Last Revision: 12 March 2008
For my 3 Band transceiver project (see different article here in TechMat), I have been looking around for a transverter from a shortwave band to 50 MHz. I found someone selling the TenTec 1208 transverter which uses 14 MHz for IF. This low IF makes it easy to modify to 10 MHz, where I want to use it.
The TenTec Manual is well written and following all advice, the transverter is quickly built. There is nothing special and it ran almost immediately after completion. I mechanically broke the driver transistor (2SC1970) and I had to replace it. I was lucky to be able to use a spectrum analyser and signal generator for tuning, this makes things easier especially when there is some tendency for oscillation in the PA. This oscillation was stopped by resoldering many contacts in the PA.
I have "tuned" the low-pass filter in order to get the output power and reduce the 100 MHz harmonic. This tuning was made by making a short between the winding of the coils (L22, L23, L24) with solder and by bending the coils a little bit. See image above.
Originally this transverter is set up for an intermediate frequency (IF) of 14 MHz using a 36 MHz crystal. In Funkamateur 2/98 p182 is a description of a modification for 28 MHz published. For a 10 MHz IF however I need a 40 MHz oscillator. Thanks to Thomas DJ5LY I got a highly stable 40 MHz clock oscillator with CMOS output (Pletronics P1100-HC 40 MHz DIL). Additionally I needed a 7805 5V regulator for supplying the oscillator. I removed L8 and the crystal and feed the oscillator output via a 470pF to the base of Q7. The oscillator itself was soldered on the side of the printed circuit board (PCB). Beside the oscillator I placed the 7805 regulator and a 47nF capacitor on the +5V output. With a small lead I connected the regulator input to 12V at R27. The only other modification to make is to solder 47pF in parallel to C7 in order to get the receiver output in resonance on 10 MHz. This is now easily tuned to maximum output while receiving. The conversion gain of the transverter is ca 15 dB now.
40 MHz Oscillator installed
Power Output with 17 dBm drive: ca 23 dBm (200mW)
max measaured + 38 dBm (ca 7W).
40 MHz oscillator at output: -35 dBm (ca -70dBc at full power)
100 MHz 2nd harmonic: -60 dBc
Receive Conversion gain: 15 dB
This seems all OK to me ... The transverter is prepared for my project ...