Eico Model 324 Signal Generator

Next to a multimeter, a signal generator is the most useful piece of test equipment for radio restoration.

I picked up this old kit-built generator for $10 at a church rummage sale. The unit looked little-used, and it included the original construction manual with schematic.

When I got it home, I gave it the usual cleanup and spent a little time reading the manual. I powered it up, and it seemed to work just fine. Since that time, I have used it to align many of the radios seen in this website.

Eico test equipment is very commmon, and can often find units like this for $20 or less. If you find one without a manual, you can purchase a manual from the sources listed on our Parts page.

Description

The 324 can generate signals from 150 Khz (kilocycles) all the way up to 435 Mhz (megacycles). Most radio alignments use frequencies well in the lower part of that frequency range. For example, many AM radios have their IF stages aligned to 455 or 456 Khz.

To set the frequency, you first select the appropriate band, with the range containing the desired frequency. Then you turn the big knob to dial in the exact frequency.

The generator can produce a modulated or unmodulated signal, and it can accept an external modulator. The lower right knob lets you adjust the strength of the signal output. Some jobs demand a stronger signal, while others require a weaker one.

Double-Checking the Frequency

As with many old tube-powered devices, the 324 cannot compare in accuracy to modern digital equipment. I use a modern digital radio to double-check the 324's settings.

This is easy to do. All you need is a couple of feet of insulated wire and a digital radio. Form about three turns of the wire into a loop roughly six inches across—the dimensions are not critical—and solder two insulated alligator clips onto the ends of the loop. Now you have a simple loop antenna.

To check the signal generator's output, connect the ends of the loop to the output leads on your signal generator and turn on the generator. Place your digital radio next to the loop and turn it on. Dial both devices to the desired frequency, then adust the 324's dial setting until the sound heard from the radio is as loud and clear as possible. The sound that you hear will be a low hum.

Now the 324's output is set exactly to the frequency shown on the digital radio. Disconnect the 324 from the loop antenna, set your digital receiver aside, and you are ready to use the 324 to align the radio that you're restoring.

If your radio isn't capable of receiving a certain frequency, try setting it to a multiple of that frequency. For example, my digital radio, a Grundig Yacht Boy 400, can't be tuned to 455 khz. However, it can receive 910 khz, which exactly twice the desired frequency. In addition to the main frequency, the 324 generates harmonics, or multiples, of that frequency, so with the radio set to 910 Khz, I can hear the 455 Khz generated signal just fine.

A Cheap Home AM Transmitter

If you want to have some fun, you can also use this device as a flea-powered AM transmitter, to broadcast programs to radios in your house. This trick takes advantage of the 324's audio input terminal.

Try connecting the headphone output from a cassette player to the audio/ground inputs (lower left in this photo) of the 324. You will also need to connect some kind of antenna wire to the RF output terminal (lower right).

Turn on your 324, then turn on a nearby AM radio and tune it to a quiet spot on the dial. Now set the bandswitch on the generator to the correct range and tune it to the same frequency as the radio's quiet spot. You should hear the signal from your cassette player broadcast through the radio.

If you're interested in a higher-quality AM transmitter, you can find complete plans for building one at our Li'l 7 page.

Important Note: FCC regulations place strict prohibitions against broadcasting radio signals beyond a very limited range (essentially, beyond your own dwelling) unless you have a government license. Please resist the temptation to connect a giant antenna to your Eico in hopes of broadcasting a signal over great distances. It won't work well anyway, and the last thing you want to do is invite a visit from the Men In Black!