HP 973A & 974A Multimeter Service Manual anyone? - Page 1
Forum section: electronics / testgear

author: AMR Labs
author_rank: Frequent Contributor
timestamp: December 30, 2018, 04:01:02 pm
content: I recently acquired a HP973A DMM as an addition to my other three fully working HP974A's as in my opinion these are outstanding instruments, even though the "970" series (971A, 972A, 973A, and 974A) dates back to around 1994-2000 they will easily hold their own against any comparable Fluke, and remarkably only at a fraction of the cost. All of them except for one where basically still dead on calibrated after all these years. Built quality is excellent, both inside and outside, as with any HP product. Incidentally these where manufactured in Japan. Maybe by a major Japanese electronics company under contract for HP? Or under license?So far I have tried to locate any web reference to a schematic or service manual for these DMM's without any luck. Same goes for the HP/Agilent website, and on the HP-IO group's messages. At this point I am not even sure there even where any service manuals that got released, as all hits always only seem to point to the user manual or brochures/spec sheets. The user manual (is more like a leaflet), in comparison to conventional HP manuals, is quite limited on technical and maintenance information, and contains no schematics or diagrams other than the location of the internal cal points and a calibration sequence.Does anybody (that may have worked at HP, or otherwise) know if there where actually any service manuals for these DMMs that where made available? In my mind I would think there must be something out there as HP would most definitively have created some kind of detailed service literature, or at least a schematic for these DMMs, but maybe due to cost cutting measures the service information was excluded from the standard user manual. Then again these where manufactured in Japan, so maybe the company that made them under the HP brand actually never released any service literature in normal HP tradition.Anyway, I would be very interested in anything technical related to the 973A/974A, other than the standard user manual or spec sheets. Or even to find out about the story of why this 970 series of DMMs where manufactured in Japan, which seems a bit odd for an HP product.BTW the HP970A DMM is not really in any way similar to the rest of the 970 series I am interested in, but is more like a hand held probe style meter, for which there is some information available on the net.Thanks for any leads.EDIT: Added JPG image.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: December 31, 2018, 06:33:23 am
content: Hewlett Packard was in a joint venture with Yokogawa Electric from 1963 to 1999:https://en.wikipedia.org/wiki/Hewlett-PackardSpecialty meters and impedance analyzer equipment were some of the key products from there:www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1966-09.pdfIt may help to look to see if there is an equivalent Yokogawa Electric product.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: December 31, 2018, 12:08:17 pm
content: Thank you very much for that info, very useful. A step closer to possibly finding what I am looking for.Sounds possible that these 970 series multimeters might be an original product from the Yokogawa company that where relabeled as HP.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: December 31, 2018, 12:46:16 pm
content: I did send an email inquiry about the service manual to Yokogawa, hope they will reply back.

author: Kosmic
author_rank: Super Contributor
timestamp: September 15, 2019, 01:27:27 am
content: Look like the HP 970 series is similar to the Yokogawa 7537 seriesYokogawa 7537:http://almateks.com/doc/dmm-e-150.pdfMy HP 974a:

