HP 204B Audio Oscillator

Brooke Clarke, N6GCE


This is a solid state (both Ge and Si transistors) instrument made using single sided printed circuit boards.
It has very good frequency calibration and amplitude stability as the frequency is changed.
The second harmonic of a 1 kHz output is down 52 dB and from 6 kHz and up they are down more than 60 dB.
Was used in the HP 3350 Carrier Test Set (AN/USM-181 Telephone Test Set TM 11-6625-602-40) that consisted of a 3 wide Combining Case (-hp- Model 1051A or 1052A), the 204 Oscillator, a 353 Patch Panel and the 403? AC Voltmeter, HP 3553 Meter & Patch Panel??

The 204C adds a SYNC input/output so that multiple oscillators can all be set to the same or harmonically related frequencies.

HP History (Wiki)

The first HP patent was for an audio oscillator (200A) and made use of a pilot lamp as a stabilizing element. 
2016: I received an inquiry about a Bell System Technical Journal article Bridge Stabilized Oscillator by L.A. Meacham, presented June 6, 1938, published Proc I.R.E. Oct 1938.
Was this prior art to the hp patent?
Variable frequency oscillation generator, Hewlett William R, Filed: Jul 11, 1939, Pub: Jan 6, 1942

So at first glance it appears that the BSTJ article is prior art.  But I don't see it that way.  Here is my response:

"The first line in the BSTJ article has the phrase "constant frequency oscillator" and the title of the patent is "Variable Frequency Oscillation Generator".
I think this is the key difference.  But there are other significant differences:
The BSTJ circuit is a bridge with a crystal setting the frequency whereas the patent uses an RC circuit to set the frequency over many decades.
The BSTJ circuit has the lamp in the bridge whereas the patent has it in the cathode circuit of the tube.

It's very possible that Bill read the article and applied the lamp in a new and unique way.  But from where I sit it is a very different circuit.

I have not learned how to follow court cases related to patents. But in this case there would not be a patent vs. patent suit, but maybe a case for a competitor of HP to invalidate the patent.  For example the regenerative receiver suit between Armstrong and DeForest.
Sometimes when a patent is re issued  (RE prefix) you can learn what's really important (often related to the class numbers used).

I've currently learning about combination locks and reading a lot of patents about them.

Again and again I learn things from patents that are not to be found anywhere else."


There was no output when the unit was turned on to any of the frequency ranges.
By spraying the OFF-RANGE switch contacts with Radio Shack Tuner - Control Cleaner & Lubricant ( RS-64-4315) the output appeared.
The frequency accuracy and amplitude stability are very good.
The power supply is a separate assembly on the back outputting +13 and - 13 VDC.
The 204B uses the old style power cord, not the modern IEC type.
There is a version that used 4 each mercury batteries to allow floating from the AC power line.
The output is DC coupled and there is an adjustment on the back panel to zero the DC component.

Source Resistance

By using a Agilnet (HP) 34401A DMM and a decade resistance box to see how the external reesistance loads down the open circuit voltage from the 204C it's output impedance can be calculated to be 636.2 Ohms.  But if all the impedance values of a TS-585C/U audio output meter are used as the load for the 204C and the assumed source resistance (Rs) of the 204C adjusted for the lowest error, then the Rs value of the 204C looks like 632.5 Ohms, these two values agree within 0.58%.  Excel spreadsheet with the measured and calculated results.

Output Power

This oscillator was intended for use in testing telephone type equipment where the more interesting power levels were those near the noise.  The max open circuit voltage from the 204B is about 5 volts or 2.5 volts into a 600 Ohm load that's 10.4 mW.  The amplitude control shows 10 MW at full scale.

So this oscillator is not suitable, as it stands, to provide a 1 Vrms sine wave into the 50 Ohm input to the FTS 4060 Cesium Beam Frequency Standard since that takes 20 milliwatts.


Manual available from: Lewis Porter
The 204C Manual is TM 11-6625-1589-15-1

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Page created 13 July 2001.