DIY RF Condenser Mics

[rogs]

If there was no resonance, I wouldn't be getting hundreds of millivolts RF at the gate.

The amplitude of the RF at the gate can be quite large - even without considering any resonance.
It will depend on the difference in value betwen the capsule capacitance and C4..... In other words, the amount of 'bridge' imbalance.
It is the change in amplitude level at the gate that determines the signal output, not the level of RF present.

 

[shot]

Sadly, removing those cans from an assembled through plate PCB, without causing damage, I've found to be almost impossible.

I must admit that I had been removing cans more than ten times from the same board!
The trick was to use hot air gun! I was just heating it's pins until I was able to pull out the can using minimal force. After that I had to clean pcb thoroughly, then rewind a new core, solder it in, try to calibrate, dissapointment, unsolder all again... repeat... repeat... repeat...
So it's possible!
But very time consuming



Luka

 

[rogs]

I must admit that I had been removing cans more than ten times from the same board!
The trick was to use hot air gun! I was just heating it's pins until I was able to pull out the can using minimal force. After that I had to clean pcb thoroughly, then rewind a new core, solder it in, try to calibrate, dissapointment, unsolder all again... repeat... repeat... repeat...
So it's possible!
But very time consuming



Luka

I have manged to succeed removing cans from th PCBs .. although it's not easy with a through plate boards that has a ground plane!
It usually wrecks the can - or the board -- or both! - and as you say, it's very time consuming....

What I have been trying is to build the oscillator and inductor assembly onto a good old fashioned 'breadboard'... OK, not ideal for 10MHz RF circuits - especially from a 'capacitive leakage' point of view - but it is still possible to calibrate and test different arrangements, by just plugging the cans in and out.
Although the capacitance leakage between the 'breadboard' tracks serve to mess up the calibration somewhat, it's useful to remember that if you only change one component at a time, whatever capacitive leakage is included in the whole assembly remains the same.
Any measured differences must relate to that single changed component.

All a bit crude of course, but quite effective - and certainly better than constantly removing the cans from the PCBs, when simply trying out new ideas!