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 Post subject: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Oct Tue 22, 2019 7:36 am 
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Posts: 187
Most of the discussions on this forum are tube related but I thought that some might be interested in a simple shortwave receiver which tunes from about 3.5 to 10 MHz and covers both the 80 and 40 meter amateur bands.

Performance is very good and no external antenna is required. Strong commercial shortwave stations give earsplitting volume using standard 32 Ohm earbud headphones.

The circuit uses two tricks which improves performance and reduces parts count.

1. By connecting the 10 uF bypass cap on the LM386 audio amplifier to Ground instead of Pin 8 this increases the gain to over 70 dB which eliminates the need for an intermediate audio stage.

2. By connecting the 2 inputs of the LM386 together it allows the audio amplifier to be used as a high gain envelope detector and eliminates the need for a coupling cap to the regenerative front end.

I use 10 turns on a 3 inch ferrite rod antenna. This combined with a standard 2 gang AM PVC variable capacitor tunes from about 3.5 to 10 MHz which includes both the 80 and 40 meter amateur bands.

I use it most evenings to listen to SSB up and down the East coast of the U.S. without the need for an external antenna. The regeneration control may be used as a fine tuning control when listening to SSB.

Other stations of interest are WWCR 4.84 MHz (Alex Jones), WTWW 5.085 MHz (Golden Oldies), WBCQ 5.130, 7.490, 9.330 MHz and WRMI 9.395 MHz (Golden Oldies).

I would be interested in feedback from any who might give it a try.


Attachments:
simple regen Using LM386 Envelope Detector.png
simple regen Using LM386 Envelope Detector.png [ 8.53 KiB | Viewed 1105 times ]
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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Oct Wed 23, 2019 2:12 pm 
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Location: Cleveland, Ohio
What coil setup/windings and variable cap arrangement would you recommend when an external (i.e. 40-75 meter dipole) would be used?

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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Oct Wed 23, 2019 3:15 pm 
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On one of my receivers I use a 100 pF open frame vernier variable capacitor with 10 turns on a ferrite rod for 40 meters and 20 turns for 80 meters.

If using a external antenna, coupling must be very light so that it does not kill the oscillation or vary the oscillator frequency. A separate winding with a few turns would be a good start. Some use a single FET or transistor RF buffer to isolate the antenna.

When using a ferrite rod inductor some prefer to couple it inductively to a large loop antenna.


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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Oct Thu 24, 2019 4:03 am 
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Can you explain what makes this so successful? The amp section looks unconventional but it works at over 70 dB. Is the detection's 4 parts that special? Does the regen section do all the heavy lifting?

I'll breadboard this when the parts in my mind guide me to them.


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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Oct Thu 24, 2019 6:25 am 
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Mike Toon wrote:
Can you explain what makes this so successful?

The regenerative front end is a Colpitts oscillator which when used with a ferrite rod antenna results in a very sensitive regenerative detector. The two required Colpitts feedback capacitors in series across the tank consist of the general purpose transistor's intrinsic Base-Emitter capacitance which is typically about 20 pF and the parallel combination of the external 22pF Collector-Emitter capacitor in parallel with the input capacitance of the LM386. The values shown create the proper feedback ratio to oscillate over 10 MHz. If this tuning range is not necessary and only lower frequencies are required, the 22 pF external capacitor may be removed and the input capacitance of the LM386 will be enough to create the proper Colpitts feedback ratio for oscillation.

The LM386 has a gain determined by the ratio of its internal feedback resistors which if unbypassed is equal ro 15,000/(1350+150) = 10. This is the unbypassed single input gain. The differential input gain would be twice this value.

If there is a 10 uF bypass cap from Pins 1-8 which bypasses the 1350 Ohm resistor, the single input gain is 15,000/150 = 100.

By connecting the 10 uF bypass cap between Pin 1 and Ground this effectively bypasses the gain determining negative audio feedback entirely and the gain is 15,000/?, but may be defined by adding a small resistor in series with the 10 uF bypass cap. A resistor with a value of 10 Ohms would give a gain of 15,000/10 = 1,500.

