Adding FM to Vintage AM Radio

Here's the story of one AM tube radio's journey to FM.

Back in the late 50s, most stations broadcasted on AM only. FM did not become  prevalent in broadcasting until well into the 70s. True, FM was first demonstrated in 1936. Armstrong broadcasted via FM as early as the 40s. By the end of the 70s, approximately half the listeners in the US were listening to FM stations.

Vacuum tube radios were primarily AM only receivers through the 60s. There were some manufacturers throughout this period which made FM and dual AM and FM receivers but it was less common.

FM is a different electronic design than AM. While the amplification stage is pretty much the same, FM demodulates incoming signals by frequency while AM demodulates by amplitude. The electronic circuit design to perform one versus the other is different.

In other words, you cannot modify an AM radio to receive FM by just swapping out a few parts. You need to re-engineer the electronic circuit. It would be more time consuming to do this than it would be to replace the chassis altogether. Replacing the chassis has its downsides associated with it, and not just all about the electronics.

Like, where will you mount the switch to go from AM to FM? Are you going to drill a hole in front? Where are you going to find a matching knob?

What about the tuning dial? Are you going to re-print the scale so it has AM and FM frequency readouts? How's that going to look?

So, let me see if I got this right. You have an AM radio and you spend hours to rewire it for FM, adding additional necessary tubes, drilling into the board or chassis for needed transformers, if there's even room, replacing the variable tuning capacitor. Then, you drill out the front or back to add the AM or FM switch. No, you cannot receive both AM and FM using the same circuit so you'll need a switch to activate one circuit or the other.

So, you find a knob (or not) that matches.

Now, you need to print out an replacement tuning scale that shows both AM and FM frequencies. Probably won't look good from your home inkjet printer or laser printer at work.

Sounds like a lot of work.

Or, you can buy one of these. Mount it on the back of the radio with a toggle switch. You're off to the races! Oh, and it's somewhat reversible modification.

Sign me up.

A customer sent me this 1959 General Electric standard AM receiver. She said she wants to Add FM and Bluetooth. She also paid for Electronic Repair and Restoration. I can't add Bluetooth and FM to a non-working radio.

When I first tested it, there was silver migration disease (SMD) affecting the Intermediate Frequency Transformers (IFTs). Here's a workshop article I wrote on curing dreaded SMD. After that was repaired, I found a few resistors which were out of tolerance. The radio needed re-alignment as well. Then, it sounded good.

Tricky thing about adding FM and Bluetooth is the power supply. Some radios do not have available interior space for an additional power transformer. The typical Bluetooth receiver contains an internal LiMH battery that is good for 4 to 6 hours and recharges in 45 mins. So, not a big deal to not integrate an internal power transformer. However, there is sometimes a problem with noise.

But, this FM receiver module does not have its own internal battery, so you would always need an AC adapter to feed it 5V-12V. That's two power cords all the time for the thing to work. Not the end of the world, but it would be nicer to have an internal 5V power supply.

This particular radio has enough empty space inside for a 5V power transformer. I just need to figure out a way to securely mount it to the cabinet. And find one that doesn't generate too much noise.

This FM module is a dual function FM and Bluetooth receiver. It can be controlled via panel buttons or by remote control. The remote control unit is included. It runs on a standard 3V LiMH 2032 coin battery.

The output of this unit needs to be fed into the non-B+ side of the volume potentiometer. This enables you to modulate the volume of the module signal using the old radio volume control as well as the remote unit volume output controls.

The old radio signal and remote FM/Bluetooth are fed into a SPDT switch On+Off+On settings. Either AM radio or auxiliary FM/Bluetooth remote unit are activated.

 

Through trial and error, I found this unit sounds better:

I took out the SPDT switch and replaced it with a 2P3T switch in anticipation of having to switch off internal 5V power supply going from FM or Bluetooth to AM.

Here's the backing board with new components mounted. Yes, I realized it was mounted upside down.

Here's the back of it:

Here's YouTube video of operation:

Now, how am I going to eliminate that noise from the switching power supply affecting AM?

The cheap AC Adapters are the noisiest. The more ballast the quieter. Those big and heavy solid iron core seem to be quieter. Size and weight are disadvantages.

Wanted to avoid that second external 5V power supply with internal power supply. The power supply needed to be switched off in AM mode. No matter how quiet the transformer, the FM unit would interfere during AM operation if it was left on. Bluetooth didn't seem to matter whether it was on or off during AM mode.

Finally, noise interference was solved with non switching 5V 1A transformer. I had to try a few before I found one that was quiet enough.

Once I had everything hooked up, I was able to switch from FM, to Bluetooth, to AM and hear all inputs clearly and loudly with no interference. Switch from AM to Bluetooth to FM, however, there was a 1-2 sec pause for FM unit to power up. Once FM unit was powered up, it sounded just fine. The FM unit returned back to the last station it was last set to. It would have been less than optimal if the station was reset to 88.1 MHz every time it was powered off.

Here's YouTube of completed project. Sounds great!