Repair of HP 8569B 22GHz Spectrum Analyzer

[Jamie_A]

I'm attempting a repair of my newly acquired HP 8569B Spectrum Analyzer.

I'm seeing a terrible response in bands 2 to 6 (harmonic mixing). See pictures.

I have a working 18GHz SA and good signal generators to compare against. I believe I narrowed down the issue to the 1st mixer. I see a similar results when bypassing the fronted and injecting a (-30dBm) signal directly into the mixer. I confirmed the bias generator is nominal, and the LO is strong and flat. Additionally, I injected in a 321.4MHz IF signal into the SA, bypassing the mixer, and got a perfect flat trace.

The only suspect bias signal is the "Drain Bias" powering the IF amplifier in the mixer assembly. It is drooping to 4.6V with the mixer attached, with the mixer drawing 77.45mA into the turret. Disconnecting the mixer shows that rail raise back up to 5V (due to the series resistance in the bias supply). This means the IF FET is dissipating 1/3 W!

The other bias voltages are nominal.

Testing with a spare (used/untested) 1st mixer shows exactly the same result.

Does anyone have a working 8569B? Could you possibly measure the voltage on the Drain Bias of the 1st mixer? I have no reference to go off of, and the OSM for this unit is not as detailed as the 70s era HP gear.

Any idea what else could be going on here?

[Jamie_A]

Looking inside the mixer, not any visible damage. You can see the PIN diode and mixing diode next to each other. The small die to the west of the them is the IF FET.

[Jamie_A]

It turned out to be an YTO issue The level was 10dB too low (around 4dBm). Interestingly the level at the front panel LO output was almost in spec, which threw me off.

After replacing the YIG, I ran through the YTO calibration, and the YTF tracking calibration procedure. Now it's running beautifully.