Using transformers to extract audio in regenerative receivers can cause problems with squegging and motorboating (fringe-howl or superregeneration at audio frequencies) due to the large LC-timeconstants involved in the circuit.

You should extract the detected signals from the source. A better approach is to add a separate J-FET detector connected onto the top of the tank circuit, leave the regenerative stage as it is. Bias the detector stage near its pinch-off point by using a large source resistor which is shunted by an elco and a capacitor to decouple it. Extract the detected signal from the drain.

I would like to ask you about another receiver. I hope it's not problem.

I have built the Moorabbin receiver. It requires a 1k:8 transformer. I built a matching transformer by modifying the audio transformer of a vef 242 radio.

I took care of the turn ratio, the air gap, etc. I'm sure the transformer is OK. With the transformer, I hear a terrible scream, when I not adjust the regeneration properly, and some of the stations cannot be received, only strong scream. When I use a 32 ohm dynamic headphone, (instead of the transformer) the audio quality is good, no scream, only faint whistle, but the stations are not loud enough.

Using higher impedance balanced armature headphones (instead of the transformer) the scream is louder.

How can I resolve this problem? Something is wrong with the transformer? Or with the regeneration control? Should I use the receiver with a 32 ohm headphone (with a 70 feet long wire antenna the reception is good) or it will overload the audio stage?

All the best

Richard

Backlash or hysterese can be avoided by using an emitter resistor for stabilizing the opereation point of the receiver. The emitter resistor should be adjustable and can also be used for regen control.

Richard,

1000 uH is probably about 100 - 120 turns on the same core. Depends on your ferrite really. You can make it using air-cored coils, they will just be larger. Sounds like you have the ideal caps for the job. If you have access to litz wire I'd recommend using it for the coil over normal magnet wire.

Glad the elektor FM broadcast RX worked well for you.

Regards,

Alan

Hi Alan!

Thank you! I have a giant 1200pF variable cap.

It need a 1000uH coil. I have a 256pf variable cap for regen. control.

I wrote formerly about the elektor fm broadcast receiver. It works very well! It is very sensitive.

Thank you very much!

Richard,

You'd need to add turns to the coil to cover LW. It tunes to about 530 kHz at the moment. The polyvaricon has about 210 pF maximum capacity, so the inductance is about 430 uH. To hit 150 kHz with the same capacitance you'd need 5.3 mH! Inductance scales with the square of turns, it is 70 now, so you'd need about 253 turns (if I got my maths right)...

You might do better with a bit of fixed capacitance across the inductor, but that will limit your tuning range of course. Also, the parasitic capacitance of the windings will probably mean you need less turns. The six turn tickler can be increased in the same ratio, say 20 turns as a starting point.

Of course if you have larger variable capacitors available you can use them. My VFO calculator is useful for estimating this kind of thing.

Regards,

Alan

Hi Alan,

Can this device receive the longwave band (over 160 kHz) or it need to be modified? What is the value of the regen cap?

Thanks and Regards,

Richard