Author Topic: My HP 331A distortion analyzer thread  (Read 21168 times)

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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #25 on: December 10, 2015, 12:43:56 am »
C4 tested good.   Both in value and leakdown DC testing



C14 on the other hand...... Not so much...


As a good candidate for reformation, and because I dont have a 500uf Cap laying around.   Ive decided to give reforming an attempt.
When starting, the leakdown was less than 3 volts, couldnt measure the MA



Now its good at 10volts and climbing
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Offline AF6LJ

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Re: My HP 331A distortion analyzer thread
« Reply #26 on: December 10, 2015, 05:37:42 pm »
I meant to say C-14 since C-4 wasn't even part of the discussion. :)
Best of luck reforming that cap.
I think I would write myself a note to order one next time I put in an order for parts.
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Offline SeanB

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Re: My HP 331A distortion analyzer thread
« Reply #27 on: December 10, 2015, 06:29:31 pm »
500uF 17VDC is easy to replace, I have some 470uF 50V radials the same size that would fit.  A modern 470uF 35V one would even fit in the can if you wanted it to look original.
 

Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #28 on: December 10, 2015, 08:16:24 pm »
Im still getting several microamps of leakage at 15Volts.    A noted improvement but still not where I want to be.   I know from the IT11, which was
just calibrated that it is more than 2ua and less than 10ua of leakage.

Ill put one down for my next order, but will try the reformed cap in circuit this evening to see if there was any change (pointing towards the capacitor)
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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #29 on: December 11, 2015, 01:45:13 am »
The culprit has been found.   After installing C14 after reconditioning Id noticed the problem persisted.
It was during some testing that id noticed That the input to Q1 was the same voltage as both sides of C5.
This got me thinking and I decided to do a resistance test over C5, knowing there was no path around both ends I saw
that it read 54OHMS.





Ahhhh, C5 is toast.  Lucky for me I had bought some extras for the power supply of the same value (much higher rating) and threw it in.
Its no surprise, this unit used many large value capacitors =, and to keep the size within reason, they used ones that were just barely rated above the
operating voltage.  Im sure Ill be ordering a lot more caps before this is through.  However, I celebrate a small victory




With the repairs made, the bias was now able to be set correctly.
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Offline Vgkid

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Re: My HP 331A distortion analyzer thread
« Reply #30 on: December 11, 2015, 01:55:03 am »
Funny thing about the capacitor sizing. I have seen where manufacturers will use a 25v part in a 22v power rail, or a 120v rated cap in  opamps 15v power rail.
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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #31 on: December 11, 2015, 02:37:32 am »
Onward and forward.  I suspect Ill be doing this again once I replace all of the bad components, so this calibration will be a test run





I dont have any voltmeter calibrators laying around.  I Imagine using a function generator at 400hz to produce the voltage would be OK.  Any thoughts on this?
Id have to generate a waveform Peak to Peak at .00283 volts for 1mv RMS and .001 for .3mv RMS



« Last Edit: December 11, 2015, 02:48:32 am by jcrubin »
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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #32 on: December 11, 2015, 10:35:47 pm »
I used the fluke to do a ballpark sanity check against my oscilloscope, as well as something to load down the circuit, which it did.
It sits at the end of its measuring capibility and doesnt provide further resolution.   .003 setting is as low as I can test



The oscilloscope provides a bit more resolution, getting me to the hundredth of a millivolt.  The results look good, I can go no further, nor do i need to.
Im calling this good and moving on......
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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #33 on: December 11, 2015, 10:44:48 pm »
Im starting at step F, for obvious reasons


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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #34 on: December 12, 2015, 01:43:42 am »
this is my 400hz reference, its actually between the last dash and 1






2mhz after adjustment, is actually in the same position as the first picture, tough with the camera






3mhz is an issue, I cant balance out 3mhz with the capacitor C21 after readjusting R39

















The 200KHZ calibration was off but easily adjusted with no issues.   200KHZ is a lot more important to me than 3MHZ


400hz




200KHZ after calibration

« Last Edit: December 12, 2015, 02:20:56 am by jcrubin »
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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #35 on: December 12, 2015, 03:46:10 am »
The problems mentioned above are no longer an issue, i forgot to check the output voltage changes between 400khz 2mhz and 3mhz from my function generator.
Using the same reference voltage, the calibration was possible.  Also an adjustment to my oscilloscope made it possible to further calibrate the 400hz reference even better than before
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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #36 on: December 12, 2015, 03:58:46 pm »

