before vco quit operating on transmit/receive at 28.705
Like I said before, don't attempt to go that high with the circuit as-is, you'll throw out spurious (unwanted mixing products). Read the earlier post for the explanation.If you need to go higher (for example, to cover the 10m ham band) you need to change the 10.0525 crystal to keep it clean.
Formula: fChange = xChange * 4. For example, changing the 10.3125 will raise the thing by 1.04Mhz, giving you 28.005 on channel 1 instead of 26.965.
This will let you go cleanly up to about 29.5Mhz.
before the vco mixer looses output due to weak 21 mhz injection
Not quite, you are asking the mixer to something it can't do. On rx and tx, you'll get issues like 20khz steps and the radio getting to a point and quitting, until you either tune it back down or turn it off and back on again. On tx, a spectrum analyser screen will show fog and grass insead of nice sharp vertical lines
What needs to be done on vco and mixers to increase range above what i'm getting
I did add a second varactor bb910 in parallel
The second one won't help much - the actual capacitance isn't so important, what gives you the range is the ratio between Cmin and Cmax.
If you want more range, change the values on my diagram: 4.7pF cap to 2.2pF and 47pF cap to 100pF.
This will give more range but will also increase phase noise (increased "splatter" on rx, poor audio on SSB transmit are the 2 main symptoms of this).
It's a compromise to get it right - the more channels you cover, the worse the performance of the radio becomes.
By design, some chassis are better at this - the Cybernet tops out at 2Mhz without major work, the Uniden 858 and 8719 radios can double this.
Have a look at the export (120 - 200 channel european) versions of the Cybernet and Uniden radios - the Cybernets switch in a bunch of crystals for the "banks", the Unidens use binary adders. This is not an accident - they did it this way to keep them clean.
i cant find anywhere what the specs of the "Maxi Tune diode" were
They were made by ACS in the states, but went out of production in the mid 2000's sometime.
Over a 10V control range, they do 3.3 to 50pF, a 15:1 range, making them excellent for VCOs and tuned mixers.
They were sold locally by Hatadi Electronics here at Bayview (NSW) - we bought all of their stock of these (and many other spares / components they sold) when they went out of the radio business around 2003.