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 Post subject: Super Regen FM BCB receiver
PostPosted: Jan Mon 29, 2007 8:34 pm 
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Location: On the Left Coast
Here are some pics and schematic of a super regen FM BCB receiver I just finished. It's based on a design from John Hunter in Australia. You can view his web site here http://www.users.bigpond.com/cool386/
I added the 6T9 audio stage and a few minor modifications.

I'm listening to the receiver while writing this. Originally it tuned from 86 - 125 mhz. I was able to pick up some air traffic on the UNICOMM freq from our local uncontrolled airport. I added the 30pf SM cap in series with the 15pf tuning cap and another turn on the coil. Now it just barley goes over the FM band edges.

The dial is a modified National ACN dial from a junker homebrew rig. I cleaned and rebuilt the gearing section and add a dial skirt from a 3" vintage dial. The knob has been replaced too.

I first tried to reflex the audio through the grounded grid RF amp then fed it to the 6T9 but there was toooo much gain so the reflex circuit was removed. The reflex does work and would be fine for a one tube receiver operated with head phones. See John Hunters site for the single one tube reeceiver with reflex.

So how does it work? Great! Tuning is amazingly smooth, adjusting the regen control and tuning knob for good HiFi slope detection isn't any harder then tuning a regular well built regen for SSB or CW reception on the shortwaves.

What amazes me is the reception on a 24" telescopic antenna that was clip lead to the ANT terminal. I live 12 miles SE from Salem Ore. and I-5. I can pick up stations from Portland to Eugene, some better some worse but all intelligible.

So without further ado... The pics.

John
KB7NRN
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PostPosted: Jan Mon 29, 2007 10:29 pm 
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Wow John, thats pretty clever. Something I've always wondered with this type of circuit? Do you get any ill effects from weaker stations as they fade in and out?

-Bill


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PostPosted: Jan Mon 29, 2007 11:31 pm 
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Bill,
I haven't experenced any fast fading yet but slow fading from humidity change as things heat up and cool off are easy to deal with. Just a touch of the regen control or dial after a few hours isn't any big deal.

As far as weak signal goes the sensitivity measured with my old IFR 500A isn't that impressive, about 20 uv for 12 db sinad. Tho, I am picking up some stations full quieting at 50 miles away on the 24" antenna. If I play around with the regen and tuning on weak signals I can usually get it intelligable enough to copy. Not good for music tho but voice comes through ok. I found if I can hear it I can usually understand it after tweeking the controls.

Also, there is no blocking effects that I can detect and there doesn't seem to be any adjacent channel interferience from weak as well as strong stations.

Another thing, listening closely one can detect a limiting effect like AGC action going on. Remindes me of an AM radio. I never really noticed this with a straight regen tho I've read this effect is present.

Once again I am amazed with the humble little regen and it's variants.

John
KB7NRN


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PostPosted: Jan Tue 30, 2007 12:12 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
Superregenerative sets are known for their capability to adjust to differing signal strenghts. It is like they have an AGC built into them in a way. You may also notice that as you are tuning in a strong signal, as you get close, suddenly it is like the receiver is captured by the signal and pretty much locks in on it. Thus you get the feeling it is not drifting much. It is because the strong signal has it "locked" to that frequency.

Back when they were popular on 5 and 10 meters with hams, that was one of the major drawbacks, as if you were listening to a weak signal, everything was fine until a stronger signal came on frequency and that was the end of the weaker signal station.

Back in the days when two meter repeaters were just coming on the scene for hams, I built a two meter superregenerative set that used a 6C4 triode detector feeding a 6J5 audio amp and a 6F6 output. I think it was described in the back of one of the old RCA tube manuals, but not sure about that. I could copy the hams within a 75 mile radius, but when a stronger signal came on, that was it. Got very frustrating after a while. But it did work fine.
Curt

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Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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PostPosted: Jan Tue 30, 2007 4:58 pm 
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A brief circuit discription:

The antenna is connected to the cathode of the first triode which is in a grounded grid amplifier configuration. This is more for isolation then amplification which is low. The RF signal rides on the plate voltage in the conventional manner. Through the 4.7 pf coupling cap to the grid of the second triode.

The 330k resistor and 33pf form the quench circuit. The 15pf cap and L1 form the tuning circuit. Regeneration is produced from the choke on the cathode and the interelectrode capaictance of the tube. Kinda built in feedback. Regenration is controlled by the 1k pot in the cathode circuit that biases the cathode positive for less feedback.

Audio leaves the secod triode plate and is coupled through the interstage transformer. The 150k resistor and 470pf cap on the secondary of the interstage transformer filter out the stereo subcarrier so it doesn't beat against the quench freq producing an audible note. A 1M pot is used for volume control and the 1000pf cap from the wiper to ground filters RF and high freq osicllations to which was giving me some fits for a while.
The 6T9 is in a conventional audio amp stage and has lots of audio.

The 15uh chokes are to keep RF out of the power supply, any choke over 15uh should work fine. I used some 1/8" diameter, 1/4" long ferrite beads wrapped with 5 or 6 turns of 30 gage wire wrap wire held on by super glue. They're about 30-40 uh but work great.

John
KB7NRN


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PostPosted: May Sat 05, 2007 10:51 am 
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Location: Akershus, Norway
Hi all! :D
I have now built this receiver with a modification in the audio section (6AB8 tube). But I have a problem, and that is: When I turn the regen control towards maximum, only the VHF-hiss is inceasing and not the audio (or gain) itself, it seems like. Touching up with the tuning capacitor on a station doesn`t help
I have a 1k plastic type pot as a regen control. Could this be causing the trouble, or is it my newer russian made 12AT7 maybe?
I`m not experienced in VHF circuits so all hints could be usefull


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PostPosted: May Sat 05, 2007 9:06 pm 
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Rune,
The regen control works in conjunction with the quench circuit. The quench frequency in effect is trying to keep the detector from oscillating while it is at maximum gain. Increasing the regen control past it's optimal setting only produces more harmoics causing more hiss and desensitize your receiver.

John
KB7NRN


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PostPosted: May Sun 06, 2007 2:09 pm 
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Location: Akershus, Norway
Thanks John! :D
That clears things up a little bit. I must have misunderstood the whole thing. Because, on Hunters site he's mentioning several times the importance to crank up the regeneration "to the point where it almost oscillates" (just to paraphrase).


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PostPosted: May Sun 06, 2007 6:32 pm 
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Rune,
When I first learned about super regen receivers years ago I remember reading an article that said that the quench circuit would allow the detector to approach the point of oscillation then as the quench voltage increased it would stop or quench the detector from oscillating.
Well, that's not quit accurate, or it may be my memory is off. What is happening is the detector is oscillating but is being quenched at the frequency that is set up by the quench circuit. The detector is now a modulated oscillator, thus producing a double sideband signal. As the regeneration control is increased so does the side bands and this decreases selctivity.
Tho sensitvity is increased with greater regeneration, the wider band width allows more noise. Also most quench circuits produce saw tooth wave forms and this causes harmonics in the side bands of the detecor which mulitply as the gain is increased and desensitzing your receiver.

Charles Kitchin explains it here in his article for a 6mtr super regen http://www.arrl.org/tis/info/pdf/9712039.pdf

A great little circuit that I plan to build one day.

A note on my super regen; I replace the hand wound 15uh chokes with some factory made chokes and the perfromance improved. Signals at the bottom of the band come in stronger and clearer.
I bought these chokes off Ebay. They are the size of a 2 watt resistor. I got 30 chokes for about $4.00 sent first class mail.

John
KB7NRN


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