EICO Model 324 Signal Generator

Next to a multimeter, a signal generator is the most useful piece of test equipment for restoring vintage radios and TVs.

I picked up this EICO 324 generator for $10 at a church rummage sale. The unit looked little-used, and it included the original operating and construction manual.

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 fine. However, I opened it up to clean the controls and give it a basic recapping, standard practice for any tube device of that vintage. Since that time, I have used it to align many of the radios seen in this website.

EICO test equipment is commmon and you can often find units like this for $20 or less. The BAMA website has free manuals for many test devices as well as boatanchor radios. You can also purchase manuals from the sources listed on our Parts page.


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.

This 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 procedures demand a stronger signal, while others require a weaker one.

When using the generator for alignment, turn it on for about 30 minutes before starting the procedure, to make sure it has stabliized at normal operating temperature. Do the same for the radio that you're aligning.

The RF output jack accepts a simple shielded cable with alligator clips on the end. The alignment instructions for a particular radio will sometimes tell you to connect a small capacitor or resistor in series with the signal lead, for a particular procedure, so read those instructions carefully.

What's a Signal Generator Good For?

A signal generator has various uses in radio restoration. If a radio has become misaligned, the generator can be used to set it right again.

You can also use a signal generator to diagnose radio problems. The basic idea is to inject a signal with a particular frequency at the input portion of a circuit and then measure the output farther downstream, comparing it to the ideal.

To take a simple example, if a radio is completely silent, you can inject an audio-frequency signal in the audio output circuit to see if the generator's tone is heard from the speaker. If the signal comes through loud and clear, you cross the audio output circuit off the list and shift your attention farther upstream in the radio.

One of my favorite service books, Elements of Radio Servicing by Marcus & Levy, makes extensive use of the signal generator and explains how to use it in many different contexts. The book went through several editions and it's readily available in the used book market and in libraries. You can also download an electronic copy of the first (1947) edition from the Antique Radios archives.

Double-Checking the Frequency

The EICO's signal is stable enough for everyday workshop use, but its dial is not very accurate, compared to modern devices. I use a modern frequency counter or a digital radio to check the 324's dial settings.

In the following photo, my 324 is sitting on top of my B & K Precision model 1801 frequency counter. The generator's output is set to 465 KHz, the IF frequency used by the Stewart Warner 1865 radio on the workbench.

The readout on the frequency counter shows 465 KHz exactly.

You can also double-check the frequency using a modern radio with digital tuning. I use a Grundig Yacht Boy 400, a multi-band receiver.

Form a couple of turns of the wire into a loop several inches across—the dimensions aren't critical—and connect the ends of the loop to the output leads on your signal generator. Place your digital radio by the loop and tune it 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.

If your radio isn't capable of receiving a certain frequency, try tuning it to a multiple or fraction of that frequency. For example, my digital radio can't be tuned to 455 khz, but it can receive 910 khz, which is exactly twice that frequency. In addition to the main frequency, the 324 generates harmonics. With the radio set to the 910 KHz harmonic, I can hear the 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 uses the 324's audio input terminal.

Try connecting the output from a tape player or other audio source 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 at lower right.

Turn on your 324 and 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.

Note: FCC regulations prohibit 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!

©1995-2014 Philip I. Nelson, all rights reserved