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 15, 2019, 03:08:30 pm
content: Interesting information, thank you for posting it here. So in the end the suspicion was confirmed that the HP97xA lineup was a rebranded Yokogawa product.So it seems their model 7537 "4" 20KHz True-RMS and 0.1% accuracy specs are equivalent to the HP973A, one down from the HP974A 100KHz True-RMS & 0.05% accuracy top of the line model. And apparently there is no Yokogawa 7537 model that would have been equivalent to the 974A. The above internal view pictures are exactly the same as the HP version I have, so no doubts left regarding it being the same product. Rebranding not withstanding, they certainly perform very well and are fully up to par to carry the HP logo in terms of quality and long term reliability. None of the 4 HP974A's (5 by now - found another brand new one and still in the box so could not resist) I own is off by as much of a percentage fraction of the reading from either my HP3478A/3468A, even after all these years since they where last manufactured, and without having as so much to touch up on any calibration at all.The only one from the batch I own that I actually needed to recalibrate was the 973A as it came with quite a bit of battery corrosion inside that had fortunately not spread beyond the battery holder but the acidic vapors had over time apparently deposited on some areas of the PCB causing lower resistance paths here and there not allowing the instrument to zero on some ranges etc. Spot cleaning with alcohol did not help. The solution was to wash the whole PC board with some dishwasher detergent and a soft tooth brush, then dry out with moderate heat. Then recalibrate according to the procedure in the owners manual. It has worked flawlessly ever since.I wonder why there doesn't seem to be any 974A direct Yokogawa equivalent in their 7537 lineup. A model "5" would probably been the denomination given. Perhaps it would have competed against some other of their own top of the line model they already had on the market...In any case still no luck finding a schematic for these, even at the Yokogawa website the search for the 7537 series raises no results. When I contacted Yokogawa some time ago to ask about the HP97x service manuals or anything they would have on that lineup, I finally got a reply from one of their Technical Support & Calibration Engineers in Europe (after about a month and 3 different messages sent to them) that they did not have any records left about the HP lineup or the Yokogawa equivalent models as they had been "long discontinued", and instead only sent me the copies of the HP owners manuals that are already circulating online. So what company (huge company) completely and absolutely disposes of records for their not so really that "very long ago" discontinued products, and bear in mind this was a product (at least the HP version) that was still widely available late 90s/early 2Ks. Oh well, guess Yokogawa did it, maybe did do not have the storage space, or they just plain don't feel like releasing their schematics or service info that is perhaps still lingering inside some boxes or file cabinets somewhere around the factory.Thanks again for this lead, and I will post any updates if I am able to find any further information online.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: September 15, 2019, 03:45:15 pm
content: Quote from: AMR Labs on September 15, 2019, 03:08:30 pmI've seen this quite a bit, both generally and at companies I have worked at. The dynamics are typically something like:1. Marketing specifies some market segment. Cursory research is done, and an engineering and manufacturing team is put together.2. It gets designed and manufactured, sold for a while, then obsoleted.3. After some time, perhaps 5 years after obsolescence, the remaining repair parts are disposed of.4. Schematics and other drawings are not generally accessible by customer support. From their point of view, it's all gone even if copies may be somewhere, whether on computer or microfilm.5. All the people involved with the project have long since moved on.6. From a business case point of view, there is no ROI (Return On Investment) by maintaining access to "old, obsolete" data that long.7. Managers tend to have very little passion for obsoleted products.Classical Hewlett Packard equipment, where a fair percentage of documentation remains available, is an outlier compared to others.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 15, 2019, 03:55:59 pm
content: Yokogawa is now part of Keysight. There are other instruments made in Japan that are more or less identical under Yokogawa and HPAK name.While a good quality DMM, 3.5 digit DMMs are usually not really serviced if there is something seriously wrong (e.g. more than fuse or battery contacts). So in this case no really important data missing.In the process of going from paper base documentation to electronic form some documents may get lost. This also happened with early computer systems - so the docs may be still there on some 8 inch floppies or old MO drives or write only tape. The merger could also be a reason to get rid of old paperwork.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 15, 2019, 05:42:22 pm
content: To Tomorokoshi:Following are my personal views and opinions, and I fully understand your points and don't wish to argue about them. Just wanted to air my "grievances" to the point and say my peace. I have been into electronics both professionally and as a hobby most of my life, and it has been a rare occasion (specially with the advent of the Internet) that one cannot find service information or even a simple schematic of any given electronic equipment in order to do my own repairs. Even sometimes finding something that is "similar" will do the trick and save the day. But in the case of these Yokogawa 7537 | HP97xA line of DMMs it is the first time in my life that it has been so far impossible to locate absolutely anything at all, other than the user manuals, and everything else seems to be shrouded in a veil of mystery. Maybe the service manual has written TOP SECRET across it? ;-)How does it hurt the company ROI to just voluntarily release the service information to the public domain of a discontinued product, specially if the company does not want to do anything further with it. It might even possibly save them more than a few service calls! It is understandable that the company does not want or it even makes sense to any longer manufacture and keep stock of parts for older discontinued products beyond their "in-support" period. No spare parts available anymore? fine, let me deal with it and there will always be parts mules out there, or creativity, ingenuity and improvisation might take place as long as it is worth the effort. But why hoard, hide or conveniently "misplace" or even destroy any technical service information? Again: how disclosing this information at the appropriate time can hurt the company ROI? They will always count on their sales to corporate client purchases which have their own in-house obsolescence and replacement cycles in place, and that definitively will keep on buying from the same source no matter what, as long as the deal numbers are right. I would think those sales are the ones that really matter the most to the ROI.I don't think it would hurt the [real bottom line] sales figures that in general giving a limited percentage of here and there owners possibly the ability of fixing up their discontinued equipment. That instead of them running to the store to buy a newer model from the same brand, which I doubt happens consistently anyhow just because one cannot fix the current one anymore due to lack of information from the manufacturer. I would think that a good part of the long standing Bill & Dave's HP brand tradition success to this day and well beyond the HP era itself is because of easy access to service information even for older discontinued products and keeping them going while they are still getting the job done (talk about boosting YOUR own ROI). Plus just the fact that the service information for most existing HP models was released at some point to the general public (probably around the time when the first name change came into the picture) comes across as a good faith "nice guy" action that will not only bring more luster to the brand, but also more individual (non-corporate) customers would actually consider buying again from the same company. In any case I would think that the bulk of sales for a big company like Yokogawa does come from corporate purchases that don't give a fig about service information being available at the time or in the foreseeable future. So why hurt the individual user by not making this service information available.To me in particular, and speaking in general terms, if test equipment that was purchased as NEW that came from a company that at some point ends up not being cooperative in disclosing service information about a product that is clearly serviceable outside of the factory, and with the exception perhaps of some custom parts can otherwise be repaired with common off the shelf supplies, this does not exactly foster and promote the desire in me to buy any further equipment of the same brand in the future when forced to replace any now unserviceable equipment because of their refusal or "inability". And let alone talk about recommending it to other peers. Again speaking in broad terms, I recognize that it is the manufacturers choice to withhold the service information, but ultimately it is my/our choice at large to continue buying and recommending that brand.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 15, 2019, 05:59:44 pm
content: Quote from: Kleinstein on September 15, 2019, 03:55:59 pmYou must be referring to factory service and then only "decent quality" series models, or those DVM-in-a-chip types. As for the HP972A-974A series they have lots of discrete thru-hole and SMD replaceable parts inside on the main board. And if it ever came to it (so far fortunately all my DVMs are working fine) it would be much easier to troubleshoot them if one had access to the schematic and possibly also to a parts layout diagram. So basically I have been looking for an schematic or service manual for this series mostly out of piece of mind and curiosity, as they are very well build and engineered for long term reliability unless the batteries are left inside for a long time without attention.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 15, 2019, 06:00:48 pm
content: I can't speak to how either of the companies involved made their decisions about document retention, but I have encountered a phenomenon that definitely affects publicly held companies in the US. Product liability lawsuits are a real thing. And deleting documents relative to a pending lawsuit is obstruction of justice. So lawyers at some companies have recommended that ALL documentation be destroyed after a fixed period. If the documents don't exist they can't be used against you, and if it is a general policy not aimed at a specific lawsuit it is difficult to prove that the intent is obstruction rather than just eliminating storage costs or other valid reason. The recommended fixed interval varies but I have seen numbers ranging from 5 years to 25 years.One more thing to thank our legal system for. I am a firm subscriber to the observation that 99.3% of lawyers spoil it for the whole profession.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 15, 2019, 07:25:36 pm
content: Speaking of lawyers...Another thing that I was wondering about is why did HP not disclose any technical information on these rebranded Yokogawa DMM series during the period when a lot of other documentation of older discontinued equipment was being made public by HP themselves. After all this was a product sold in the US with a HP badge. Maybe HP never even got to see schematics or service manuals at all, or they where only provided by Yokogawa with a convenient NDA in place.

