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THD measurement on a budget - DIY HP 331A repair & restore

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Well done. :)

The HP-331A is a classic unit, absolutely beautifully built, and a testament to US manufacturing at the time. They don't build em' like they used to.
 
Really nice repair and calibration. They really don't make anything like this anymore. HP and Tektronix TOTL test equipment. Have a pair of Tek 475 scopes to check out.
 
Great video! I didn't notice you nulling out the fundamental using the frequency and balance controls. Did you edit that out?
 
JRUBIN said:
If you are not taking measurement professionally , there is an option which I took to simply purchase a less than fully functional 331A for a song and repair it, this movie documents these events.


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Modern computer sound cards have lower residual distortion than HP331A. If used with specialized software that can be purchased for few hundred dollars, they can be employed for all audio frequencies (below 40kHz) measurements at levels reaching -150dB.
 
for_p1 said:
Modern computer sound cards have lower residual distortion than HP331A. If used with specialized software that can be purchased for few hundred dollars, they can be employed for all audio frequencies (below 40kHz) measurements at levels reaching -150dB.
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Sure they can, but that's not what the thread is about is it? This is about a classic piece of test gear, picked up for peanuts, that will continue working long after any computer based solution dies.

Most real techs have all types of gear, computer based, old hand-me-downs, classic pieces, things they've built themselves, and ones they spent a heap of money on- I know I have.

The HP-331A is simply a military grade built, classic piece, and worthy of a place on any decent tech's bench, even if just used as a reliable backup. No, I haven't got one, I've got another old classic- the Leader LDM-171. I love that old girl. She never lets me down.
 
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restorer-john said:
Sure they can, but that's not what the thread is about is it? This is about a classic piece of test gear, picked up for peanuts, that will continue working long after any computer based solution dies.
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It will continue working, but job can not be done with it. What is the purpose then? Everything has the end of its useful life.
 
There are modern solutions that render vintage test gear obsolete, but just like vintage audio can be fun, and vintage cars can be fun, and vintage motorcycles can be fun, and vintage wine can be fun, and pretty much vintage anything can be fun, vintage test gear is fun.
 
I was just bidding on a defective 331A the other day from the Dutch CL-alike marketplace and I just got an email I have it for 30 euros. I even did not ask what is wrong and just take my chances. Fortunately for all this old stuff one can find the manuals on the web.

I will watch that video for sure.
 
Have one of those on the bench, like me the various controls need an occasional extra tweak or two to perform properly but I sure like it, in many ways easier to use than my big ST distortion analyser.
-Lee
 
Is it me or does anyone find it a bit ironic that someone feels the need to inject hot air into this thread with the 'why bother' attitude in a forum such as this, probably having been passed off an Ipod as a sufficient application for audio a thousand times over and told there is no need for anything else as technology has surpassed the need for what they might possess from 2 or more years ago? Im reading this and Im thinking to myself..... Wow, If you don't fancy the 331A, close the thread, and move along, because while an argument can be made as to the usefulness of the 331a given the price point and restoration of the unit, i don't feel as I should have to now take the time to defend it just because a foolish remark is made by someone with nothing better to do.
 
JRUBIN said:
Is it me or does anyone find it a bit ironic that someone feels the need to inject hot air into this thread with the 'why bother' attitude in a forum such as this, probably having been passed off an Ipod as a sufficient application for audio a thousand times over and told there is no need for anything else as technology has surpassed the need for what they might possess from 2 or more years ago? Im reading this and Im thinking to myself..... Wow, If you don't fancy the 331A, close the thread, and move along, because while an argument can be made as to the usefulness of the 331a given the price point and restoration of the unit, i don't feel as I should have to now take the time to defend it just because a foolish remark is made by someone with nothing better to do.
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Just ignore the trolls. It was an excellent tutorial that will no doubt help the folks that don't have the cash or can't justify the expense of a modern distortion analyzer. I used a 334 for years until I got my Keithley 2015. It did the job just fine.
 
dr*audio said:
Just ignore the trolls. It was an excellent tutorial that will no doubt help the folks that don't have the cash or can't justify the expense of a modern distortion analyzer. I used a 334 for years until I got my Keithley 2015. It did the job just fine.
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I posted in another forum that im restoring a Heathkit IM-12, you wont believe how many trolls that stirred up.... Its like repairing a Heathkit to use to measure THD is considered a personal attack on peoples family or something.
 
If we do something because we like it and we do not expect a traditional reward it is supposed to have non physical benefits . Tacking on more negativity just to be noticed isn't one of them. But everyone is different ....and come from different places.

I just wanted to know the circuit differences in early HP Distortion analyzers in the 33x series.
 
Davie O'brien from Mcintosh Clinic fame was capable of consistently measuring distortions down to .o2 % using a modified version of the HP 202 Oscillator. But there was a particular tube in the 202 Davie had to keep and eye on he told me once. So he checked the generator distortion directly to the HP analyzer in the morning before the clincs started and then again in the after noon after a lunch break. But he was very happy when the Sound Technology units came along because he didn't have to deal with problem tubes in his HP test equipment. He also used a HP VTVM voltmeter. I don't remember the number or the type right at the moment. I do know it gave true RMS values in the AC mode, and I know it was accurate over the audio spectrum and as high as 100 KHZ. He also had some precision potentiometers with 10 revolutions each so he could tell quickly when one channel of an amp was out of balance left to right to left or across the spectrum from 20 to 20,000 HZ. He also had a channel selection box when checking stereo amps. He made his own load resistors at first. Then companies started to make ones that would handle up to 500 watts and finally over 1000 watts rms. Remember you need some way of setting the AC voltage to the unit being tested. As all units are not measured with the same AC voltage. MOST of the time he made the tests with the volume controls out of the circuit so he would have to deal with potentiometer mis tracking.
 
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