Eico 320 Signal Generator

A vacuum tube signal generator, about 60 years old, with new capacitors, new connectors and a clean signal again.

A gray metal Eico Model 320 signal generator on a workbench, with a large half-round tuning dial printed in red and black, three black knobs, and a carrying handle with the power cord wrapped around it — The Eico Model 320 as it came to me

This relic is an EICO Model 320 Signal Generator. EICO stands for Electronic Instrument Company. A signal generator makes a steady radio signal at whatever frequency you set on its dial. You feed that signal into a radio to test it or to tune it up.

It uses 2 vacuum tubes and wax paper capacitors. I had never actually seen wax capacitors until now. It is always fun opening these old pieces of equipment up. In the winter of 2025 I replaced the old capacitors and the connectors, then calibrated and tested it.

2 tubesa 6SN7 and a 6X5

10 capacitorsreplaced with new ones

400 Hzaudio test tone

7 bandsstarting at 150 kHz

1. A look inside

One tube, a 6SN7, does the real work. Half of it makes the radio signal and the other half makes a 400 Hz audio tone. The second tube, a 6X5, is the rectifier in the power supply. The dial starts at 150 kHz and is split into seven bands, lettered A through G, with markings up to 102 MHz. The brown coils set the band, and the yellow parts are the wax paper capacitors.

The chassis pulled out of its case, seen from below: yellow wax paper capacitors, an orange electrolytic capacitor, a power transformer, brown coils and a tuning capacitor with metal plates — Under the chassis, all original

A closer view of the underside of the chassis with yellow wax paper capacitors wired between the tube sockets, coils and controls — The yellow wax paper capacitors

The top of the chassis with two vacuum tubes, a row of brown coils wound with copper wire, and a small transformer — Top side: two tubes and the band coils

A glass vacuum tube marked 6X5 GT, with Made in U.S.A. for EICO by Sylvania printed in gold on its black base — The 6X5GT rectifier tube, made for EICO by Sylvania

2. New capacitors

I replaced all of the old wax paper capacitors. In went 8 metallized polypropylene film capacitors, 1 electrolytic and 1 mica. I didn't replace any resistors, as all of the resistors tested within 5 to 10% of their values.

The signal generator on the bench with small plastic bags of new yellow film capacitors, two black electrolytic capacitors and small red mica capacitors laid out in front of it — The new parts, ready to go in

The chassis with new yellow capacitors installed, and in front of it nine old capacitors, two old round connectors and a small white part laid out on the bench, with soldering irons behind — The old parts that came out

The underside of the chassis after the work, with bright yellow film capacitors, a small red mica capacitor and a black electrolytic capacitor wired in where the old parts were — After: new film capacitors, a new electrolytic (far left) and the red mica capacitor

3. BNC connectors

I also replaced the old screw-on panel "mic" connectors with more modern 50 ohm BNC panel connectors, so standard test cables plug straight in.

A close view of the front panel with the two original round screw-on connectors marked Audio Out and R.F. Out on either side of the band switch — Before: the original screw-on connectors

The same part of the front panel with a new BNC connector at Audio Out and another at R.F. Out — After: BNC connectors

4. A mystery part

I believe this signal generator was slightly modified some time in the past. The original schematic and parts list call for a 50 picofarad mica capacitor, which wasn't there. Another component was in its place: a small white cylinder.

I suspect it was done to add or change some of the higher frequencies. On the dial face you can see new frequencies written in by hand.

A small white cylinder with wire leads soldered between a tube socket and the band switch, next to an old yellow wax capacitor — The white part as I found it

The small white cylindrical part lying on a green cutting mat after it was removed, with a wire lead at each end — Out of the circuit

A close view of the center of the tuning dial, where extra numbers from 60 to 170 and a new scale line have been written in by hand in black ink — Frequencies written in by hand on the dial

Peter, KF7PSM, thought the white part may be an inductor, and that it looks like someone wanted to get up to 2 meter frequencies. With older equipment like this, the higher you go the more stability you lose, so that was probably as high as they could reasonably get.

Regardless, I replaced it with what the schematic originally called for, a 50 picofarad mica capacitor.

A small dark red mica capacitor soldered in between the band switch and a terminal strip — The red part is the 50 picofarad mica capacitor I installed

5. Calibration and testing

The next day I calibrated and tested it. It supplies a fairly accurate and clean signal for being about 60 years old. The oscilloscope shows the audio output, which is close to the standard 400 Hz, as a clean sine wave. On the frequency counter I checked radio signals at 5 MHz, 10 MHz and 260 kHz.

The signal generator connected by cables to a frequency counter reading 5.004 and to an oscilloscope showing a smooth sine wave — 5 MHz on the frequency counter and the audio tone on the oscilloscope

The signal generator with its red pilot light on, beside the frequency counter reading 261 — The dial set to 260 kHz, with the counter reading 261

Back to work

The Eico has already earned its keep. I used it to align my Realistic DX-160 shortwave receiver.

Parts

Qty	Part	Type
1	50 pF	Mica
1	0.002 µF (2 nF)	Film
2	0.01 µF	Film
1	0.05 µF	Film
2	0.1 µF	Film
1	0.25 µF	Film
1	0.5 µF	Film
1	16 µF or greater, 350 V	Electrolytic
2	50 ohm BNC female, panel mount	Connector

The film capacitors are metallized polypropylene, rated 400 volts or more.