HP 339A Nonfunctional Auto-Nulling

[busaboy]

Pulled out my HP 339A after about a decade of non-use. The notch will not auto null. Oscillator output is connected directly to the analyzer input. I've gone through the various notch circuits using an an O'scope. Unfortunately the manual is a bit lacking in what signals I should be looking for. Can't seem to find anything obvious wrong.

A few clues:

1. Frequency LEDs indicate a low frequency.

2. At 1KHz, the oscillator is putting out 950 Hz.I don't know if the notch will null with 50Hz error.

3. If I adjust the Frequency Vernier to max, I can get a partial null. 20% distortion.

 

[JonSnell Electronic]

You will have your time cut out, replacing all of the electrolytics! ALWAYS use exactly the same value components, after all, the designer of the 339 knew a bit more about it than the average DIYer.

 

[busaboy]

You will have your time cut out, replacing all of the electrolytics! ALWAYS use exactly the same value components, after all, the designer of the 339 knew a bit more about it than the average DIYer.

I will look at them. Hopefully there will be some physical evidence. I have a very large lab of test equipment so replacing capacitors is nothing new.

Do you have a reason to make the statement? I've replaced a ton of capacitors based on physical evidence like leakage or being shorted. Any past experience with this HP model that would point to capacitors? I have a lot of HP gear older than this without cap issues.

 

[Chamberman]

If its been sitting for that long unused then I'd suspect the rotary switches might need cleaning. I bought a 339A used a few years back and it had some weird issues. Those issues disappeared after I cleaned the rotary switches.

 

[Shinja]

CdS optocoupler may be NG. I once repaired old panasonic gear replacing optocoupler. Identical parts are hardly obtainable today. I tried coolaudio VTL5C3 but it make notch tunable range and distortion worse. So I took good optocoupler from oscillator section and put the VTL5C3 there.