Eico Signal Generator Gets A Repair

The Eico model 377 was a pretty common audio signal generator. [The Radio Mechanic] picked one up from 1956 that was in reasonably good shape, and shares a teardown and repair of the unit that you can see in the video below. The device could produce sine and square waves using a few tubes.

The unit was a bit different inside than expected because there were several versions made that shared the same model number. The bottom of the case had some goo in it, which is never a good sign. Unsurprisingly, the culprit was an old capacitor.

The oscillator uses the Wien bridge circuit that incorporates an incandescent light bulb as a dynamic resistance element. This was a common low-distortion audio oscillator circuit. The bulb acts as an automatic gain control since its resistance is lower when cold.

We will warn you: the end of this video isn’t a success — it is something of a cliffhanger. But part two is right below the first video. If you want to understand the lightbulb better, [Jeri Ellsworth] took us through that circuit before. We’ve even simulated them virtually.

5 thoughts on “Eico Signal Generator Gets A Repair

  1. Heyyyy I recognized that circuit immediately! Just found the exact same model at a thrift store and fixed it up. And I found that same circuit diagram printed on a folded-up and yellowed piece of paper taped to the inside of the case. I really appreciate when previous owners and servicemen leave a (possibly amended) diagram behind for the next one. Very respectful.

    1. Yep! I was delighted when I opened mine up and saw that bulb down in there. Had a friend over who saw it and went “why is there a lightbulb hidden where you can’t see it?” and of course I got to go into the whole strange story of using a bulb as a dynamic resistor for reasons unrelated to producing light.

  2. An Eico I probably would have left on the heap. An old HP I would rescue. I have one oddball and I forget who made it but it is tube based and does sines and squares as well as triangles and sawteeth.

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