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HP 333A Analyzer


John Warren

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The first photo is a unit I picked up at an MIT flea market back in the late 90s with plans to repair it.  It's in good physical condition and sort of works.  I have a "donor" unit also (second photo) thus prompting my renewed interest.  

 

They're not worth saving unless, of course, you're interested in repair for the pleasure of it.

 

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With time permitting, I'll share some of my findings here.  

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  • 4 years later...

Did a complete overhaul on this 333A.  New Al- and Ta-electrolytics, many new transistors.  The goal was to achieve like new performance.  I have many of the HP special BJTs including the Germaniums from this era.  

 

The 333A and 334A were popular distortion analyzers from 1968 to about 1980.  They're actually very useful for measuring THD+N in tube amps but good luck trying to find one in good working order.  They're straightforward to rework but getting the best out of them isn't so simple.  

 

The key to getting these units to operating "like new" is the voltmeter circuit, it must measure accurately to the micro-Volts level and that's not so straightforward.    

 

How does the analyzer work?  A signal is applied to an amplifier to be tested for distortion, say a 1000Hz sine wave signal (the fundamental).  The amplified signal is then sent to the analyzer where the operator "tunes" a Wien bridge to "notch out" the fundamental leaving the harmonics and noise for measurement by the meter circuit.  The accuracy of the analyzer is governed by the accuracy of the voltmeter and the accuracy of the meter circuit over the bandwidth of the instrument (5Hz to 600kHz).

 

Below is the signal source, a 100uV, 400Hz sinewave signal which requires specialized instruments to source.

 

100uV_out.jpg.c3107d4cf6868fbeed1263041e3e901a.jpg

 

And the measurement of the 100uV sinewave at the 333A is below.  The meter range knob (lower right) is set at .0003 which is 300uV or about 1/3 of a millivolt at full scale.  The magnitude of the voltage is read at the 0-3 graduated scale.  Note the pointer is at 1 (= 100uV).   

 

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The 333A and 334A units are useful to about 0.03% THD+N which is about 30X higher than current state of the art units, like the Krohn-Hite shown above which is capable of 0.001% THD+N.

 

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I have a 331A, which I think is the same unit but without the mode switch or high pass filter.

It is in good working order as it was acquired from a retired audio engineer.

 

I have a HP 654A test oscillator as well.

While it does have an issue, (dodgy range switch?) if you let the unit sit powered up for a few hours it will work fine and puts out some beautiful sine waves.

I'm currently trying to revive a HP model 205AG audio signal generator, which is an older Navy unit.

 

Like you mention, the older HP distortion analyzers are good for use with tube amplifiers.

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Here the analyzer is measuring a Crown D-75.  Input is a balanced, 10kHz, 0.5Vrms sinewave from an ultra-low distortion source (<0.001%).  Output from the amplifier is measured across an 8 Ohm power resistor.  The gain is set to realize 10Vrms across the resistor.  The amp stewed for about 30minutes before measurement.  Testing at 10kHz will drive the amp to get very warm to the touch. 

 

The 333A is reporting 0.042% THD+N (0.1% FS, pointer a bit to the right of 0.04%).  

crown_residuals_1.jpg.dbbe68b53f452316decb9e69797968f0.jpg

 

The signal is simultaneously fed to the Krohn-Hite 6900B analyzer that has a current calibration sticker (performed at Krohn-Hite, NIST traceable).   The 6900B is reporting 0.046%THD+N indicating the 333A is a bit off from the actual magnitude.  

 

crown_residuals_2.jpg.66def9367a3e6d013610d1863403f46f.jpg

 

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Both the 333A and the Krohn-Hite output the residuals of the signal being measured.  The output of each is shown below taken between 500Hz to 60kHz, the bottom plot is the 333A.  First thing to note is that they're similar, a good thing.  The fundamental is 10kHz, attenuated to -60dBV by each instrument leaving the harmonics and whatever else is left to measure.  At this testing extreme, the Crown D-75 exhibits some instabilities at 20kHz, the second harmonic.  The amp gets hot as hell testing at high frequencies and it's not a mystery that thermal distress will show up in the residual spectrum. 

 

crown_residuals.thumb.jpg.c33245d78bb7d641e85605d60b20a12b.jpg

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22 hours ago, EpicKlipschFan said:

looks brand new 

Have the original HP shipping carton, plastic wrapping and service manual too.  It just didn't work.  Does now.   

 

My ULD signal generator shown above developed a problem, sent it back to SRS for repair yesterday, it's still under warranty.   I have another ULD unit but the SRS is insanely low distortion.  

 

 

 

 

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  • 1 year later...

I could use a bit of advice regarding a 333A which a local resident has offered to sell me at a fairly reasonable price which is still under negotiations. It is in rather good shape although the Frequency Range rotary switch may need a cleaning as would the Meter Range Set level. I gather a high quality cleaner would do the trick? The 333A is functional, rail voltages are fine,but has not been calibrated for some time. I do not have access at the moment (specialized instruments) to produce 100uV, 400Hz sinewave signal nor any other high level calibration instruments, but may have acess in the near future. Having done a complete overhaul (J) which looks phenomenal and working as new would anyone have any suggestions for an immediate attempt to bring this unit, if purchased, up to some form of of reasonable performance? I will mainly be utilizing it for measuring THD+N in tube amps/guitar and other forms of experimentation along these lines. Thank you! 

Edited by M-HZ
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