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.
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