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 Post subject: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Mar Sat 16, 2019 7:53 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5945
Location: Indianapolis, IN
Hello All,

Just over four years ago got the idea for adding an "infinite-impedance" detector stage to one of my good performing radios to hopefully improve the sound quality of weaker signals. Frank512 was also interested, so both of us began researching & experimenting. Found several FET circuits on-line and initially built one which required two, using one for a constant-current source. Tried a dual-FET IC I had in junkbox and it didn't work, so just installed a single 2N5163 N-channel JFET which was also in "stash" using a fixed resistor with a pot in series for source leg to adjust for best performance.

Was curious about the sensitivity of this FET circuit and added an adjustable loopstick, polyvaricon tuning cap, and an earphone jack to see what would happen. Plugged in headphones, connected battery, and could actually hear strong local stations with just a couple feet of antenna ! Then plugged in a patch cord to AUX input of stereo receiver and could receive more with excellent fidelity---and NO antenna ! The polyvaricon which I'd had since childhood was kinda scratchy while tuning, so replaced it with an air-variable tuning cap which did MUCH better... Now could receive a local using headphones without external antenna !

Installed basic FET circuit in radio and it seemed unstable and grossly overloaded the audio driver. Didn't have enough motivation at the time to do more modification to the circuit and radio, so gave up... The novelty wore off, more project radios were coming in, so back into bench cabinet it went---until a couple weeks ago...

Since that single-FET circuit was so sensitive as a simple receiver, thought it might make a good, external tunable AM BCB RF-amp booster to use with the poor performing transistor radios (or tube sets with ferrite rod antennas) in collection. Had recently found a simple one-transistor circuit for such a device in the September, 1959 Radio-Electronics magazine and was curious on how this very simple high-gain FET circuit might work in that application. Simply replaced the adjustable loop-stick used for input with a ferrite-rod antenna from a junked radio (using "antenna" winding) and added a .01 uF disk cap in series with the "antenna" winding of a scrapped miniature ferrite-bar across output. After applying power, placed "output antenna" (L2) about an inch from top of a poor performing set while tuned to a weak signal that was just at noise level at full volume and tuned (C1) to same frequency---the signal "boomed" in so loud that I had to quickly turn down the volume ! WOW !!! Station was now received as strong as a local ! Adjusted pot (R2) for best quality. Then tried it with a good performing radio which has a tuned-RF amp stage and it also made a noticeable improvement---station now sounded clearer/cleaner and a bit louder. All this with absolutely no added antenna !

To insulate and protect the "output" ferrite antenna bar and wiring, covered it in two layers of heat-shrinkable tubing. This also keeps it from damaging cabinet of the "boosted" radio...

This circuit might also be great in the front-end for a home-brew receiver and am thinking about trying it as an internal RF-amp in one of my poorer-performing radios...

Was also curious about the current draw which I thought would be very low, so checked with DMM---.07 mA (or 70 micro-Amps). Battery should last a LONG time !

Was so excited and impressed by how well this worked, thought that others here might like to "play" with it also... This was a quick & easy experiment and know that it could be improved upon, so welcome others' experiences.

Did schematic below using Paint last night. "Snagged" the symbols from an old Zenith print. J1 & C5 were left in circuit so that headphones or amplifier could be connected to use circuit as a receiver.

PARTS LIST:

Q1 2N5163 (N-channel JFET) others may work ???
C1 Variable cap (tuning cap which was used with salvaged ferrite antenna in junked radio would be ideal)
C2 47 uF electrolytic 16V
C3 220 pF disk ceramic
C4 .01 uF disk ceramic
C5 .1 uF disk ceramic (optional to use circuit as receiver)
L1, L2 Ferrite rod or bar antennas salvaged from junked transistor radios (use "antenna" windings)
R1 22K 1/4W
R2 Used a miniature 1M pot as had several in parts box...
J1 Earphone or headphone jack---your choice (optional to use circuit as receiver)

Other than the FET (which probably could be easily substituted), the parts for this very simple circuit should be easy to come by for those with a decent junkbox.

Now for the "quirks"... Some radios will require the "output" rod be an inch or so away from cabinet to work properly. Tuning is somewhat touchy and there are points of oscillation and spots where stronger signals are "transmitted" into radio from booster. Start with R2 pot about mid-adjustment and when booster properly tuned, decrease until performance maximized. Remember---this was a simple experiment and am sure that circuit could be improved...

If anyone builds this, please post about how it goes !

Picture of my "prototype" below---am thinking of how to "package" it better using a smaller, single-gang tuning cap or good polyvaricon...