author: dseyst
author_rank: Contributor
timestamp: September 15, 2019, 08:38:35 pm
content: I have a 974a also. You can find a user/calibration manual at xdevs.com but I haven't been able to find the schematics yet. Yokogawa made a 7544 02 model which is very similar to the 974a. There is one 7544 02 for sale on ebay right now but the asking price is $199.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 15, 2019, 10:02:14 pm
content: Thanks for the input. However the user manual (or rather leaflet) that includes a very short calibration procedure is widely available from many other sources, if I remeber correctly even from HP themselves. But as you mention unfortunately not so for any service manual or even schematics, hence the reason for this post.I tried searching on ebay for "yokogawa 7544" but nothing relevant came up, even by adding the last "02" to the model. Can you provide a link or ebay item number? Out of curiosity I would like to compare it with the 974A. The price you mention seems way out of line to the usual norm, should be more like between $50-100 and that is for the one with the HP badge, complete with yellow holster, test leads etc. I doubt that a Yokogawa even if in good shape "like new" condition will ever sell higher than the HP counterpart, and definitively not at that price level.I have been monitoring ebay for 974A's for the better part of 4 years, passively as of late, as I do not think I will need to buy yet another one but always curious of what will come up next. On the other hand if something really nice out of the ordinary comes up like the last one in pristine like new never used condition in the original box with test leads and at the right price I could simply not pass on it, hence 974A #4. That one now sits in the closet as a back up unit, as I really don't have much more need or space on the bench for it besides the other four.

