Receiver VFO Controlled CW Transmitter

After working a contest or two it doesn't take a wise amateur long to understand that time is a premium. A kilowatt helps to bring contest points, but with the advent of highly selective receivers and crowded ham bands high power often goes unheard. Like snatching cola bottles out of the cases as the delivery truck drives by, contest operators have to be fast, smooth, and on frequency. And needless to say if we have our transmitter exactly on frequency at all times and are. taking up only one frequency on the band there is that much more room for QKM free operation of other stations. With this in mind I realized that I needed a rig that was not commercially available.

There are a few store bought rigs that have transceive operation but most are designed exclusively for SSB or are designed for SSB and include CW operation as an add-on feature which has been given little consideration. Shortly before beginning construction Oil the rig described herein I practically gave away a $2000 transceiver with atrocious VOX keying and a transmitter that was always 1.3 kc oil frequency.

A glance at the block diagram of my transmitter (Fig. 1) will bring to visualization what is happening inside the transmitter. The

Receiver Block Diagram
Fig. 1. Block diagram of the receiver VFO controlled CW transmitter.

variable crystal oscillator generates a signal which can be varied several kc either side of the frequency which brings the transmitter in exact coincidence with the receiver. This we will call the center-frequency. In my particular case I am using a Drake 1-A receiver with a 4.6-4.0 me vfo. This signal source is tapped directly at the grid of the mixer tube it is feeding and cabled out of the receiver by means of high impedance microphone cord and phonograph connectors. Capacitive coupling is used to bring the if to the signal grid of V2. If you don't like the idea of delving into your receiver to pull out the sample of rf needed to excite the mixer you can easily modify your existing vfo, or any external vfo to operate on 4.6-4.0 mc, or whatever frequency you desire, other than one in the ham bands, and still have the advantage of a fine heterodyne, grid-block keyed exciter. Many will be interested'in using both the receiver vfo and a separate one for cross-band operation, chasing DX etc.

By applying the VXO output to the injection grid^of V2 we get the desired operating frequency along with various image signals and harmonics of the two inputs. When figuring the proper VXO crystal center frequency for use in conjunction with your receiver be sure that none of the harmonic or image frequencies fall much closer than 10%-15% of your operating frequency. To determine the VXO crystal center frequency if your receiver vfo is above the received signal frequency, subtract the received frequency from the vfo frequency, If it is below, add the received signal frequency to the vfo frequency. For example in the case of the Drake 1-A, when its vfo is tuned to 4.6 me the received signal frequency is on the low edge of the ham

Fig. 2. Schematic of the transmitter. Note that many of the parts used come from a Geloso VFO, but can easily be found elsewhere.

Electron Tube Vfo Mhz

V3 serves as both an amplifier and a device to filter out imdesired signals with both grid and plate tuned circuits.

At this point it might be interesting to note that I he sub-assembly chassis that V3 is on was originally a Geloso all-band vfo, and the 5763 was used in it as an amplifier stage. The tube socket that V2 is in was original and held a 6CL6 previously. VI was mounted in an empty space forward of the 6BA7. Other components supplied by Geloso are, the slug-tuned coils, their associated trimmer condensers, the VXO tuning capacitor, which was first a vfo grid circuit part, and assorted resistors and by-pass capacitors which just happened to be heeded in the new circuits. For those who have an unused Geloso vfo kicking around you might employ it's compact chassis in the construction of your! version of this transceiver. If you don't you might go out and buy one, they are relatively cheap. The V2 plate circuit slug tuned coils were left untouched as far as taking off any turns or re-winding them. However, they were taken out of the single ended circuit and used in a pi-network with a 140 pf capacitor added on the plate side to tune them to resonance, and a fixed 100 pf capacitor placed

Front view of the transmitter, bands, in order to determine the VXO crystal center frequency on 40 meters we add 4.6 mc and 7.0 mc to get 11.6 mc. As the vfo is tuned lower in frequency the transmitter frequency will raise, 4.6 mc gives a transmitting signal of 7.1 mc, and 4.2 mc brings us to the top edge of the band.


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