Eico 320 Signal Generator Restoration

I’ve taken a liking to the blue art deco paint scheme on Eico’s old test equipment.

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This Eico Model 320 signal generator is a good example. 

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Internally, it is a very basic two-tube signal generator, with a 6X5 rectifier and a 6SN7 dual triode. One section of the 6SN7 is the RF oscillator. The other is the AF oscillator.

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Under the chassis, it’s full of wax paper capacitors. The label on the Sprague “tubular condenser” (at the bottom of this photo) is pretty cool, though.

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This was a kit. The soldering was pretty heavy-handed, with big gobs of solder on some of the terminals.

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The single-section electrolytic looks original, along with the masking tape that’s holding it in place. I doubt the tape was mentioned in the kit instruction manual, though.

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Note the metal-cased glass-sealed oil and paper capacitor at the right of the photo.

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There are two of them in the 320. I tested their insulation resistance with my Simpson 383A Capacohmeter, which applies 300 volts or so. They tested fine, so I left them in place.

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I cleaned the band switch with DeOxit D-100 on the contacts and on the contact leaf on the rotor shaft of the variable capacitor. I shot some WD-40 into the potentiometers, and resoldered all the connections to the band switch.

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This is the layout of the wiring for the 6SN7 tube socket. As usual, I took lots of photos to help me replace the components correctly. I don’t depend on the photos, though. I trace the schematic for every connection I make, and then double-check against the photos.

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Here’s the view from another angle.

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The two reddish capacitors at the lower right are the AC power line filter capacitors.

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The power transformer is mounted under the chassis, right next to the big electrolytic. I could have used a few more photos of the wiring layout of the 6X5 tube socket.

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Here I’ve started clipping out the components. The layout was too crowded to replace them one at a time. The insulation resistance of the mica capacitor at the top of the photo tested good, so I left it in place.

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That was a mistake. After replacing the necessary components and re-soldering many of the solder joints, the 320 worked OK except at the high end of band D, where the RF oscillator was squegging. I asked for help on the Antique Radio Forum, and eventually decided to try replacing C1, the 50 pF molded-mica grid-leak capacitor shown in the previous photo. I replaced it with a 47 pF dipped mica capacitor, and that fixed the problem.

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I replaced most of the resistors and all of the capacitors except for the glass-sealed paper-in-oil output coupling capacitors.

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The smaller size of the modern metallized-film capacitors really opened up the layout.

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I used 0.0047 µF type Y safety capacitors to replace the AC line filter capacitors.

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There was plenty of room to mount a modern radial electrolytic capacitor as the power supply filter capacitor. No masking tape needed this time.

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The completed job. Note that there is only one trimmer capacitor for all five of the bands. So you can’t expect great accuracy on the dial scale for all the bands. 

The Eico 340 was an upgraded 320 that added individual trimmers for each coil. I believe they are otherwise identical.

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I checked the dial accuracy on band B, which covers 400 kHz to 1300 kHz.

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Surprisingly, the 455 kHz calibration was spot-on!

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Next I tested the 1,000 kHz point on the band B scale.

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The 1,000 kHz calibration was accurate to less than 1%. Not too bad for such a basic signal generator.

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Here it is all buttoned up in its cabinet. I kept the Amphenol 75-MC1M microphone connectors. While Amphenol no longer makes them nor the 75-MC1F mating connector, Switchcraft still produces them. The Switchcraft 2501F will mate with the Amphenol 75-MC1M. It’s readily available from Mouser.

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The insulation on the old line cord was cracking, so I replaced it with a new cord.

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The cabinet is a little rough, with some scratches in the paint that have rusted.

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But I don’t mind equipment with a little patina.

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© Steve Byan 2011-2016