Restoration of HP-21 alike Cavity Oscillators

HP-21 legend of HP8640B

HP8640B is known as a high performance signal generator, extremely low noise, benefit from it's cavity oscillator. The most detail information only a picture from HP journal. I wonder why no teardown been founded, might because afraid to damage it.

Refer to hpmemoryproject HP-21:

The cavity is less than 1/4 wavelength long so that short at bottom end causes opposite end, near tuning plunger, to appear inductive. Capacitance between tapered tuning plunger and center post resonates with resulting inductance at selected frequency. Moving plunger changes capacitance and thus changes resonating frequency. Varactor diodes are in series with capacitance between varactor end cap and plunger; total capacitance thus formed is in parallel with plunger capacitance.

Feedback for oscillation is by collector and emitter loops coupled magnetically through the cavity. AGC, necessary for stable oscillation, is accomplished by modulating the conduction angle with self bias developed by rectification in the oscillator base-emitter junction.

Anonymous replica

I'm lucky to rescue one cavity oscillator from a hobbyist in another city. It did not work while arrive. So it's a change tear down a cavity oscillator. The pre-owner said it's may be hart of a Japan device, but not sure which device it was.

By totally tear it down, I found it is definitely a replica of HP-21, but without the fine tune mechanical.

Oscillator Overall

Detach the Cavity Oscillator From it's Buffer and power module.

The cylinder part it's all the oscillator. The sealed diecast cases is oscillator power/buffer modules.

This side is the buffer amplifier. The small heavy gauge wire pick up oscillator energy from the cavity. This monster has same frequency coverage as HP-21: from 250 Mhz to 500 Mhz.

Deep dive the Cavity Oscillator

Before tear down further more, the bit Hex nut need be dissembled first, and pull out the transitory out in this nut window.

Cavity contains 2 crone, One of crones had slow motion reduce gears, it's main frequency adjust facility.

The fix crone mounted the oscillator, and a FM varactor.

FM varactor mounted on the top of this fixed crone.

Oscillator mounted on very bottom, contain only 4 resistor 2 capacitor and one transistor.

Oscillator Details

Oscillator mounted on a plastic frame, use golden plate copper couple the RF energy out and inject the energy in.

resistor connected to feed through capacitor, bottom external copper pad connected to the Transistor emitter.

Top copper connected to collector.

both collector and emitter connected to feed-through capacitor, and another side of the copper connected to GND( the metal) via a low value resistor and a small capacitor.

This box provide DC voltage and current limit resistor for oscillator, and FM varactor control wire.

Schematic of the oscillator. Almost same as HP-21.

buffer

Restoration the Oscillator

2019/9/10

It's mysterious why it won't work and why it works again. Use multimeter checking all the circuits component, nothing seems wrong. Change a good transistor replace the original one won't' help anything.

And I happened connected wrong polarity from power box to the oscillator, but it's not a disaster, because there is resistor limited the overall current, about 20mA.

I realized it and cut off that in time. And unbelievable, it's starting oscillating while i reattach the power in corrected polarity.

I really don't know why. Few friends tell me they run into such things too while try to rescue other old stuff.

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