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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #37 on: December 13, 2015, 02:52:29 am »
This was the toughest of all calibrations so far.  But it worked out perfect after about 2 hours of work and some math.
Everything was spot on, didnt make any changes


Using some voltage dropping resistors and adding those values into the equation I was able to get my function generator to operate in the parameters of the test
RMS voltage measured at 331a with oscilloscope

1: Using a 10meg resistor between the frequency generator and the 331 I found that to get the desired position on setting 5:
2: POSITION X [SETTING 5](43.2mVRMS @ 10MOHM) = -27.290325 DbV ------> +10db = -17.290325 DbV
3: -17.290325 DbV @ 10MOHM = 136.6mVRMS
4: More selector switch to SETTING 4 ------> Increase voltage till oscilloscope reads 136.6mVRMS, adjust capacitor so needle is at position X from setting 5

So basically, needle position for SETTING 5 @ 43.2VRMS with 10MOHMS = needle position for SETTING 4 @ 136.6VRMS with 10OHMS


The following resistors were used, so the equation needs to be recalculated each time

0db ---> +10db         10MOHM          SETTING 5 to SETTING 4
10db --> +10db         2.7MOHM        SETTING 4 to SETTING 3
20db --> +10db         330KOHM        SETTING 3 to SETTING 2
30db --> +10db           0 OHMS         SETTING 2 to SETTING 1






10DB sweep shown on the meter between two settings:

-10db on setting 4




to 0db on setting 3



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Offline AF6LJ

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Re: My HP 331A distortion analyzer thread
« Reply #38 on: December 13, 2015, 03:45:16 am »
Very Cool. :)
 :-+ :-+
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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #39 on: December 13, 2015, 02:31:30 pm »
Next up




FIG 5-3
« Last Edit: December 13, 2015, 02:41:08 pm by jcrubin »
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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #40 on: December 13, 2015, 10:14:26 pm »
Bridge balance was done today, The knob is slightly off but not worth adjusting

Set up the input at 2dB



Note that this is not a distortion test, because the reference is at 2dB and not 100%
The lowest expectable value from my test set read .0005, Im good with that

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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #41 on: December 13, 2015, 10:18:49 pm »
The final calibration, distortion adjustment



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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #42 on: December 13, 2015, 10:41:14 pm »
Apparently this procedure has been removed from later models, including mine. There is only one image which shows R30.
It is not found on my board




Sure enough.......






This officially ends calibration.........
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Offline fivefish

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Re: My HP 331A distortion analyzer thread
« Reply #43 on: December 13, 2015, 10:45:28 pm »
Hey, on your output jack, does it only show 0.1Vrms for a full scale 1Vrms reading? 
 

Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #44 on: December 14, 2015, 01:18:46 am »
Hey, on your output jack, does it only show 0.1Vrms for a full scale 1Vrms reading?

Yes, you are correct....




As observed.....

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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #45 on: December 15, 2015, 03:29:48 am »
Demonstration of the 331A using test reference

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Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #46 on: December 18, 2015, 02:00:14 am »
Practical demonstration of operation using an oscilloscope in conjunction with the 331A
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Offline AF6LJ

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Re: My HP 331A distortion analyzer thread
« Reply #47 on: December 20, 2015, 11:15:28 pm »
Looks Good
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Offline GregDunn

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Re: My HP 331A distortion analyzer thread
« Reply #48 on: May 22, 2018, 03:45:37 pm »
Sorry to reawaken this old thread, but I just wanted to thank the OP for the detailed repair info.  I have a 331A which is beginning to age and needs work, so I will be referring back to this as I crack it open and resolve the issues.  I have the service manual, but need to check my S/N to be sure I'm using the right parts list / schematic references.

I picked my unit up at a hamfest back in the 1980s for about $30.  At the time, it was in good condition except that some previous owner had removed the balance control + knobs and I had to fix that before using it.  Rather than delaying my checkout of the unit by trying to track down the original parts, I substituted a high precision 10-turn pot and vernier dial which works fine.  Now I expect I'll need to open it up for the second time and start replacing caps!
 

Offline jcrubinTopic starter

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Re: My HP 331A distortion analyzer thread
« Reply #49 on: May 24, 2018, 02:01:50 am »
Hello,

OP still here doing all sorts of restorations.   I've also done a few Heathkit THD analyzers since then and conducted a shootout against the 331A.   The HP won.

Glad the video was helpful to you.

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