My HP 331A distortion analyzer thread - Page 1
Forum section: electronics / testgear

author: jcrubin
author_rank: Frequent Contributor
timestamp: November 26, 2015, 08:16:16 pm
content: This thread is to chronicle my HP 331A repairs. So far, in the inspection stage no power yet applied.Found 2 bad solder connections on the power supply board and one of which was a sad C4 capacitor.Both Filter caps measured about 200uf and had an ESR of 0.22OHMS. I still think they are ready to go.The mini documentary movie - added here after completionThe thread starts here.....Power supply boardBad solder jointBurned capacitor C4Serial

author: jcrubin
author_rank: Frequent Contributor
timestamp: November 26, 2015, 08:38:53 pm
content: First thing is to check the component chagelog in appendix C to see what steps have been done. Mine include steps 9 through 18Mine might be exempt from Step 9 as well

author: jcrubin
author_rank: Frequent Contributor
timestamp: November 27, 2015, 02:14:31 am
content: Many of the single pin connectors to the board are loose, a light crimp should fix them. Not too worried about the capacitors on the other boards.Though they will be tested as well, Ive pre-ordered replacements for all electrolytic on the main power boardOn the ESR tester

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 08, 2015, 02:58:39 am
content: Capacitors replaced, power supply is goodThe following photos are before any calibration, and just testing the operation and controls of the unit.Coax to unshielded alligator clips, no 600ohm terminating resistorTHD = .21% at 1KHZ 1VTHD = .205% at 5KHZ 1VTHD = .215% at 10KHZ 1VTHD =.315% at 20KHZ 1VTHD =1.2% at 40KHZ 1VTHD =2.2% at 50KHZ 1V using x10K * 5THD =2.2% at 50KHZ 1V using x1K * 50THD = 3.6% at 60KHZ

author: radhaz
author_rank: Regular Contributor
timestamp: December 08, 2015, 05:01:18 am
content: Thanks for posting this. I have a 331 as well, but I bought a Panasonic VP-7723A audio analyzer shortly after that, and the 331 is on the shelf.The 331 is very easy to use, and I love the feel of the switches and the tuning capacitor.What are your plans for using the 331? checking audio amplifiers?

author: oldway
author_rank: Super Contributor
timestamp: December 08, 2015, 12:54:11 pm
content: I also have a 331 and I like it.It is great as most of HP test gears of those times.First thing I have done is to change the ac power connector by a standart one.For more precision, I measure output voltage with a true rms multimeter because analogic display of 331 is NOT true rms.It is usefull to check tube audio amplifiers (adjust of bias for instance) and sine wave oscillators.

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 08, 2015, 02:00:51 pm
content: Quote from: radhaz on December 08, 2015, 05:01:18 amHere is a collection of videos of my current project. I generally work on Dyna equipmenthttps://www.youtube.com/playlist?list=PLK_zQ6_j4TF13qvrS3i2fw-p9trdjUhVm

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 08, 2015, 02:07:31 pm
content: Quote from: oldway on December 08, 2015, 12:54:11 pmInteresting... If its not true RMS what is it exactly?

author: oldway
author_rank: Super Contributor
timestamp: December 08, 2015, 02:20:14 pm
content: Like multimeters, it measure the average value of the rectified ac voltage.

author: dom0
author_rank: Super Contributor
timestamp: December 08, 2015, 02:56:53 pm
content: Only the later versions (339A for example) have a RMS converter.I think these have a residual / monitor / filter (whatever it is called on the unit) output, so you can connect an external RMS meter (e.g. 3400A, DSO, DMM) anyway.

author: AF6LJ
author_rank: Unknown
timestamp: December 08, 2015, 03:35:29 pm
content: Good Thread Thanks for starting it.

author: oldway
author_rank: Super Contributor
timestamp: December 08, 2015, 09:14:44 pm
content: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1966-04.pdf

author: TimFox
author_rank: Super Contributor
timestamp: December 08, 2015, 11:02:13 pm
content: I relied on a 334 for many years, until I obtained a 339.The 333 and 334 have an extremely useful auto-balance function, which servos the fine frequency and fine phase controls to achieve null.The 339 is great: a very low distortion generator built in, and the analyzer automatically tunes to that frequency. Most importantly, if the signal level changes within the selected range, the percent distortion function automatically re-normalizes and gives the distortion. With the 331 to 334, if you are adjusting for minimum distortion you need to re-check the input level (manually adjusting for 100%).

