The PHILCO Phorum

So for the last few nights I have been looking at AM transmitters and poking around the junk boxes wondering if I have anything that could be pressed into service until i get something proper.

Initially I thought of the old fm->am car radio converter box thing my dad gave me to play with as a kid. Sadly, after much searching, I came to the conclusion that I must have finally thrown that out.

Then I poked around on the web a while looking for ultra-simple transmitter plans. But they all involved either a resonator I didn't have (no 1mhz ones, only much higher, sad) or coils which i didn't want to make.

So then I was talking to a guy at work about the hum and distortion my radio is producing. He asked if I had a scope. I said yes and he said well then you can poke through the circuit (carefully) and find where the signal goes bad. he mentioned it was best to have a signal source of some sort. i mentioned i had a signal generator that could probably do it.

So I get home that evening and spend about an hour searching for the d**ned signal generator. Finally find it!

And immediately remember that it has several 'external modulation' modes. Hmm... Does it have an AM external mod mode? It does!

First though, will it send a signal my radio can hear? So I set it up for an internal 1khz modulated AM signal at 550khz. Poke around on the radio a bit, and there, very faintly, is the signal!

I realize quickly that the wire i am using as an 'antenna' is shielded, so replace that with a....paperclip...wrapped around the end of the radio's antenna wire. Signal now comes in loud and clear.

Poke around with the generator a bit. Something rolls off the audio about 8khz very sharply. There is also a drop at 4khz.

Then I spend some time finding the right cable and adapters to attach my laptop to the external modulation input of the generator.

Set the mode to AM external modulation at 550khz.

Hit play on the Country Joe and the Fish mp3

And....It works!

Tuning around a bit shows that slightly off-frequency results in better high frequency response (why is that?).

So, here is the set up:

Laptop with 'bithead' external DAC, the internal audio on this laptop is absolutely horrible.

http://www.flickr.com/photos/audin/2404075665/" title="Bithead - where does that wire go? by Audin, on Flickr"


Output from bithead into back of signal generator:

http://www.flickr.com/photos/audin/2404075827/" title="Whats that wire? by Audin, on Flickr"


Front of signal generator showing mode and 'antenna' setup:

http://www.flickr.com/photos/audin/2404075995/" title="Signal generator in External AM mode... by Audin, on Flickr"


Generated signal on the scope:

http://www.flickr.com/photos/audin/2404076161/" title="Generated AM signal! Ready for tuning by tube radio. by Audin, on Flickr"


The manual for the generator claims the external modulation signal can be either 35khz or 50khz wide. Either way I think that counts as a 'hi-fi' am signal. The audio quality is surprisingly good.

anyway, it isn't a permanent solution, but i think it may be one of the most expensive hack ways to broadcast an am signal.

I'm curious to see how its external fm modulation mode works, but unfortunately i don't have anything that can do fm audio at 20mhz (the highest carrier frequency this generator will do).

>>slightly off-frequency results in better high frequency response (why is that?

You're peaking the radio's passband on one of the transmitted sidebands, and off the other. Both sidebands carry the same signal, so you can afford to lose one.

A lot of RF generators have external modulation inputs, but the distortion level is usually high and the maximum modulation is around 30%. Some older ones will do better. I have a 1937 Ferris for instance that uses a small transmitting tube in its output stage (an 802), plate-modulated by a pair of push-pull 6L6s. That thing is practically a broadcast transmitter (and about as heavy as one too).

Using one of those 1960's Heathkit sig gens with external AM worked wonders at a museum display a few months ago. One addition that I made to the set-up was a step-up transformer (1:4?) from the CD player output to the AM input. Distortion was surprisingly low and not detectable to the ear.

In a pinch I use a double balanced diode mixer (hp 10514) as a modulator- this arrangement gives you very low distortion but you are limited to about one milliwatt max. This comes in very handy when you need to generate a modulated IF or RF signal and you only have a CW carrier source and a separate audio source.

That certainly is a cool, and "expensive" solution if you didn't already own the generator, but I have another solution that hits the oposite end of the spectrum...

We can call it the "lo-tech...Red-neck" solution.

You will need an old, or new, earphone with a mini connector on the end.
Crack open the earphone, or cut the speaker end off.

Strip back the wires... ground and source.

open your radio and solder the ends of the earphone wires to the two contacts on the speaker... (of if you really want to be "redneck" about it, just use duct tape and bubblegum)

Close your radio and leave the mini connector end outside the case.

Plug in your MP3 player and play it...

It will work, but I don't recommend it. I just wanted to show an opposite end solution.

Thanks.