John


Attachments:
ARF EXTERNAL TUNABLE AM BOOSTER 3-15-19.jpg
ARF EXTERNAL TUNABLE AM BOOSTER 3-15-19.jpg [ 125.41 KiB | Viewed 1174 times ]
ARF EXT TUN AM-BCB BOOSTER PROTO.jpg
ARF EXT TUN AM-BCB BOOSTER PROTO.jpg [ 226.25 KiB | Viewed 1174 times ]
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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Mar Sat 16, 2019 9:16 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 31995
Location: SoCal, 91387
Very interesting John, I may have to try building one. Now, how does it compare against using a tunable loop instead?

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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Mar Sat 16, 2019 10:55 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5945
Location: Indianapolis, IN
Hey Richard,

Don't have a tunable loop, but the signal level increase using this is fantastic ! And you probably have everything you need (except maybe the FET) in your junk stash...

Need to make some type of adjustable support for the output antenna, though, since it needs to be slightly away from some radios to work properly---wonder if a large loop would work with it ? Want something compact enough to blend into the backaground...

John


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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Mar Sat 16, 2019 11:51 pm 
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Joined: Sep Tue 30, 2014 6:08 am
Posts: 4215
Location: Norfolk, VA
xrhonda91 wrote:
Hey Richard,

Don't have a tunable loop, but the signal level increase using this is fantastic ! And you probably have everything you need (except maybe the FET) in your junk stash...

Need to make some type of adjustable support for the output antenna, though, since it needs to be slightly away from some radios to work properly---wonder if a large loop would work with it ? Want something compact enough to blend into the backaground...

John


John,

Nice work - I built one as a kid using a dual-gate MOSFET, from a magazine project. I used silly putty to hold my loopstick to my Channel Master (hey, I was 9!). We picked up WSM, and some Miami station. Without it, we could already pick up DC and Boston.

It seemed (to me, at least) that every magazine in the 60s and 70s was required to publish at least one "BCB Booster" project.....

Tunable loopsticks are harder to find these days - I usually salvage them from older 250 or 300-in-one electronic kits.
:evil:
https://www.ebay.com/itm/Vintage-NOS-tu ... Sw-0RbFTY0

https://www.ebay.com/itm/Vintage-Archer ... SwDgJcROom

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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Mar Sun 17, 2019 12:50 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5945
Location: Indianapolis, IN
Thanks Brian !

Remember Silly Putty very well---kinda neat it would pick up the print off of a newspaper including the color comics on Sunday...

I've seen those adjustable loopsticks on Ebay---am sure going to hold on to the couple I have ! Used on a lot of projects as a kid...

Boosting reception was a big deal many years ago---imagine interest started during the infancy of commercial radio.

Do you remember any details of your MOSFET circuit ?

John


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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Mar Sun 17, 2019 8:43 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5945
Location: Indianapolis, IN
Played a bit more with this on other low-performance radios in collection with very good results. Did find that it was more unstable & touchy on the upper frequencies and with some radios---especially the position of "output bar"...

Below is a picture of it and a 1957 Miller Transistall which was a kit radio that only uses three transistors. The Transistall typically only receives strong locals at listenable volume, but the booster brought up the signal level of my favorite low-power DX station at 1230 kHz to just a bit below their volume level !

John


Attachments:
ARF TRANSISTALL BOOSTED.jpg
ARF TRANSISTALL BOOSTED.jpg [ 186.42 KiB | Viewed 1067 times ]
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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Mar Sun 17, 2019 8:50 pm 
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Joined: Sep Tue 30, 2014 6:08 am
Posts: 4215
Location: Norfolk, VA
xrhonda91 wrote:
Thanks Brian !

Remember Silly Putty very well---kinda neat it would pick up the print off of a newspaper including the color comics on Sunday...

I've seen those adjustable loopsticks on Ebay---am sure going to hold on to the couple I have ! Used on a lot of projects as a kid...

Boosting reception was a big deal many years ago---imagine interest started during the infancy of commercial radio.

Do you remember any details of your MOSFET circuit ?

John


They tied the two gates together. :shock: Dad had all of the parts except the MOSFET. Our local RCA dealer subbed it with a 40673, and gave dad the eval kit for the 40673 - seems they weren't best sellers. We built it on a Saturday night, and had to wait until the following Friday night to check it out. My antenna was a wire run from my room to a pine tree about 20 yards from the house. We even tried a big wire mesh "separator" in the attic (we lived in military housing, and they used chicken-wire barriers to separate the attic into storage for the 4 apartments), but all that picked up was buzzing and hum.

