Vintage teardown - General Radio 723-C Vacuum Tube Tuning Fork

[JuiceKing]

I was thrilled to see one of these come up on eBay and even more thrilled that no-one challenged my starting bid, which was itself only a little more than the cost of shipping.



Inside the box is an electro-mechanical oscillator, whose frequency is controlled by a tuning fork. This one has a resonant frequency of 1000 Hz. There was also a 400 Hz version, and General Radio manufactured many variants for applications that called for precise oscillation.







When working, it makes an easily audible, and ultimately annoying 1000 Hz tone. At first, mine didn't want to sing at all, but after I opened it up and washed off the heavy corrosion I found on the fork, it started to come to life.

I brought mine up slowly, with a Variac, in order to avoid damaging old electrolytic capacitors inside. It was ultimately pointless, since the seller had powered it up himself to test it, and the capacitors in the power supply were toast. As a result, there was a heavy 60 Hz hum in the output signal. Replacing all four of the 40 uF capacitors in the power supply did the trick, though, and now it works great. I spent more time thinking about a way to replace the capacitors with minimal effort and damage to the original form of the unit than the repair itself. I tied the negative leads of the new capacitors to an unused common negative lug on the old capacitor, which meant that all of the original negative connections could be left alone. I clipped the positive connects and spliced them to the new capacitors with some solder, and used heat-shrink tubing to insulate the exposed leads.

There are screws inside the holes at the ends of the fork tines. I tried to turn them in order to tweak the resonant frequency of the fork, but they wouldn't turn and ultimately I left them alone.  A spot of putty (not seen in photos) on each tine was enough to shift resonance down by a hair and bring it into spec.




For all the archaic technology, performance is astonishing. After 60 minutes of warm up, frequency is off by less than 0.01%, with minimal drift and THD about 0.3%. I'm guessing that mine is from 1941. That's the date on one of the tubes. The October 1941 edition of General Radio Experimenter Magazine has a brief write up, and several other issues from the 1930's explore the evolution of tuning fork-controlled electro-mechanical oscillator technology.

[SeanB]

What I do with those old caps is to remove them, slip the sleeve off and then cut the can in half with a pipe cutter, then gut it out and clean it. Place the new units inside and glue back together and replace the sleeve. Looks original but works.

[ecat]

Always fascinating to discover things you never even knew existed.

Quite, quite lovely. Thanks for sharing

[codeboy2k]

Wow. Great find. How's the smell of bakelite?

Now we have all that big, smelly, leaky stuff packed into an smd...... MEMS oscillators !

Don't ask me how they work, I don't know. But here is a picture of the tuning fork part that was micromachined.

Discera and SiTime are leading the field here. I have not had a chance to use any MEMS oscillator in a project of mine.