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Old 19th February 2014, 06:47 AM   #1
guy75 is offline guy75  Italy
diyAudio Member
Join Date: Jul 2008
Default HP334a distortion analyzer problem.

Hi, I have an HP 334a with some malfuncitions:

In voltmeter mode there is a little bit of jitter meter and into 300uV setting the meter read 100uV without input.

In distortion mode the automatic nulling doesn't work anymore and the read distortion is high (1.5% from PC soundcard).

Thanks in advance for the advices.

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Old 20th February 2014, 01:10 AM   #2
TerrySt is offline TerrySt  United States
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Join Date: Jul 2011
Location: Colorado Springs, Colorado
I don't know if I can be much help. I do have a HP 334A although it is a different option that has all push buttons instead of the front panel dials yours has. But I think it operates the same otherwise. I don't use it very often since I also have a newer HP 339A.
On the voltmeter, if I select the 0.0003V range, with no input mine reads almost full scale (290 uV), but that is with the input open. With the input terminals shorted, it reads about 15 uV. With a 600 Ohm metal film resistor across the input terminals, it also reads about 15 uV.
The automatic nulling problem may require some troubleshooting. I have never had to work on mine, so I am not very familiar with it. But I understand it uses a pair of incandescent lamps to null the bridge that is part of the notch filter. So maybe a lamp has failed. The service manuals for most of the old HP equipment is available on the Agilent web site. The manual can be downloaded here:

The HP manuals are very good with block diagrams and troubleshooting guides.

The jitter on the voltmeter you see makes me think that maybe a filter capacitor has failed on one of the power supplies. That is pretty common in such old equipment. So a good place to start would be to measure each of the power supplies and also check them for excessive ripple.

Let me know if I can be of any further help. Good luck with fixing it.

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Old 20th February 2014, 06:34 AM   #3
guy75 is offline guy75  Italy
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Join Date: Jul 2008
Thank you very much Terry for your fast response, I'll check the lamps in the weekend.

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Old 22nd February 2014, 10:33 PM   #4
jblmar is offline jblmar  United States
diyAudio Member
Join Date: Dec 2005
Location: Brooklyn, New York
LDRs are the usual suspects.
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