UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

John G8MWF · 3rd Oct 2019, 9:13 pm

All points noted and thank you.

I went online and used a tool that calculates the component values dependent on the frequency using the formula 2*pi*F= 1/sqrt(L*C) by simply putting values into boxes.

My 'current' high band crystal is nominally 52.3Mhz tuned with a 27pF capacitor which makes L = to 343nH.

So if L is constant and we want a crystal frequency of 49.52 Mhz then C=30.4pF. a none preferred value.

So for a difference of just 3.4pF or an increase of around 12.6% it 'should' be possible to pull that circuit into range.

The BIG question is why did the manufacture change the coil values and not the capacitance?
Purely for keeping the parts list simple? Or getting a capacitor with this value is difficult - using a 33pF throws things right out!
Or is there a technical reason?

3rd Oct 2019, 9:13 pm	#6
John G8MWF Hexode Join Date: Aug 2014 Location: Dartford, Kent, UK. Posts: 435	Re: Modifying Vintage PMR Coils All points noted and thank you. I went online and used a tool that calculates the component values dependent on the frequency using the formula 2piF= 1/sqrt(L*C) by simply putting values into boxes. My 'current' high band crystal is nominally 52.3Mhz tuned with a 27pF capacitor which makes L = to 343nH. So if L is constant and we want a crystal frequency of 49.52 Mhz then C=30.4pF. a none preferred value. So for a difference of just 3.4pF or an increase of around 12.6% it 'should' be possible to pull that circuit into range. The BIG question is why did the manufacture change the coil values and not the capacitance? Purely for keeping the parts list simple? Or getting a capacitor with this value is difficult - using a 33pF throws things right out! Or is there a technical reason?