One of the saddest things about my hobby of restoring vintage radios is the absolute lack of quality programming on the broadcast bands today.
Not only is the golden age of radio long gone, today’s AM airwaves are chockful of hate filled right wing talk and syndicated FM commercial radio is mostly insipid crap music. A couple of summers ago, I built a 2 tube regen radio receiver kit with my 10-year-old niece, and while the build was quality time spent together, in the end she was left with a radio that is of no use to her.
But, while there may be very little worth listening to on the broadcast bands today, there is no shortage of excellent programming available as podcasts – good music, true crime, documentaries, comedy and even drama – free for the taking for enjoyment on your smart phone or other device. This cornucopia of content can make ‘watching the radio’ wonderful again.
There are three easy ways to play modern podcasts and other recorded programs on a vintage radio.
The first is to add an “AUX IN” to a vintage radio, like many new radios have. The circuitry is simple and with a handful of parts, including a 1/8 stereo jack, you can easily play the output from your iPod, smart phone or CD player through the radio’s AF stage.
This “AUX IN” mod is popular with radio repair folk who restore vintage radios for resale. Look at what is available for sale on http://www.radioattic.com and you’ll find many radios for sale with the mod. If this interests you, check out the YouTubevideo by D-Lab Electronics here.
The second method is to add a Bluetooth receiver wired into the radio’s AF stage like the “AUX IN” jack mod above. This mod is attractive as the listener can easily transmit podcasts and music from their smart phone to the vintage radio.
For me, the third method that I am profiling here, using a low power transmitter to ‘broadcast’ programs to my vintage radios is a bit more authentic as ‘real radio’ transmitting a modulated RF signal to the receiver where the ‘whole radio’ is being used from the RF to the IF to the AF.
So, a few years ago I decided to build the SSTRAN AMT3000 transmitter kit. It is a popular solid state transmitter that costs about $100. As of this writing, it is not clear to me whether the kit is still available or not as the www.sstran.com website still has an announcement on the home page stating that as of November 12, 2017 they are not accepting any new orders. However the catalog page and shopping cart still appear functional.
I had considered other options including a scratch build or this $40 tube based AM transmitter kit. The tube option had two solid arguments going for it – it was definitely cheaper and using a tube-based transmitter seemed a better match stylistically for most of my antique radios as they are tube based.
However, the AMT3000 won out as it had a few qualities that the tube kit did not offer. These included separate up-front controls for gain, modulation and signal compression. The AMT3000 is also easily tuned to different frequencies on the AM broadcast band by setting DIP switches.
I had also read several online reviews and discovered that the kit had a very good reputation. The fit and finish were attractive too so the SSTRAN kit won out in the end.
I recall it took me two, maybe three evenings to construct the kit. The build was easy thanks to excellent documentation, good PCB layout and no toroids or coils to wind. There was a single surface mount IC, however, but it only has 14 leads and spacing was wide enough that I had no difficulty with my middle aged failing eyesight and shakier dexterity.
In the top left view above you see there are two audio in jacks. This is a mono transmitter, but you can feed a stereo signal through the two jacks which are combined to mono. I use a stereo to RCA jack patch cable to connect the transmitter to my PC or CD player.
The transmitter comes with a wall wart step down transformer that puts out 11 VAC; the voltage regulator can be seen with the heatsink above.
A PLL synthesizer references the 4 MHz crystal to precisely set the transmit frequency. The DIP switches mentioned above to tune the transmitter is seen just below the heat sink on the voltage regulator. The manual includes a table showing the settings to tune across the AM broadcast band.
Notice the three RF chokes, which can be switched in or out of the circuit with jumpers to reduce hum caused by stray RF. Two chokes isolate the power input and the third isolates the audio input ground from the PCB ground.
On the right side of the board you will see a four position DIP switch used to switch several inductors in and out of circuit to assist tuning the indoor long wire antenna supplied with the kit. The instructions describe the construction of a base loaded vertical outdoor antenna that can be used to transmit up to a 2-mile radius which I have not built. When using the external antenna, these inductors are switched out.
For my purposes, I have setup my home AM broadcast station on the operating desk of my ham radio stationon the second floor of my house. As I amonly interested in transmitting a signal to radios within my house, I am usingthe provided long wire antenna which I have hanging out a second storywindow.
My transmitter is tuned to 1,000 kHz which is a relatively dead spot on the AM dial in New Haven County, despite the number of signals coming from NYC to the southwest and Boston to the Northeast. I get little interference day or night.
My transmitter is connected to a beater Dell laptop I bought refurbed from Walmart.com for a song. I installed Ubuntu and use the Beatbox app to manage the queue of podcasts and MP3s. I leave the station on 24/7 so now there is always something worth listening to on the radio.
I mentioned above that the AMT3000 has a compression control, which is a real nice touch. Turning down the compression increases the hi fidelity of the signal, music actually sounds better to my ears than what is typically heard from commercial AM stations.
If you are looking for an AM transmitter to bring life back to your vintage radios, you won’t do any better than the SSTRAN AMT3000. It is a real solid performer and simple enough build for anyone with moderate soldering skills. I have gotten many years of satisfaction from mine. Every December I use it to transmit holiday music to vintage radios placed throughout our house during our annual Christmas party – it’s a fine way to showcase the radios I have lovingly restored.
SSTRAN AMT3000 SPECS:
- FCC Part 15 Compliant
- Emission type: A3 Amplitude Modulation.
- Modulation Type: Series Modulation.
- Modulation Capability: 100%
- Carrier Shift: less than 0.5%
- Broadcast Frequency Range: 530 kHz to 1705 kHzin 10 kHz increments
(a 9kHz increment version is also available for non-US radios)
- Frequency Response: 10Hz to 20kHz
- Audio Input Level: 200mV for 100% modulation
- Audio Input Impedance: 4 ohms to 50K ohms
- Distortion: 0.5%
- Final DC Input Power: 100 milliwatts (full legallimit per FCC Part 15)
- Duty Cycle: 100%
- AC Ripple (Hum): Less than 0.3%.
- Frequency Stability: 0.003%
- Antenna Matching System: Pi-Network