Unilab 10m FM conversion - Transmitter VCO Board

The transmit VCO proved to be much more challenging. I started with calculated values for the capacitors and used the same biasing of the varactors as I discussed in the receiver VCO conversion.

This time I used a toroid in the tank circuit, a T50-2. I started with 12 turns but ended up removing 2 turns. I removed the resistor and thermistor and soldered a link on the source choke to ground. While I used a 91k resistor in the gate, 100k will be fine. The picture below still shows the varactor biasing diodes I put in place for testing. Remember to remove them before installation back in the radio!

Installing the VCO board back in the radio I found myself going in circles solving loop lock up followed by a poor output spectrum. Rather than bore you with the sorry tale let's cut to the chase!

The mmic buffer used is very susceptible to being over-driven and generating harmonics. Yet a small reduction in output means the pre-scaler does not get sufficient drive to work properly. I made the following changes:

• The 7p coupling capacitor is removed and I put a 2p5 and a 1p5 capacitor on the board, one on each side. This reduced the drive and the distortion on the output of the VCO board was much less apparent. 
• I then bypassed R407 with a wire link to get the pre-scaler working correctly. And finally, 
• I removed R411A to reduce the gain of this buffer and improve the output waveform still more.

If you haven't been able to find the circuit for this radio on the web here is a partial snapshot to help:

At this point I have a PLL which locks up and a nice waveform going into the remaining stages of the exciter.

Just like the receiver VCO board, I now looked at the variation in the control voltage as I locked at the top and bottom end of the 10m FM segment. The maths demonstrated that the existing loop filter was "awkward".  I ended up putting a second varicap in parallel with the existing one and changing a few capacitor values. Since making the initial mods using a toroid I had found a suitable variable inductor and I substituted that for the toroid. It makes adjustment for lock that much easier. I settled on a 3.5V variation in the control voltage and ignored the loop filter. I might revisit the loop filter after everything else is working.

The final(?) VCO component values are shown on the following schematic:

The balance of exciter board mods are in the next instalment.

73's
Richard