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Old 23rd January 2016, 05:24 PM   #1
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Default Just picked up an HP 333A got a few questions

As the title says, I found a nice HP 333A locally on Craigslist for $25 and although I already have a QA400 I thought why not at that price. The unit is quite clean and the guy I bought it from told me he picked it up from the lab that he worked at where they were getting rid of a bunch of old equipment. The RMS voltmeter is pretty much spot on and works great, the distortion meter however is not working all that great.

I tested it on one of my Aleph's and the QA400 was showing about .12% distortion under test where the 333A shows about .7% distortion. The unit nulled the signal manually and it seemed that the auto nulling was working fine as well when it was switched in.

I have searched the net a bit and I have the service manual. I noticed that a lot of people are replacing the dated electrolytics. A quick glance in mine shows a lot of 1975 date codes on the caps. I'm going to go through the steps outlined in section 5-40 of the manual for troubleshooting the distortion function. I was just wondering if some of you folks that are quite familiar with the HP equipment have any tips or things to check when the dist. meter is out of line?
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Old 27th January 2016, 11:51 PM   #2
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The question is which one tells the true the QA400 or the HP333A ?

I have a HP333A, a HP334A and a Roland Quad-Capture Sound card that I use with MatLab.

Testing a 1 KHz Sinusoidal signal at 1 VRMS from my Leader LAG-120 is giving me : 0.02% THD from both HPs and the sound card. The LAG-120 THD specification is <0.05%. So it seems consistent.. But..

The least reliable test gear out of the three for me is the sound card. I ended up using MatLab with the sound card to get a better control over the measurements. Looking at the frequency response of the QA400, I recognise my Roland Quad-Capture..

Following the HP alignment guidelines has been beneficial for me. The photoresistor and lamp alignment had a significant impact on my HP333A automatic nulling capacity.

By the way, have you opened the cover to look inside? Isn't it a nice piece of equipment !
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Old 28th January 2016, 04:04 AM   #3
RNMarsh is offline RNMarsh  United States
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It could also be that the HP 333/334 are measuring THD+N and the QA400 is indicating just the THD. The THD+N will always be higher.


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Old 28th January 2016, 04:37 AM   #4
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Quote:
Originally Posted by Chamberman View Post
. . . . I was just wondering if some of you folks that are quite familiar with the HP equipment have any tips or things to check when the dist. meter is out of line?
Although I used that instrument in some prior incarnations, I know nothing about its internal architecture nor implementation. I suggest you start familiarizing yourself with the instrument by reading through the material collected and presented by Dick Moore, e.g. http://moorepage.net/THD.html , http://www.moorepage.net/334.html et seq. Since he is recently deceased (see The passing of a great contributor - richiem-Dick Moore ) I don't know how long this excellent collection of articles will be available.

With few exceptions, H-P always made well-designed, durable instruments. While the DMM they give away at Harbor Freight may be adequate to locate a totally failed component, my (unsubstantiated) gut instinct says that it may require test equipment of comparable quality to the '333 to locate and correct the many small degradations that have accumulated over time and now compromise your '333's performance. But as Richard Marsh ( http://www.diyaudio.com/forums/members/rnmarsh.html ) documented in a series of posts scattered through the "Low-distortion Audio-range Oscillator" thread ( Low-distortion Audio-range Oscillator ), it is sometimes possible to IMPROVE some "outdated" instruments beyond their original performance levels.

The superannuated electrolytic capacitors are certainly candidates for replacement. Even if they still have "factory-new" characteristics, many modern electrolytics will be superior. Whether or not this will make a difference to your '333 is a different question.

Before I took on the task of capacitor replacement, or a full service-manual alignment procedure, I'd use my 'scope (and spectrum analyzer!) to look at the "Residual" signal output - and compare it to the residual signal from your QA400, or other instruments. This can give you a clue as to whether the '333 is being compromised by un-nulled fundamental, distortion products being created within the instrument itself, broadband noise in the electronics, or power supply noise and ripple.

And please keep us posted with what you find!

Dale
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Old 31st January 2016, 04:10 PM   #5
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Okay sorry to take so long to post back. For some reason I received no notification of follow up posts on my thread, that's never happened before.

I've had some time now to play with this 333a some more. In addition to my DIY Alephs I also tried measuring the distortion of my Hafler XL600 amplifier. It is the best measuring amplifier (recently .014% at 312W output with the QA400) in my system and it too measured about .7% distortion on the 333a. I then tried feeding the oscillator output of the QA400 at various intensities of .1V - 1V rms into the 333a and I have approx. 0.7% distortion shown at any given audio frequency from 20 - 20khz. So it certainly appears that the 333a is the source of the issue.