Some time ago I discovered after building a circuit which had anomalous behaviour that the LM386 was rectifying RF frequencies and acting as a high gain envelope detector. This is taken advantage of in this circuit by connecting both differential inputs together and using the LM386 as an envelope detector. Because the differential inputs are tied together this prevents the approximately 0.6V DC input voltage from saturating the LM386 and eliminates the need for a DC isolating coupling capacitor to connect to the front end.

For those who like to experiment, the LM386 may be used as a high gain audio amplifier which accepts both the RF and audio present at the regenerative input stage's Emitter by disconnecting Pin 2 and using a 0.1 uF coupling cap as shown in the first receiver schematic below.

The envelope detector input may also be connected directly across the tank as shown in the second receiver schematic below which will result in a slightly reduced tuning range because of the intrinsic input capacitance of the LM386 loading the tank. One of the inputs (Pin 2 or Pin 3) may be left unconnected to reduce tank loading.

The receiver may also be used in the medium wave AM band by simply using a standard AM ferrite rod inductor. Volume on local stations is earsplitting and DXing becomes possible in the evenings. Selectivity is very good and I was able to separate a local on 1150 kHz from a station on 1160 kHz which was 400 miles away with no external antenna. For use in the MW AM band the value of the 22 pF cap will likely have to be increased to about 100 pF.

Because of the extremely high gain of the LM386 it is best to build the receiver on a PC board with short leads and a good ground plane, dead bug or Manhattan style, to eliminate the possibility of spurious RF and audio feedback. This is sometimes difficult if using a plastic protoboard.


Attachments:
LM386-Audio-Amplifier-internal-diagram.jpg
LM386-Audio-Amplifier-internal-diagram.jpg [ 30.59 KiB | Viewed 966 times ]
simple regen Using LM386 Envelope Detector cap coupling.png
simple regen Using LM386 Envelope Detector cap coupling.png [ 8.95 KiB | Viewed 966 times ]
simple regen Using LM386 Envelope Detector tank coupling.png
simple regen Using LM386 Envelope Detector tank coupling.png [ 8.53 KiB | Viewed 966 times ]
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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Oct Fri 25, 2019 12:34 am 
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Joined: Oct Sun 06, 2019 1:59 am
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Nice. Clever. Thank you. -Bob


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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Oct Sat 26, 2019 1:53 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 324
Location: Michigan, 49712 - SC, 29577
Can this be modified for MW by changing L & C?


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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Oct Sat 26, 2019 2:59 am 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 187
W8EJO wrote:
Can this be modified for MW by changing L & C?

Please see my third post in this thread quoted below.

LM386 wrote:
The receiver may also be used in the medium wave AM band by simply using a standard AM ferrite rod inductor. Volume on local stations is earsplitting and DXing becomes possible in the evenings. Selectivity is very good and I was able to separate a local on 1150 kHz from a station on 1160 kHz which was 400 miles away with no external antenna. For use in the MW AM band the value of the 22 pF cap will likely have to be increased to about 100 pF.

I will add one comment which may improve the behaviour of the medium wave version.

Although not necessary, by placing a relatively low value of damping resistor across the tank (I used 8K2), this significantly flattens the regeneration control so that once set the regeneration does not have to be varied when tuning across the MW band. Your mileage may vary.

If using a standard tapped MW ferrite rod inductor you can try the version shown below. The super high gain of the LM386 is not necessary and a gain of 100 will be sufficient by placing the 10 uF bypass cap between Pins 1 and 8. If too low a value of damping resistor is used it may kill the circuit's ability to oscillate. Lowering the value of the 100K load resistor to 47K will increase the oscillator gain to compensate.


Attachments:
simple regen Using LM386 MW..png
simple regen Using LM386 MW..png [ 9.37 KiB | Viewed 827 times ]


Last edited by LM386 on Oct Sat 26, 2019 3:55 am, edited 1 time in total.
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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Oct Sat 26, 2019 3:42 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 324
Location: Michigan, 49712 - SC, 29577
Thank you!

I will give the MW version a try.