author: Kosmic
author_rank: Super Contributor
timestamp: September 16, 2019, 01:04:38 am
content: For reference, this is the Yokogawa 7544 02. Interesting that the accuracy is 0.04% and not 0.05% like the HP 974a. And the features (Averaging and etc) are not exactly the same.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 16, 2019, 02:37:44 am
content: Where did you get this picture from? Its unfortunately very poor quality specially towards the top of the DVM.From what I can make out it does not seem to be an exact equivalent of the 974A.

author: Kosmic
author_rank: Super Contributor
timestamp: September 16, 2019, 02:54:23 am
content: Quote from: AMR Labs on September 16, 2019, 02:37:44 amGoogle images. Found some pictures on a Japanese web site.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 16, 2019, 03:08:30 am
content: OK yes well as you mentioned the layout of the top buttons is definitively different than the 974A, so I doubt this is a direct equivalent. Good try though. Definitively close, but no cigar.

author: dseyst
author_rank: Contributor
timestamp: September 16, 2019, 03:12:37 am
content: I just typed in ""yokogawa 7544 multimeter"" in the ebay search box a few minutes ago and the listing came up.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 16, 2019, 03:22:24 am
content: Link please?Oh never mind finally found it.https://www.ebay.com/itm/162986352332I wonder if Yokogawa also included the yellow holster, not shown on the listing.

author: Kosmic
author_rank: Super Contributor
timestamp: September 16, 2019, 03:26:41 am
content: Don't think so. The yellow bumper on my 974a was made in the US. So was probably added by HP.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 16, 2019, 03:39:16 am
content: Right you are. I'm now looking at one of mine:Hewlett PackardBoot Made in USA.Beautiful bright yellow boot. Can't even get one of those anymore unless its on a Fluke.They registered it as a trademark. Ridiculous this was even granted.

author: Kosmic
author_rank: Super Contributor
timestamp: September 16, 2019, 03:50:49 am
content: Quote from: AMR Labs on September 16, 2019, 03:39:16 amHaha didn't know that

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 16, 2019, 04:28:30 am
content: That is why you now see a lot of blue, red, black, and other colored boots. All except bright yellow. Only allowed now on Fluke DVMs.Guess they had a good team of lawyers to win the case and be the only ones to be able to sport a yellow boot. Or maybe the counterpart's lawyers failed to make this valid point:The yellow color of the protective boot is standardized to conform to industrial safety norms, and among others to make it easier to locate your instrument by sight. Its called "Safety Yellow" for a reason. Not meant to just look nicer, and Fluke claimed it to be "their proprietary image look". Its ridiculous that they where even allowed to claim exclusive use of the yellow boot. I would compare it to only a single company being able to manufacture bright yellow life vests, all others have to use some other less eye catching color. Guess which one will be more effective.

author: artag
author_rank: Super Contributor
timestamp: September 19, 2019, 08:21:25 pm
content: My 970A multimeter is clearly marked 'Made in USA'.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 19, 2019, 08:36:07 pm
content: Your HP970A is a completely different DMM, its more like a hand-held probe type of instrument which was made by HP in the US.No relation to the HP971A-974A series made in Japan by Yokogawa.HP should have given the later multimeters a different model designation.

author: artag
author_rank: Super Contributor
timestamp: September 22, 2019, 11:11:49 am
content: It's also a little earlier .. 1973

