Simple VXO - Rock Mite 20 Meters Dec 2002

I decided to try to VXO my Rock Mite so it would have the ability to tune a little above and a little below the 14.060 Mhz crystal frequency. I first tried a 5-60 pf variable NPO capacitor. I lifted the crystal lead from the junction of R10 and soldered the variable cap in series with the crystal and R10. The oscillator would not tune below about 14.061 Mhz which was unsatisfactory since the QRP calling frequency is 14.060. I then tried placing a 4.7uh molded choke in series with the variable cap and R10. The oscillator tuned from about 14.057 to about 14.065 Mhz. The 8 Khz tuning range was more than I expected, and it was now both above and below 14.060. However, the low end frequency reading was roughly 2.5 Khz below the crystal's fundamental frequency and the high end was roughly 5 Khz above the crystal's fundamental. Since adding the 4.7uh inductor in series with the variable cap shifted the frequency down below 14.060, it seemed reasonable that additional inductance might shift it down more. So, I tried a 6.8uh inductor, just to see what would happen. The low frequency went down to about 14.054 Mhz and the high end went to about 14.064 Mhz. Not only did the low frequency shift down but the total tuning range increased. It was now about 10 Mhz. The distance away from 14.060 was now about 6 Khz on the low side of 14.060 Mhz and about 4 Khz on the high side. My curiosity was aroused. Would adding more inductor produce an even wider tuning range? I wired an additional 2.2 uh choke in series with the existing 6.8uh for a total of 9uh. The tuning range ended up much broader than I expected. The lowest frequency was now about 14.048 Mhz and the highest frequency was about 14.063, which is about 15 Khz of tuning range 15 Khz seems like a lot of tuning range for a single crystal VXO. I wonder if I have an exceptionally active crystal? Anyhow, I gave up experimenting at this point. I decided to use the 60 pf trimmer and the 6.8uh choke for my VXO. Once the installation was finalized, I listened for stations on the air and found I could hear signals throughout the trimmer cap tuning range. There is a noticeable peak in the center of the receiver crystal filter. However, signals on either side of center come though plenty loud enough to work off center stations. This suggests that an audio filter might be in order to give this excellent radio more selectivity.

I Initially had difficulty tuning my Rock Mite to a received signal in order to be at the proper offset audio frequency when I transmitted. I got the answers I needed from the article Rock Mite VXO Mods by Wayne NB6M. Wayne explained that he first zero beats the received station using his VXO. The Rock Mite function switch, the same one that adjusts the keyer speed, is then pressed "one" time switching the Rock Mite to its other receive frequency. This operation set's the Rock Mite for proper audio offset while the transmitted signal is dead on the other station' s frequency. I strongly suggest you read Wayne's excellent article. You might be more interested in his wide range VXO. It can be found at the Rock Mite files web site

I checked my transmitter's steady state RF output into a dummy load as I varied the VXO's trimmer cap from one extreme to the other. I still had full RF output and the slight changes I saw as I varied the trimmer were insignificant.

If you decide to try this mod for your Rock Mite, remember that the trimmer cap must be isolated from ground. I was able to isolate my trimmer by gluing it, with epoxy, to the plastic rotor inside an old potentiometer housing:

Ron - K3PF

RF Gain Control - Rock Mite (20 Meters)

I tried a 100K pot between C1 and D1 as the Rock Mite instructions suggested for an RF control and found that gain variations were not smooth throughout the pot's travel. The volume jumped from full loud to a noticeably lower level with a small change of the pot's resistance. Further pot changes did not reduce the volume much until I adjusted the pot near its extreme end. I checked the pot with an analog ohmmeter and it looked good. The problem was somewhere else. I tried a 50K, a 10K, a 5K and a 1K pot and found that the 1K pot worked best. There was a smooth transistion as the volume went from full loud to minimum. I noticed some noise as I varied the pot, but it was not too bad.

If you want to add this circuit to your Rock Mite, you must first cut away the PC board track between C1 and D1. If you carefully look at the board, you will notice that there is a very short track between C1 and an unused pad. The unused pad is one that can be used for a 1K resistor if you have local broadcast station interference. That same unused pad also goes to D1. So, cutting the short track between C1 and the unused pad will separate C1 from D1. Solder a wire from D1 to one side of the new 1K pot. Solder another wire from C1 to the wiper. The other side of the pot goes to ground. My board had the 1K broadcast interference resistor already added in. I did not try the new RF gain pot with the 1K broadcast resistor removed. I checked for changes in the transmitter's RF output level with the new 1K pot in the circuit. I still had full RF output and the slight changes I saw as I varied the pot were insignificant.

Ron - K3PF