UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Another "Pantry" Rig for the IC-allergic

mictester · 14th Jan 2020, 6:28 pm

Further news:

I made a PCB layout for this circuit. It's not the smallest it could be, but allows the use of several alternative parts in some places (like for the VFO coil). I'll put it up here when I've translated it from my notebook scribbles.

I have experimented with making the output filter tunable, to get the best possible match into the wire aerial. The tobacco-tin boxed prototype doubled its already too effective output, and gave me a range of almost two kilometres to my car radio when hung from the tree in my garden! Getting the match idealised isn't a good idea if you want to restrict the output.

This little circuit is very open to experimentation - a friend built one using an LM386 as the modulator, and got great results. He also took some of the modulator output through a diode pump circuit (to derive a DC voltage proportional to the modulation) and used this to control the gate of a 2N7000 FET shunting the input to the '386. This gave (very basic) compression to the modulation, and with a bit of value tweaking, he made it impossible to over-modulate the "PA". He also added a diode clipper to the input of the '386 to prevent overshoots.

Another constructor built one into a defunct CB SWR bridge box, and used the meter to show RF output, match (approx) and modulation level, using a three position toggle switch to select meter function! I think that this was rather overdoing things!

14th Jan 2020, 6:28 pm	#20
mictester Triode Join Date: Dec 2007 Location: Sometimes Suffolk and other times Limburg, NL Posts: 37	Re: Another "Pantry" Rig for the IC-allergic Further news: I made a PCB layout for this circuit. It's not the smallest it could be, but allows the use of several alternative parts in some places (like for the VFO coil). I'll put it up here when I've translated it from my notebook scribbles. I have experimented with making the output filter tunable, to get the best possible match into the wire aerial. The tobacco-tin boxed prototype doubled its already too effective output, and gave me a range of almost two kilometres to my car radio when hung from the tree in my garden! Getting the match idealised isn't a good idea if you want to restrict the output. This little circuit is very open to experimentation - a friend built one using an LM386 as the modulator, and got great results. He also took some of the modulator output through a diode pump circuit (to derive a DC voltage proportional to the modulation) and used this to control the gate of a 2N7000 FET shunting the input to the '386. This gave (very basic) compression to the modulation, and with a bit of value tweaking, he made it impossible to over-modulate the "PA". He also added a diode clipper to the input of the '386 to prevent overshoots. Another constructor built one into a defunct CB SWR bridge box, and used the meter to show RF output, match (approx) and modulation level, using a three position toggle switch to select meter function! I think that this was rather overdoing things!