Dragon TRC485 VCO Moddfication

[fracu]

Hi.
I own a Dragon TRC485.
I already did the trick to get 400 channels using this:
http://www.radiomods.co.nz/trc485mod.html
But "To transmit / receive on all of the 400 channels it is necessary to adjust the VCO."

I've adjusted the VCO tuning the resonance coil but in that way i can only move the window to an upper or lower frequency, but i can get the same excursion (1,7Mhz, e.g. from 26.705 to 28.475).

I would like to get at least 2,5 MHz of covering. So i modified the VCO:



I putted a second varicap with a different capacitance curve in order to use it to work in other frequencies. I turn between original varicap and added varicap using an electronic switch. I build the mod it in a very small area near to the original varicap and isolating everything from annoy RF interference from other radio parts. Then I adjusted the trimmer to avoid varicap's bands superposition.

I tested and it was OK, PLL working OK, I've got an 2,6 MHz covering BUT transmission is very noisy due to the poor quality carrier that the VCO is producing after my modification I think. I even add 0,1uF capacitors in parallel to the switching diodes but i only got a band reduction.

Someone can please suggest me another way to do this? Maybe adding a brand new VCO to the radio?
Please help me!
THANKS IN ADVANCE!!!

[Rick]

There are 2 types VCO, see diagram you will need to use all the same varicaps. Hope this helps you.

Rick (Euro Radio Co).

[fracu]

Thanks Rick for your attention !!!
My Dragon has an OLD TYPE VCO. Just one KDV251.
I've added a BB405 varicap just because it has a lower capacity compared to KDV251 at the same tension (In order to get higher frequencies).
What do you mean with "use all the same varicaps" ? Should I add another KDV251 in paralell with the original one or something?
There's something wrong with my mod?

THANK YOUUUUUUUUUUUUU

[Rick]

You need to modify your old type VCO to the new type VCO to get required bandwidth from your VCO. What I mean by use all the same Vericaps, if you are to use BB405 then use all 4 the same, I do not know the Capacitance range of the BB405 but the KDV251 Capacitance range is 19pf to 38pf which is a very good Capacitance Ratio about 1.7. So you need to know the Capacitance range of the BB405 before you use them. Components to change R134 (56K) to 82K, C126 (68nF) to 100nF, C128 remove (68pF) put a link in its place, C132 (47pF) to 20pF. Hope this helps. Let us know how you get on.

Rick.

[fracu]

THANK YOU! I'll try it out!!! I'll let you know later.

So you're saying if i change that components will I enhance the radio's frequency range? (More than the original 1,7 Mhz e.g. from 26.705 to 28.475?) I had never heard about that new VCO. Has it a wide range?

And should I add three more varicaps?
Is not the same to use just one KDV251 instead of use four in that configuration? I mean...



Sorry for the inconvenience!

[Rick]

If you change to the new type VCO you will have a bandwidth of 4 MHz. It is really easy just change all the components to the NEW TYPE VCO that all there is too it.

I would only use SVC251 or KDV251 varicaps; there varicaps have a very high Q and high Capacitance Ratio which makes them ideal for use in VCO circuits.

Using 4 varicaps will increase the capacitance ratio, if you have 4 varicaps in Parallel the formula is like so C = C1 + C2 + C3 + C4. This is the reason why you need 4 varicaps so you can not get the required capacitance ratio.

Rick.