Morse super-regenerative oscillator circuit - but not... - UK Vintage Radio Repair and Restoration Discussion Forum

6SN7WGTB · 25th Jun 2022, 7:05 pm

I came across a very simple oscillator circuit to use with a morse key - which will supposedly transmit 1MHz RF a few feet (although "detectable all over the AM dial")

See below.

Well, I can't get it to do any such thing.

The coil was described (and pictured in the article) on a 5/16" ferrite - I used 10mm. It was shown as very loosely wound (over about 2") - I tried that and also then bunched the windings up. Still no joy.

It only seems to draw about 6µA when energised.

Can anyone see a flaw in the design?

kalee20 · 25th Jun 2022, 8:05 pm

It's a Hartley oscillator - it should work!

The 150kΩ resistor, after allowing for the base-emitter voltage drop, will have about 1V across it. So current through will be 6.67μA and that lines up with what you have.

But the transistor will have a current gain of at least 100, so you should have an additional current of 667μA, assuming it's not oscillating. Is your transistor OK?

I'll bet the time constant in the base circuit is chosen for it to be a squegging oscillation, probably at an audio rate, in which case there will be rather a lot of audio sidebands. But even so, I wouldn't expect it to spread right across a waveband.

6SN7WGTB · 25th Jun 2022, 9:24 pm

Thanks for confirming the base current, but I too cannot seem to get any 'gain' happening.

I tried a few transistors, but to no avail.

Will rebuild it again tomorrow in case I have missed something obvious.

The original build did seem to have thick coil wire - whereas I used 30 SWG which I had to hand. Could this affect it?

kalee20 · 25th Jun 2022, 11:19 pm

Hardly! Measure the voltage on the three leads of the transistor, with the key closed, relative to battery -ve.

Emitter should be at zero; base about 0.6V; collector 1.5V.

Aub · 25th Jun 2022, 11:29 pm

I would think you'd need a lot more turns on the rod. I would try a two inch winding length of 22swg close wound. You could also try putting the coil tap in the centre.

Cheers
Aub

Aub · 25th Jun 2022, 11:31 pm

Try the 30 gauge if that's what you have.Try a 1.5 inch winding length.

kalee20 · 26th Jun 2022, 1:05 am

The coil may indeed leave something to be desired, but there's something far more fundamental going on.

If it's not oscillating, why is the current only 6uA? It should be around a milliamp.

Until the DC conditions, and the current gain, are right, you could fit the best coil in the world and it won't oscillate.

6SN7WGTB · 26th Jun 2022, 8:42 am

Confession time:

I have found a duff cap - there's a break somewhere around the point the lead enters the case. Checked all components before assembly, and it measures fine when held a certain way but when wiggled and inserted into the breadboard it disconnects.

One of those "haven't seen before" faults.

Sorry to waste everyone's time.

Bottom line - it works as advertised... The rudimentary coil is fine - loud and clear from 500kc/s to 1Mc/s, audible at 1.5Mc/s.

25th Jun 2022, 7:05 pm	#1
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	Morse super-regenerative oscillator circuit - but not... I came across a very simple oscillator circuit to use with a morse key - which will supposedly transmit 1MHz RF a few feet (although "detectable all over the AM dial") See below. Well, I can't get it to do any such thing. The coil was described (and pictured in the article) on a 5/16" ferrite - I used 10mm. It was shown as very loosely wound (over about 2") - I tried that and also then bunched the windings up. Still no joy. It only seems to draw about 6µA when energised. Can anyone see a flaw in the design? Attached Thumbnails

25th Jun 2022, 8:05 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: Morse super-regenerative oscillator circuit - but not... It's a Hartley oscillator - it should work! The 150kΩ resistor, after allowing for the base-emitter voltage drop, will have about 1V across it. So current through will be 6.67μA and that lines up with what you have. But the transistor will have a current gain of at least 100, so you should have an additional current of 667μA, assuming it's not oscillating. Is your transistor OK? I'll bet the time constant in the base circuit is chosen for it to be a squegging oscillation, probably at an audio rate, in which case there will be rather a lot of audio sidebands. But even so, I wouldn't expect it to spread right across a waveband.

25th Jun 2022, 9:24 pm	#3
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	Re: Morse super-regenerative oscillator circuit - but not... Thanks for confirming the base current, but I too cannot seem to get any 'gain' happening. I tried a few transistors, but to no avail. Will rebuild it again tomorrow in case I have missed something obvious. The original build did seem to have thick coil wire - whereas I used 30 SWG which I had to hand. Could this affect it?

25th Jun 2022, 11:19 pm	#4
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: Morse super-regenerative oscillator circuit - but not... Hardly! Measure the voltage on the three leads of the transistor, with the key closed, relative to battery -ve. Emitter should be at zero; base about 0.6V; collector 1.5V.

25th Jun 2022, 11:29 pm	#5
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,039	Re: Morse super-regenerative oscillator circuit - but not... I would think you'd need a lot more turns on the rod. I would try a two inch winding length of 22swg close wound. You could also try putting the coil tap in the centre. Cheers Aub __________________ Life's a long song, but the tune ends too soon for us all.

25th Jun 2022, 11:31 pm	#6
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,039	Re: Morse super-regenerative oscillator circuit - but not... Try the 30 gauge if that's what you have.Try a 1.5 inch winding length. __________________ Life's a long song, but the tune ends too soon for us all.

26th Jun 2022, 1:05 am	#7
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: Morse super-regenerative oscillator circuit - but not... The coil may indeed leave something to be desired, but there's something far more fundamental going on. If it's not oscillating, why is the current only 6uA? It should be around a milliamp. Until the DC conditions, and the current gain, are right, you could fit the best coil in the world and it won't oscillate.

26th Jun 2022, 8:42 am	#8
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	Re: Morse super-regenerative oscillator circuit - but not... Confession time: I have found a duff cap - there's a break somewhere around the point the lead enters the case. Checked all components before assembly, and it measures fine when held a certain way but when wiggled and inserted into the breadboard it disconnects. One of those "haven't seen before" faults. Sorry to waste everyone's time. Bottom line - it works as advertised... The rudimentary coil is fine - loud and clear from 500kc/s to 1Mc/s, audible at 1.5Mc/s.