author: Compulsive Fixer
author_rank: Contributor
timestamp: September 16, 2020, 04:20:03 am
content: Greetings AllMy first post on this forum.I came here seeking information on the HP 973A multimeter after mine recently stopped working.I have had it for 25 years after winning it for renewing my subscription to a well known Australian electronics mag and it has been an excellent piece of equipment which has worked faultlessly all that time.A couple of weeks ago it stopped working so I pulled the back off and found the batteries leaking and covered with a blue deposit on the ends. I was rather surprised at this as I wrongly assumed that Duracells were reasonably leak proof. Fortunately the battery holder contacts cleaned up well with no signs of pitting or corrosion and I optimistically fitted new cells, but the meter powered up and presented a normal display as soon as I pushed the second battery into place and could not be powered down again using the ‘Power’ Button.The first thing I did was clean the switch contacts on the board and re-coat the button contact with a thin coat of powdered graphite rubbed in well but still no luck!So out with the magnifier and low power microscope to examine all of the tracks and components in the general area of the battery holder, but all looked good and well protected by the conformal coating. Using my cheapie multimeter, which I carry in the car glovebox, I was able to tediously trace a path from one of the switch contacts through a couple of resistors and diodes to the general area of IC7. The other contact carried +3V.My next suspicion was that some of the corrosive liquid from the cells had seeped in under the chips on that end of the board so I soaked just that end in warm water and detergent with some brushing, washed and soaked again in several changes of fresh water then into denatured alcohol and thoroughly dried with warm air. I had previously un-clipped the screen and was careful not to wet the rest of the board as I did not want to get water into any of the trimmers.All of this still failed to get the ‘Power’ button working, although all of the other functions of the meter seemed to be working normally.With nothing to lose I decided to look at the most likely culprit – IC7, a 74HC132A (a quad 2 input NAND Schmidt trigger) surface mount IC on the back of the board under the LCD screen as this had a clear path to the batteries through a hole for the battery holder locating pins. With the hot air rework gun I lifted this chip off the board and found it to be wet underneath with a film of pale green liquid. Close examination showed that the area around the solder pads had been masked off from the conformal coating but a small section of copper track from the pad for pin 11 to a via under the middle of the chip body had been dissolved away by electrolysis.Full of hope, I carefully bridged the gap in the track with a thin strand of copper wire and re-assembled everything but, alas, still the same problem.Once again I lifted the IC and re-examined the area under it with the microscope. The only slightly suspicious thing I could see was a tiny tab on the end of the pad for pin 10. There was no signs of any track leading from it to any other pad or via.Still convinced that I was on the right track I started drawing out the circuit around this chip but eventually gave up due to the number of fine tracks and vias in this area – it was going to take too long!So, here I am hoping someone can help with a circuit diagram or a photo of the tracks under this chip from a scrapped meter.I have spent quite a lot of time looking online for a low-cost replacement for this meter but nothing seems to compare, so I an really keen to fix this one.Any help or suggestions from forum members would be much appreciated.

author: helius
author_rank: Super Contributor
timestamp: September 16, 2020, 05:35:57 am
content: Quote from: Compulsive Fixer on September 16, 2020, 04:20:03 amOh, brother.I don't think schematics for this series ever appeared. However, they do have an unusual and distinctive feature, the soft power button. Most DMMs use a rotary switch position to turn off and disconnect power. The 973A has a key that switches power on and off: this means that it must have a path from the battery to the key, and a latching mosfet circuit that is switched on and off by the key. To facilitate latching it probably uses a capacitor as well. Another DMM with this design feature is theProtek 506, for which schematics are available and should be similar in this area.

