 vac from a filament string or about 8 volts DC, it has a rectifier built-in for ac. The board measures 0.75 by 1.5 inches, very small and can fit almost anywhere. I have used the filament string for my tap-off to a half-wave diode and filter, then I can supply DC voltage to the Digital-Dial using RG-174 coax along with the output cable for the counter out the back of the radio.
Rear View of the Digital Dial Circuit Board