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.
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.
Ah, the good old Wien bridge oscillator! I’d recognize that light bulb anywhere – such a simple, elegant solution to amplitude stabilization.
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.
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.
Thanks for the in-depth troubleshooting. I have three of these units in the repair queue.
Interesting side-bar: the 377 was part of the prop setup on “The Time Tunnel” and “Lost in Space”, used to generate the Lissajous figures on a B&W picture tube. The other AC source was a 6.3V filament transformer.
http://q7.neurotica.com/Q7/scifi/Tunnel/Chase-8L.jpg