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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Sat 02, 2019 4:30 pm 
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Joined: Aug Wed 25, 2010 3:23 pm
Posts: 1335
Location: WA 98407
I breadboarded the SW version with parts on hand. The transistor is a 2N3906. I’m using just one section of the two-gang tuning cap which has a max capacity of 310 pf per section. The ferrite rod is 3 7/8” long and the winding is 10 turns of Litz on a cardboard tube near one end of the rod. It howls like a banshee over most of the range of the pot, and the howling breaks in very sharply—no transition from a background hiss. I’ve been unable to hear any sort of signal with it. Frustrating. Oh, yes -- the LM386 is set in a 16-pin socket. It's all I had. :oops:

Tom
Attachment:
P1090090.jpg
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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Sun 03, 2019 1:27 am 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 187
Very nice build!!!


From your photo it appears that you have built one of the two schematics shown below. Are Pins 2 and 3 of the LM386 connected?

Is that a 0.1 uF cap between the 100K and Pin 2 of the LM386 or just a jumper?

As a first step it would be worthwhile putting a 100 Ohm resistor directly in series with Pin 6 of the LM386 along with a 100 uF cap from Pin 6 to Ground to further filter the supply.

You can then use a coupling cap instead of the jumper as shown in the first schematic below. A large value 0.1 uF allows both RF and audio from the front end into the LM386. It would then be worthwhile trying a much smaller value (100 pF) to allow only RF to be amplified.

When using the coupling cap, try using both input pins connected and then interchanging the input pins using only one of them to alter the phasing of the resultant audio output. If this doesn't remove the oscillation it would be worthwhile adding a choke (10mH?) in series with the headphones.

You can also try adding a 220 pF cap between Pin 8 and ground to further suppress RF feedback.

The gain of the LM386 may be reduced by placing a small value resistor (100 Ohms) in series with the 10 uF cap from Pin 1.

It would be best to have the variable capacitor ground connection, the ferrite inductor ground connection and the 22 pF feedback cap ground connection at the same place on the ground plane to minimize parasitic effects. The leads to all the front end tuning components should be as short as possible.

Finally, it would be worthwhile orientating the ferrite rod horizontally. I use pieces of hot glue sticks for standoffs. The inductor is directional and its changing its horizontal orientation allows focusing in on a desired station.

Please report back.


Attachments:
simple regen Using LM386 Envelope Detector cap coupling.png
simple regen Using LM386 Envelope Detector cap coupling.png [ 8.95 KiB | Viewed 591 times ]
simple regen Using LM386 Envelope Detector.png
simple regen Using LM386 Envelope Detector.png [ 8.53 KiB | Viewed 591 times ]
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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Sun 03, 2019 3:52 pm 
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To be clear, I built the version originally posted; i.e., the second of the two you just posted. Although hard to make out in the photo, pins 2 and 3 are indeed connected (all connections at the socket of the LM386 were confirmed with an ohmmeter). There’s no 0.1 uf cap in my build, so that’s just a jumper to the 100K resistor.

I will go ahead and try the hints you offered in the order you listed them. I might get to it today; if not, certainly tomorrow. FWIW, where you originally showed a 33 uf cap off pin 6, I used a 47 uf because I don’t have any 33’s.

Lastly, although I have no training, it seems to me that the howling I hear is audio oscillation, not RF. But who am I to say.

Many thanks for replying,

Tom


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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Sun 03, 2019 8:42 pm 
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allthumbs wrote:
it seems to me that the howling I hear is audio oscillation, not RF.
Tom

In your original post you indicated that you were getting sudden "howling" as you increased the regeneration control.

What likely is happening is that as the receiver begins to oscillate, RF energy is being fed back to the ferrite rod antenna which induces audio oscillation.

The suggestions made were to isolate the problem, ranging from power supply filtering, phase reversal of the LM386 input and the addition of a choke in the output to eliminate the possibility.

With the circuit as you have built it, because the tuned circuit has two different ground connections between the variable capacitor and inductor, energy could be introduced into the tank through the common ground plane (the ground plane with all the random current flowing through it as it is presently wired now forms part of the tuned circuit). It is best at these frequencies to use very short leads with a common ground connection for the RF front end components.