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 09, 2015, 02:55:09 am
content: Voltage portion of the calibration this evening........-25V rail +/- .25v violet+25V rail +/- 1v red (adjusted by negative rail)-11.3 +/- .1v A2TP1 Problem here.... adjusting pot A2R3 has no effect. Pot tested good on resistance+8.2 +/- .5v A2TP2+2.65 +/- .2v A3TP1-20.2 +/- .3v A3TP2The next board to work on......

author: Vgkid
author_rank: Super Contributor
timestamp: December 09, 2015, 03:03:55 am
content: It is impressive that those rails are still in spec. Though something is amiss with the -11v rail.

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 09, 2015, 03:22:57 am
content: They weren't... that was post cal. They were off, out of spec, but nothing terrible

author: AF6LJ
author_rank: Unknown
timestamp: December 09, 2015, 03:34:17 am
content: It will be interesting to see what is wrong with the -11V source.

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 09, 2015, 03:39:34 am
content: See my post again.... Its probably C4 or C14 or Q1-4My guess is C4 or Q1

author: AF6LJ
author_rank: Unknown
timestamp: December 09, 2015, 03:50:09 am
content: Quote from: jcrubin on December 09, 2015, 03:39:34 amI am thinking C-4 leaky or shorted.

author: AF6LJ
author_rank: Unknown
timestamp: December 09, 2015, 03:26:04 pm
content: Quote from: jcrubin on December 09, 2015, 02:08:41 pmAs I remember the asterisk means the transistors are selected for specific performance properties.

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 09, 2015, 10:00:14 pm
content: lets hope its a capacitor.That being said. And granted I only work on tube equipment, there must be a transistor that meets or exceeds the specification of the ones in the device

author: dom0
author_rank: Super Contributor
timestamp: December 09, 2015, 10:06:56 pm
content: Most certainly. But sometimes they are not as easy to come by, for example HF Si-PNPs were common when Germanium transistors faded... but today they aren't really common anymore.

author: fivefish
author_rank: Frequent Contributor
timestamp: December 09, 2015, 10:35:33 pm
content: Thanks for posting. I'll be following this thread... also have a 331A, sitting unused at the moment.Got the 331A but then found a deal on a 8903B so the 331A never got turned on beyond checking that it's working.But would like to at least maintain and calibrate my 331A.

author: AF6LJ
author_rank: Unknown
timestamp: December 10, 2015, 12:20:15 am
content: Quote from: jcrubin on December 09, 2015, 10:00:14 pmI am confident one could be found.A friend I know on the radio has an old Collins 75-A4 receiver from about 1954 or so. He had overhauled the radio back about 30 years ago replacing all the parts that are known for failure in said receiver. The other day he was using it and had to change to another set up because the receiver started developing problems.The trouble was a ceramic disc cap that had gone open. He was reminded by a friend that "there are no good 60 year old caps".While the distortion analyzer isn't that old... (yet) My experience tends to lead me to the passives before considering transistors when the problem is not obvious, but marginal like this one....

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 10, 2015, 12:43:56 am
content: C4 tested good. Both in value and leakdown DC testingC14 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 MANow its good at 10volts and climbing

author: AF6LJ
author_rank: Unknown
timestamp: December 10, 2015, 05:37:42 pm
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: December 10, 2015, 06:29:31 pm
content: 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.

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 10, 2015, 08:16:24 pm
content: 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 wasjust 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)

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 11, 2015, 01:45:13 am
content: 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 sawthat 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 theoperating voltage. Im sure Ill be ordering a lot more caps before this is through. However, I celebrate a small victoryWith the repairs made, the bias was now able to be set correctly.

author: Vgkid
author_rank: Super Contributor
timestamp: December 11, 2015, 01:55:03 am
content: 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.

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 11, 2015, 02:37:32 am
content: 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 runI 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

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 11, 2015, 10:35:47 pm
content: 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 testThe 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......