Attachment:
BCB Booster.gif
BCB Booster.gif [ 146.53 KiB | Viewed 1064 times ]


https://www.americanradiohistory.com/Ar ... Summer.pdf

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Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Mar Sun 17, 2019 8:51 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: SoCal, 91387
Image

Now THAT ^ is a beautiful picture!
(Trade you a Regency TR-1 for the Miller...) :wink:

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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Mar Mon 18, 2019 3:36 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5945
Location: Indianapolis, IN
Thanks much, Guys !

Brian---downloaded magazine and read about that project. Very interesting ! Would think with all that gain it would overload the radio as this one does on certain sets if output bar not positioned just right. And this one works great without any external antenna...

Richard---that Transistall is probably one of the last radios I'd part with since it took so long to find a good one... Actually have it covered with a piece of napkin on display shelf to keep off the dust ! Need to find a glass cover... Do you remember how I tried to find a tuning cap to rebuild the one I "trashed" during childhood ? Have seen a couple others pass through Ebay over many years and though not cheap, sold for about half of a decent TR-1...

John


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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Mar Mon 18, 2019 4:23 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: SoCal, 91387
Hey John, yes I do remember your searching for a component for yours.

Now consider; the ones you've seen on The Bay go for half of a TR-1. I'm offering you a whole TR-1! :wink:

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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Apr Fri 12, 2019 6:52 am 
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Joined: Aug Fri 29, 2014 6:17 pm
Posts: 2575
Location: Vincennes Indiana
I have a ferrite loopstick from a aa5 along with tuning condenser and with just some twinlead connecting the two it makes for a dandy passive signal enhancer on ambc. I presume your lack of performance with the polyvaricon is due to q being less than with the metal cap? Neat stuff as always!


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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Apr Sat 13, 2019 3:46 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5945
Location: Indianapolis, IN
db gain,

The old polyvaricon was "scratchy" while tuning either due to the worn poly between plates or bad rotor contact. Didn't try another, but imagine a good one would do fine.

Tried it again recently and it does do a great job---need to neaten it up and find/make an enclosure. Thought it would be kinda neat to mount the "output" ferrite rod into a section of tubing and mount it to a length of small diameter "gooseneck" so position could be easily adjusted around any size/shape radio (with internal ferrite rod antenna)...

John


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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Apr Tue 23, 2019 3:48 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5945
Location: Indianapolis, IN
Hello All,

Just ran across the C.Crane Twin Coil Ferrite AM antenna signal booster the other day---had no idea that these even existed and would be great to see a schematic ! Imagine it's much more stable than simple circuit I threw together, but watching a demo on YouTube makes me think that performance is comparable as far as gain is concerned...

Anyone have one ? Is schematic or even basic circuit diagram available ?

John


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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Apr Tue 23, 2019 4:47 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 31995
Location: SoCal, 91387
Hey John,
I have a C Crane AM/FM/SW radio with the Twin Coil Ferrite antenna system. It is a very good sounding radio, with a large speaker and separate bass and treble controls, but AFA DX sensitivity, certainly lags behind my garden variety GE SR-2,
if that's any help.

You might contact C Crane, and ask if they would send you a schematic of it. Worst they could do is say no.


Attachments:
C Crane radio.JPG
C Crane radio.JPG [ 112.38 KiB | Viewed 578 times ]

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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Apr Tue 23, 2019 10:40 am 
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Hi John,

The "Twin Coil" is a catchy name to describe an ferrite antenna rod where the tuned coil is split into two identical halves which are placed near the ends of the rod. Signal is extracted from the antenna by a broadband transformer which combines the out-of-phase signals from the two coils as shown is this figure:
Image
Whether this antenna system performs better than the plain old antenna rod where the primary tuned coil is spread over the entire length of the rod is subject to a debate. A schematic diagram as well as a description of the antenna can be found in this 2003 US patent:
https://patents.google.com/patent/US6529169B2/en

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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: Apr Wed 24, 2019 1:29 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5945
Location: Indianapolis, IN
Thanks Guys !

Richard---might try that !

Binh---very interesting info ! Added it to bookmarks...

Below is the actual product from C.Crane site that I was referring to---an actual production booster similar to my simple home-brew device. Was really surprised to find it the other day as I wasn't aware that anything like that was commercially produced...

John


Attachments:
ARF ccrane ant booster.jpg
ARF ccrane ant booster.jpg [ 141.7 KiB | Viewed 541 times ]
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 Post subject: Re: Super-Duper Simple single-FET external AM-BCB booster...
PostPosted: May Fri 03, 2019 7:02 pm 
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Location: Ohio 45177
I have one of those Crane boosters and I tried it with an SPR-4 and it seemed to work OK. Except for the annoyance of tracking the tuning of the device and the radio. Two hands like a neutrodyne. IF you knew what station you were searching for and could tune up on that particular frequency that is helpful but band scanning is work. I have mislaid the interconnect for a radio so right now it is setting.

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