- March 9, 2007 at 3:57 am #6860T.ALLREDParticipant
Total posts : 47
I read one of the posts inthe forum topic about the retro-style antenna design. Lee mentioned turning table top radios into transmitters and such. Is there any websites that have details on the how to’s of table top radio modification?
-TravisMarch 9, 2007 at 4:54 am #15008WILCOM LABSGuest
Total posts : 45366
I used some old table top tube type radios back in the day. You want to avoid the ones without a power transformer as one side of the chassis is hot at line potential presenting a shock hazard! And they all have high voltages inside,so be forewarned! Having said that,I would look for a screen grid on the mixer and tap the LO signal there for the antenna with a series DC blocking cap. Then,I would feed the audio in the first grid with a DC blocking cap. Different radios had different places to tap and I found them by experimentation. Try this little trick,it works with all receivers. You will need two radios close to each other,set one to mid band,1000kc’s and tune the other one slowly downband about 455 kc’s lower and you will hear the local oscillator,offset by the i.f. frequency. A Bearcat scanner becomes a signal generator by entering a frequency 10.245(some 10.455) higher(maybe lower?) than the frequency desired. Have fun! Regards,LeeMarch 9, 2007 at 5:09 am #15009radio8zGuest
Total posts : 45366
**edit Lee posted as I was posting. Sounds as if he and I took similar approaches. End edit**
I don’t know how Lee did it but for AM I tapped into the local oscillator of a tube set and attached an antenna to the plate circuit. For modulation I applied audio to the screen grid circuit. It worked, but not up to todays standards.
For FM, I tapped the LO tube circuit using a rooftop TV antenna and applied the audio to the control grid. This worked well but the range scared me. My cousin 3 miles away could hear the signal so I quit. I lived near a major Air Force base and I knew they listened all over the bands and the risk wasn’t worth it.
These were vacuum tube radios and unless you know your way around these circuits you will need to do a lot of learning. You might start with a search for the “all American five radio”. This will show you how a typical circuit appears. I have never seen a site where this modification is described but maybe you can search for one.
You might be able to do this with transistor sets, but I have never done it so I can’t advise.
NeilMarch 9, 2007 at 5:36 am #15010RattanGuest
Total posts : 45366
At least for FM radios, most FM receivers already do transmit, just not very far. That’s why they have a part15 statement on the back.
If you tune one FM receiver fairly low on the band, and tune with another one around a little over 10 Mhz higher, you’ll hear an empty carrier. I’ve never seen a website on it, but it used to be a “parlor trick” to hack into an old table radio to find the spots where you could add a bit of modulation to that carrier. Usually one of the resistors near the tuning capacitor would do reasonably well with any sort of line level signal like the earphone output of an old casette deck.
Basically it was a parlor trick, though. Range of maybe 10 or 20 ft. Handy if you needed to move a signal over a few feet without wires or for practical jokes. Back when I was in high school we used to mess with such things a bit.
(PS via edit) Ok, I’ll take it back about it being a parlor trick. Lee’s bit about using a scanner as a signal generator would be useful enough to be out of the realm of parlor tricks. LOLMarch 9, 2007 at 6:20 am #15011kyradioGuest
Total posts : 45366
Disclaimer, the following is strictly experimental information only and no one should EVER experiment with anything that they are not familiar or comfortable with. You are responsible for yourself and any damage to any equipment or property . And if you exceed FCC power limits, you are responsible for it. If you have no idea what you are doing , then don’t. You are on your own.
Having said that, here goes.
GM and Ford Analog Car radios from the 70’s are the best for transmitting AM with.
I have a Ford AM/FM transistor car radio I used for transmitting AM with . When you open the radio up. There are two circuit boards. Looking at the topboard vertically facing up, (thats the board you want. You will see these notches with wires wrapped around it. The 7th notch is where you put audio plus. Audio negative is usually the case.
The antenna connection is the trimmer type of capacitor (on the top board with the notches), it is the only capacitor of its type that you will see ( it is located lower right on the top vertical board).
The top part of that trimmer capacitor lead is where the antenna connection goes. Use several feet of antenna and a good ground. Sometimes you will get a buzz if you use a 12 volt power supply, experiment with a gel-cel for hum elimination if needed.
Some car radios have a 262 khz IF, most others are 455 khx IF. the advantage is that you can transmit a little lower on the dial with the 262 IF.
So if your (now) modified car radio (transmitter) has a 262 khz IF, the dial says 530 khz, you are actually putting a signal out on 792 Khz.
The frequency drift isn’t all that much and the sound quality is OK. AM car radios can be had for as little as $3 at flea markets.
Some advice when experimenting like this , always use a cheap throwaway audio source ( like a cassette walkman) . If you blow the audio amp on it, no great loss.
I started transmitting with FM and AM receivers way back in the mid 70’s. I got across the street and several houses over on FM using my old CB antenna back in the late 70’s early 80’s.
FM transmitting is easier with your older average analog transistor portable radio receiver. Stick with older transistor analog car radios for AM.April 9, 2007 at 7:56 pm #15227RattanGuest
Total posts : 45366
While the output of older devices and tube gear might be in excess of what is allowed for part 15, the “table top” trick might still have some uses in putting together something that is part15 compliant.
Some people do still run “tuned tank” LC type circuits even on AM to avoid having to buy a crystal or going to PLL if they want a transmitter that is frequency agile. Drift and hand capacitance make that a bit of a nightmare.. But.. AM BCB recievers aren’t much affected by hand capacitance so far as I’ve seen and don’t seem to drift much. So I’ve started toying with the idea that they might be practical to use as a VFO for just over 1 mhz to 1700 khz. If one added a modulator and final rf amplifier where one could conveniently check the voltage and current and adjust them so the rf amp was only getting 100 mw to work with, it could be part15 compliant.
Well, assuming the little oscillator in the receiver that was converted to VFO is clean enough, anyway.
Stability and etc are going to depend on the quality of that section of the receiver, I would think, but it’d still likely be a step up from some LC tuning schemes on some AM transmitters from the inexpensive kits and etc?
Might at least be a possibility for some who want a house/yard system and have something around like an old component stereo that has a blown audio amp or something where they just replaced it rather than attempting a repair. Or maybe a small portable walkman or transistor radio could be adapted to a VFO if they have a part15 AM rig that is crystal based and they wanted it to be frequency agile? The actual boards in some of those puppies are very small, and would probably fit into whatever case they were already using.
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