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 11, 2015, 10:44:48 pm
content: Im starting at step F, for obvious reasons

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 12, 2015, 01:43:42 am
content: this is my 400hz reference, its actually between the last dash and 12mhz after adjustment, is actually in the same position as the first picture, tough with the camera3mhz is an issue, I cant balance out 3mhz with the capacitor C21 after readjusting R39The 200KHZ calibration was off but easily adjusted with no issues. 200KHZ is a lot more important to me than 3MHZ400hz200KHZ after calibration

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 12, 2015, 03:46:10 am
content: 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

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 13, 2015, 02:52:29 am
content: 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 changesUsing 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 testRMS voltage measured at 331a with oscilloscope1: 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 DbV3: -17.290325 DbV @ 10MOHM = 136.6mVRMS4: More selector switch to SETTING 4 ------> Increase voltage till oscilloscope reads 136.6mVRMS, adjust capacitor so needle is at position X from setting 5So basically, needle position for SETTING 5 @ 43.2VRMS with 10MOHMS = needle position for SETTING 4 @ 136.6VRMS with 10OHMSThe following resistors were used, so the equation needs to be recalculated each time0db ---> +10db 10MOHM SETTING 5 to SETTING 410db --> +10db 2.7MOHM SETTING 4 to SETTING 320db --> +10db 330KOHM SETTING 3 to SETTING 230db --> +10db 0 OHMS SETTING 2 to SETTING 110DB sweep shown on the meter between two settings:-10db on setting 4to 0db on setting 3

author: AF6LJ
author_rank: Unknown
timestamp: December 13, 2015, 03:45:16 am
content: Very Cool.

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 13, 2015, 02:31:30 pm
content: Next upFIG 5-3

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 13, 2015, 10:14:26 pm
content: Bridge balance was done today, The knob is slightly off but not worth adjustingSet up the input at 2dBNote 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

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 13, 2015, 10:18:49 pm
content: The final calibration, distortion adjustment

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 13, 2015, 10:41:14 pm
content: 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 boardSure enough.......This officially ends calibration.........

author: fivefish
author_rank: Frequent Contributor
timestamp: December 13, 2015, 10:45:28 pm
content: Hey, on your output jack, does it only show 0.1Vrms for a full scale 1Vrms reading?

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 14, 2015, 01:18:46 am
content: Quote from: fivefish on December 13, 2015, 10:45:28 pmYes, you are correct....As observed.....

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 15, 2015, 03:29:48 am
content: Demonstration of the 331A using test reference

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 18, 2015, 02:00:14 am
content: Practical demonstration of operation using an oscilloscope in conjunction with the 331A

author: AF6LJ
author_rank: Unknown
timestamp: December 20, 2015, 11:15:28 pm
content: Looks Good

author: GregDunn
author_rank: Frequent Contributor
timestamp: May 22, 2018, 03:45:37 pm
content: 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!

author: jcrubin
author_rank: Frequent Contributor
timestamp: May 24, 2018, 02:01:50 am
content: 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.

author: GregDunn
author_rank: Frequent Contributor
timestamp: May 28, 2018, 08:27:28 pm
content: Allthe power supply voltages are way out of spec (as in the ±25V supplies are asymmetrical and reading about 3-5V; no bueno). There's no hint in the manual, but I suspect I should replace all the likely power supply caps (most of which can be tested pretty easily) and look at CR7 before moving on to the transistors.This thing worked fairly well a few years ago and hasn't been abused, so I am going to attack the caps first, as they're more likely to have degraded/failed. I'll post back when I get the parts.

author: AF6LJ
author_rank: Unknown
timestamp: May 29, 2018, 12:38:12 pm
content: An in circuit ESR meter is your best friend right about now.

author: jcrubin
author_rank: Frequent Contributor
timestamp: May 30, 2018, 12:50:20 pm
content: Ive only really found value in ESR readings at those values when there are multiple caps of the same value to compare against. Or a return value so obnoxious that it has to be bad. MEH values are not so easy. I found my bad cap as I recall with the ESR meter but confirmed it with my IT-11. The IT-11 is the final judge in these matters!I have added my original documentary movie for this project to the original post on page 1

author: AF6LJ
author_rank: Unknown
timestamp: May 30, 2018, 03:57:10 pm
content: Quote from: jcrubin on May 30, 2018, 12:50:20 pmIt is a fast way to weed out the marginal caps, especially when there are a lot of tantalum caps involved.

author: GregDunn
author_rank: Frequent Contributor
timestamp: June 02, 2018, 10:33:32 pm
content: All the questionable parts in the box are aluminum electrolytics as far as I can tell from looking at them. The A1 board unplugs, leaving most of the caps already disconnected from the circuit, so any good meter should be able to sort them out. I have found several with excessive ESR - enough that I'm just going to replace every 'lytic on the A1 board before proceeding. A few identified as diodes in my multitester - that can't be good.