I then started on the distortion function troubleshooting section of the service manual which led me to the wein bridge checks. A couple of the resistances were off for the wein bridge by about 30% so I spent an evening getting that adjusted back to no more than about 15% at the highest. Boy that was fun! I ended up turning the lights out in the shop and wearing my headlamp with red LED illumination to perform that setup. However even after going through that the distortion readings remain the same.

I decided to check the RMS meter calibration to see if that was working properly and aside from a small adjustment that was needed at the 400Hz setting it measures RMS volts quite well all the way out to 2.2 MHz, which is the limit of my el-cheapo benchtop oscillator. I had a to use my scope to match output levels from the benchtop oscillator to the 333a at the various measured frequencies.

Just to quell any questions nagging at me from the back of my mind about the power supply. I checked the +/- 25vdc outputs with the scope. The +25v seemed a good bit noisier than the -25v. I pulled the 40uF cap that was right after the +25v regulator and it was measuring bad at < 1uF, where the -25vdc cap was still measuring about 41uF. I changed both of them out with a pair of new 47uF axials that I had in my spare parts bin. Noise on both rails seemed now equal and low. I also went ahead and changed out the 200uF/50V caps right after the bridge rectifier with some 1000uF/80V caps. Unfortunately this had no effect on the distortion measurement, still ~ 0.7% displayed.

One thing I noticed yesterday about the time I needed to walk away from the 333a for the day was that the auto tuning is not working at low frequencies. I tried switching the auto tuning in at 400hz and it quickly mistunes. At 1khz and above the auto tuning holds the fundamental fine. The bad thing is that I'm not sure if this was present before I began my adjustments or not because I only tried auto tuning at 1khz and above in the early stages.

In skimming through most of the cal procedures it looks like I will have to be creative for some of them with what I have available to me here at the house. In addition to the aforementioned QA400 my current benchtop test equipment includes a Hitachi V-1060 100MHZ scope, Uni-T 61E and Fluke 77III DMM's, Beckman FG2 2 mhz oscillator and a BK precision 878 LCR meter. The FG2 oscillator is anything but low noise, so I have to rely on the output of the QA400 either directly or fed through an amplifier for my distortion measurement purposes.

DAsutton - You are correct this HP is a very nice piece of equipment, build quality on the interior matches the exterior. It looks like an old rugged piece of ex-military gear. Anyway, Now if I can just get the distortion measurement to look as good as the aesthetics I'll really be happy.
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Last edited by Chamberman; 31st January 2016 at 04:29 PM.
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Old 3rd February 2016, 01:37 AM   #6
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I ran a check on the 333a tonight with the QA400 oscillator connected to it. I attached the picture of the residual output to the QA400 from the 333a. The left channel is of course the residual and the right channel is showing the QA400 in loopback.

Nasty yes?
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File Type: jpg 333A_residual.jpg (157.5 KB, 147 views)
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Old 3rd February 2016, 02:55 AM   #7
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By any chance, do most of those spectral lines fall at odd multiples of the power line frequency?

Dale
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Old 3rd February 2016, 03:25 AM   #8
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Originally Posted by dchisholm View Post
By any chance, do most of those spectral lines fall at odd multiples of the power line frequency?

Dale
Dale,

Just a quick glance at it looks like yes they do fall on the odd power line multiples.

I ran through the wein bridge balance procedure again which brought the fundamental down a good bit now after tuning. Picture attached.
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File Type: jpg 333a_residual_C3_a.jpg (158.2 KB, 145 views)
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Old 4th February 2016, 03:26 AM   #9
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Hi Chamberman,

I am away from home during the week. I can't make any measurements right now that we could use for comparison.

This weekend, I will try to reproduce the residual output measurement that you made. I will post it for comparison purpose.

I looked for the THD specification of the Hafler XL600. From what I found, I think that it is more likely to measure around 0.08% THD or a little bit better from this amplifier.
Since that the XL600 is of a certain age, I guess that the 333A measurement would match with that kind of performance.

Measuring 0.014% with the QA400 from the XL600 is questionnable for me. I am not sure if it is realistic and what it represents. It seems far out. At a point that I don't know if it is a valuable measurement.
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Old 5th February 2016, 02:53 AM   #10
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DAsutton,

That would be great if you could post your residual measurements. The .014% reading on my XL-600 was at 312 watts out, at 200 watts it was .011%.

On a good note I made a little bit of headway tonight. I decided to go ahead and replace ALL of the power supply capacitors. The residual cleaned up quite a bit. The two pictures posted are just before and then right after the PS capacitor replacement.

Also the low frequency 400hz auto mistuning issue I spoke of earlier cleared up with the second wein bridge balance procedure that I performed the other evening.
Attached Images
File Type: jpg 333a_1.jpg (139.9 KB, 119 views)
File Type: jpg 333a_pwr_sup_caps_repl.jpg (133.6 KB, 117 views)
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