I have built many versions of the receiver using the LM386 as an envelope detector in its high gain mode and care must be taken, especially at these higher frequencies, to insure that any possibility of feedback must be eliminated especially if using a ferrite rod antenna.

It would be worthwhile shortening the tank leads and moving their ground connections as shown below.


Attachments:
P1090090.jpg
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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Wed 06, 2019 2:05 am 
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No luck with the suggested changes. FWIW, here are the voltages at the pins of the LM386:

With pot turned all the way clockwise:
1 - 1.50v
2 - 0.37v
3 - 0.37v
4 - 0.00v
5 - 2.28v
6 - 8.69v (battery)
7 - 4.43v
8 - 1.55v

With pot turned all the CCW:
1 - 1.39v
2 - 0.01v
3 - 0.01v
4 - 0.00v
5 - 4.19v
6 - 8.69v (battery)
7 - 4.33v
8 - 1.39v

All recommended ground leads at a common point. Everything mounted on a small sheet of aluminum.

Tom

Attachment:
P1090113.jpg
P1090113.jpg [ 364.93 KiB | Viewed 439 times ]


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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Wed 06, 2019 3:19 am 
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allthumbs wrote:
No luck with the suggested changes. FWIW, here are the voltages at the pins of the LM386:


From the voltages that you measured it appears that you still have the receiver configured as in the original schematic with Pins 2 and 3 connected and joined to the Emitter with no coupling capacitor. From the picture, Pin 2 looks like it is still connected to the jumper and Pin 3 looks like it has a yellow capacitor connected to I don't know where.

A schematic with the actual circuit configuration that you are using or a description would be helpful.

As suggested, using a coupling capacitor instead of the jumper and unjoining Pins 2 and 3 and connecting the coupling cap to each one alternately to determine the best position can only create a circuit which doesn't have audio oscillation. When you say "No luck" does the circuit still have audio howling or is it not oscillating at the tank frequency?

It would also be best if the 22 pF feedback cap were connected directly across the transistor from the transistor's Emitter to Collector.

The 0.37V input voltage when the pot is fully clockwise is low. It should be about 0.6V. Did you separate Pins 2 and 3 and using a coupling cap connect to only one of them as previously suggested? After making the changes shown in the schematic below, if the circuit does not oscillate at the tank frequency its gain may be increased by reducing the value of the 100K resistor to 47K.

The schematic below contains 2 of the suggested changes, power supply filtering and capacitive coupling to either input Pin 2 or 3 (whichever pin works best).

What type of headphones are you using? If they are 32 Ohm stereo earbuds are they connected in parallel or in series? If there are still remaining audio issues it would be worthwhile connecting them in series for a load impedance of 64 Ohms by connecting only the 2 outer pins of the stereo audio output jack (one of them to Ground) and leaving the normally used ground lead of the jack disconnected.

The only other possible addition in the unlikely case that there is still audio howling would be to add a 10mH choke in series with the output and the headphones.


Attachments:
simple regen Using LM386 Envelope Detector cap coupling.png
simple regen Using LM386 Envelope Detector cap coupling.png [ 9.8 KiB | Viewed 426 times ]
P1090113.jpg
P1090113.jpg [ 382.27 KiB | Viewed 393 times ]
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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Fri 08, 2019 2:30 am 
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Update: I built it this time according to the the last schematic you posted, i.e., this one:
Attachment:
Image1.gif
Image1.gif [ 4.81 KiB | Viewed 330 times ]

Attachment:
P1090115.jpg
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There is no sign of howling, but there is no sign of anything else, either. I don't think it's wired incorrectly but perhaps something is poorly connected (I'm not Allthumbs for nothing). I've gone over it with a magnifying glass and don't see any sign of bad connections, either. I've abused the pad where the chip socket is mounted enough this week and so will try again when I get some more (I have them on order). In the meantime, here are my voltage readings with the newly revised circuit:

Voltages with pot turned all the way clockwise:
1 - 1.38v
2 - 0
3 - 0
4 - 0
5 - 3.92v
6 - 8.21v
7 - 4.09v
8 - 1.38v

With pot turned CCW: identical readings. Maybe that's a sign of a fault or mistake.
The 'phones are Radio Shack model Nova-35. Only one side works any more.
I'll report back in a few days when my order comes in and I can rebuild it with due care and neatness.