author: GregDunn
author_rank: Frequent Contributor
timestamp: June 07, 2018, 05:26:46 am
content: It turned out to be a good idea to replace all the power supply caps; a couple of them measured in spec but were clearly not OK when powered up. After replacing all the A1 caps I got roughly ±20V out of the power supply and it slowly decreased after a few minutes of warm-up; I couldn't get it to ±25VDC.So I went ahead and replaced the caps on the A2 board. I'm not sure what those big Sprague caps were covered with (some kind of semi-transparent plastic/rubber sleeve), but it was gooey and unpleasant after all these years. After fixing up A2, I not only got good ±25VDC with plenty of adjustment, but the -11.3 and 8.2VDC supplies came in spec very nicely with just a little tweaking of the appropriate pots. I'm going to attack A3 next, but a quick check of the voltmeter function showed it to be working OK and it didn't exhibit the slow oscillations I saw before fixing A2.It was very interesting to see how much smaller the average cap is than it used to be when this unit was originally built. I bought only quality parts, incidentally - I don't want to have to do this again in 20 years.

author: jcrubin
author_rank: Frequent Contributor
timestamp: June 08, 2018, 02:19:36 am
content: Quote from: GregDunn on June 07, 2018, 05:26:46 amWhat single measurement did you use to make this assertion? Did you mean that somehow the capacitance on a capacitance meter was good, but the leakdown at the rated voltage was a horror show? Id buy that. Or did you mean the ESR was ok in circuit but the leakdown at max voltage said otherwise?

author: GregDunn
author_rank: Frequent Contributor
timestamp: June 08, 2018, 07:01:37 pm
content: What I was trying to say was that the capacitance and ESR of the old parts measured similar to the replacement caps (both out of circuit) but when I installed the new parts on A1, the power supply immediately started working properly (all 6 test point voltages in spec). I can only assume that when the old caps were subjected to the higher voltage in-circuit, the leakage increased dramatically on at least some of them, pulling the voltages down. I don't have a capacitance tester which can be made to use higher voltages at this time, so they were tested somewhere around 5VDC. Obviously that's one of the limitations of a low cost multi-function tester.After upgrading all the electrolytics, I was able to perform a distortion measurement using my upgraded Heath IG5218 signal generator. Spot checks yielded <0.05% @ 1 KHz and <0.02% @ 10 KHz. I think that's pretty good.

author: bitseeker
author_rank: Super Contributor
timestamp: June 09, 2018, 01:34:52 am
content: Quote from: jcrubin on May 24, 2018, 02:01:50 amI had wondered about that kind of a shootout. Thanks for the results. In what ways or by how much did the HP win out?

author: bitseeker
author_rank: Super Contributor
timestamp: June 09, 2018, 01:37:05 am
content: Quote from: GregDunn on June 08, 2018, 07:01:37 pmYep, that sounds reasonable. Capacitance and ESR tests are only part of the picture. A capacitor leakage tester is worth having if you plan to refurbish more old gear.

author: jcrubin
author_rank: Frequent Contributor
timestamp: June 10, 2018, 12:55:55 pm
content: Quote from: GregDunn on June 08, 2018, 07:01:37 pmThe only conclusive test in these situations is leakdown. That is why my IT-11 is the most used Heathkit I have on the bench. The power supplies running at full tilt cant deliver when capacitors tied to ground are providing a direct path for DC. I see this often as the single biggest cause of PS and transformer / rectifier failure.

author: jcrubin
author_rank: Frequent Contributor
timestamp: June 10, 2018, 12:58:01 pm
content: Sadly in the worst way possible......When the source is a solid state sine wave generator versus tube. When you start getting into 5th order harmonics.... There were constraints that didnt exist or were not thought of when the Heathkit was designed.I realize im asking for trouble when I inject subjectivity into a topic of test equipment.... but ill post it anyway...

author: Jacampb2
author_rank: Newbie
timestamp: November 16, 2019, 06:38:42 pm
content: Hi jcrubin, is there any chance you would be willing to fix your pictures? It appears all the links went dead and I would very much like to review this repair as I am beginning work on a 331a myself.Thank you!Jason

author: jcrubin
author_rank: Frequent Contributor
timestamp: November 18, 2019, 02:50:39 pm
content: i will have a look and see what went wrong