Thanks very much,

Tom


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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Fri 08, 2019 3:34 am 
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allthumbs wrote:
With pot turned CCW: identical readings. Maybe that's a sign of a fault or mistake.

With this configuration there is no DC connection between the regen pot and the LM386. Varying the regen pot will have no effect on the LM386 voltages.

It's difficult to tell from the photo but are the connections to the 16 pin socket correct?

Is there a Ground connection to Pin 4?

With the LM386 connected in high gain mode you should at least hear some background audio hiss.
allthumbs wrote:
The 'phones are Radio Shack model Nova-35. Only one side works any more.

Although you indicated that you had howling previously, because the phones are in parallel and one side is not working why not try connecting another set of earbuds to determine if the nonworking side of the Nova headphones is shorted?


Attachments:
P1090115.jpg
P1090115.jpg [ 466 KiB | Viewed 323 times ]
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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Fri 08, 2019 5:02 am 
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Maybe it sounds weird in this day and age, but I don't have any other 'phones or earbuds except for an old-fashioned 2K ohm pair that I use for tube radios.

Yes, pin 4 is grounded. To the common ground used for the inductor, varcap, etc. But there is no characteristic hiss of a set in operation. So like I said, I'm going to rebuild it when my parts order comes in. Should be simple enough :)

Thanks,

Tom


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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Fri 08, 2019 8:55 pm 
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Posts: 113
Location: Texas, U.S.A.
For the given frequency range of around 3.5 to 10 MHz and a two gang roughly 360 picofarad per section variable capacitor with the sections connected in series, around 12 microhenrys would be required for the coil inductance.

I would be inclined to concentrate on the transistor stage first.
The idea for a regenerative amplifier stage for radiotelephone reception is to produce RF amplification without oscillation.
I would remove the LM386 while testing the transistor stage.
Vary the regeneration control pot over its range while measuring the dc emitter voltage. The emitter voltage should be variable from 0 to around 0.7 volts dc.
An oscilloscope of at least 20 MHz bandwidth with a 10X probe could be used to look at the emitter RF signal. The probe capacitance will have some effect on the circuit.
Couple a known frequency, variable small amplitude RF signal to the coil.
Vary the regeneration control and vary the RF signal source amplitude while observing the signal at the emitter.

Or possibly, instead of a 'scope, the input of CW or SSB receiver could be loosely coupled to the transistor circuit to monitor evidence of amplification or oscillation.

There is not any voltage gain from base to emitter, so apparently any RF amplification would be due to positive feedback and the Q of the tuned circuit.

----------
WB5HDF


Last edited by infzqi on Nov Sat 09, 2019 3:30 am, edited 1 time in total.

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 Post subject: Re: Simple High Performance Shortwave Regenerative Receiver
PostPosted: Nov Sat 09, 2019 1:23 am 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 187
infzqi wrote:
For the given frequency range of around 3.5 to 10 MHz and a two gang roughly 360 picofarad per section variable capacitor with the sections connected in series, around 12 microhenrys would be required for the coil inductance.

===================

Or possibly, instead of a 'scope, the input of CW or SSB receiver could be loosely coupled to the transistor circuit to monitor evidence of amplification or oscillation.

----------
WB5HDF

Only a single gang is being used and 10 turns with the intrinsic capacitances of the transistor should give an upper tuned frequency of about 10 MHz and a lower tuned frequency of about 3.5 MHz.

Because of the ferrite rod antenna, the receiver also makes a good transmitter and when the regen control is advanced and the circuit is oscillating, bringing a shortwave receiver within about 1 foot will result in a heterodyne tone in the shortwave receiver at the tuned frequency. I frequently use this technique for adjusting the tuning range.

I have built this receiver too many times to count and posted it because of its simplicity and performance. Because the LM386 is configured in ultra high gain mode I am having difficulty in understanding how it is possible to have a circuit with no audio out unless it is a somewhat trivial problem such as using defective earphones.


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