author: Compulsive Fixer
author_rank: Contributor
timestamp: September 16, 2020, 08:09:18 am
content: Thanks for that info helius,I had a quick look at the Protek 506 circuit (Hung Chang) and it appears to use the same IC so I will sit down and study it in more detail as soon as I get the time.The link to the Duracells was also most interesting. Mine were date stamped for Mar 2017 so not terribly old. They don't mention any guarantee on covering equipment damage but they were made in China so could even be counterfeits.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 16, 2020, 01:59:11 pm
content: I had a similar problem when I got the 973A off ebay when it came with old long time dead batteries installed and the usual blue-green corrosion on the battery clips. I actually had to remove the battery holder by unsoldering the connecting tabs from the main PCB (remove LCD display first), as it was really the only way to be able to completely remove the whole mess which in the end cleaned up quite well after using some diluted acetic acid (common vinegar), an x-acto knife as a scraper and a small brush. After that the DMM would seem to function but the lower uA and mA current ranges where not displaying zero without any input, instead displaying some value, but everything else seemed to work. It appeared that the outgassing of the old batteries had been spread around leaving a seemingly invisible layer of conductive and corrosive electrolyte in some areas and was bypassing or creating some unwanted electrical paths.After some careful inspection also fund a couple of very thin copper tracks had been partially eaten away and still had tiny blue spots at random points on them, located somewhat near the top of the board near the display, and also between the display and the rotary selector switch area. I had to use a 10x magnifier in order to get rid of all the really almost microscopic tiny green spots here and there along those tracks as they where so small that they where virtually invisible otherwise. I first cleaned all those areas with Qtips and alcohol, but some of the tiny corrosion spots really required some gently scraping off with the sharp tip of a x-acto knife, and then proceeded to patch up one or two spots on the traces which looked suspect.Unfortunately after all that work the current ranges where still not zeroing, but at least I had moved forward in cleaning and fixing those traces. Next step I took out the whole main board, removed the display and battery holder, and carefully washed the whole board down with a solution of water and dish wash liquid (Palmolive) with a soft tooth brush and gently going in circles letting the soap do its thing. Finally rinsed off everything under running water and thoroughly let it dry off. After reassembling the unit, finally this time the current ranges started to zero properly, but then I noticed that some of the other volt and ohm functions where a bit off calibration. So did a full calibration on all functions and ranges with the help of the trimmer adjustment locator in the manual, and ever since the DMM has been performing well so far.One of my 974A also came with old corroded batteries inside, but for this one I only had to remove/unsolder the battery holder to remove the blue stuff off the contacts, and do some general cleanup with alcohol/Qtip around the battery area for good measure, and that was enough to make everything work. BTW I now do have total four 974A units after not being able to resist buying a brand new one some time ago at a very reasonable price on ebay still in the box with original test leads, and which I have set aside as a future "spare" unit. Unfortunately nowadays all the asking prices seem to have gone through the roof ($100-150+) even for a no-leads, as-is 973A. This in comparison of what they where selling for perhaps 4-6 years ago rarely going over $50-$75. I remember actually paying just $99.99 for my first used mint 974A back in 2014, including the original box, test leads, and user manual. Now in more recent times, probably the resellers have been getting wise to the real quality of these DMMs, and as usual are trying to further milk the cow.After having some bad experiences with supposedly good quality brand name alkaline batteries like Duracell and Energizer, I no longer trust any of them. They are either not made as they used to, or it would appear that the chinese have managed to flood the global markets with counterfeit batteries that look exactly the same as the originals so one never knows what you are really getting. Anyway, to be on the safe side, and also to cut down on recurring battery cost, for some time I only use AA or AAA NiMH rechargeable cells from well known brands like Panasonic/Eneloop on all my important portable electronics, also including all my HP Led calculators and the HP41CX N-Cells which I nearly also lost to some battery corrosion that formed only weeks after replacing the batteries with some "fresh" ones. The HP973/974 DMM's really don't seem to care about the difference of 1.25V against the regular 1.5V per cell, and even if there is some difference in running time, they usually still last for months before needing a recharge, so really who cares.I am always amazed of how accurate these old DMMs still are, even after all these years since they left the factory in Japan. Specially the 974s, they all are almost dead on compared to my HP3468A/78A 5.5 digit MMs. Too bad the schematics where never released, probably forgotten in a drawer somewhere in Japan under the Yokogawa brand name.

author: Compulsive Fixer
author_rank: Contributor
timestamp: September 23, 2020, 10:01:20 am
content: YAY - It's Alive!!!!After spending countless hours trying un-successfully to fault-find my 973A and then starting to re-draw the circuit I finally got it working.I had patched up a dissolved section of track under the 74HC132 smd chip but stupidly created a dry joint when re-soldering the 4066. I used a low power stereo microscope to examine all of the solder joints and find it.A touch with the iron and it was all good, much to my relief.After the experience of nearly losing an old friend I have vowed to never touch another Duracell!Thanks to all who offered help & advice.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 23, 2020, 01:02:54 pm
content: Excellent news! Congratulations on fixing your 973A.Did you by any chance take any pictures of the fix? If so it is always welcomed to publish them here for future reference for others.

author: Compulsive Fixer
author_rank: Contributor
timestamp: September 25, 2020, 12:46:24 am
content: I'll see what I can do with a couple of photos as soon as I get time. I will have to pull it apart again. I thought of taking some at the time but got so excited at finding the fault and keen to test it out that I just re-assembled the whole thing. My most significant discovery was with the presence of a clear path for battery electrolyte from around the battery holder through one of the holes for its mounting lugs to the other side of the board where it could wick in under the surface mount chips. With the smd chips sitting flush to the board they readily hold the liquid and allow electrolysis to slowly do its work on any unprotected tracks. No amount of soaking and scrubbing is likely to remove the liquid from such a narrow space under the chips - the only effective way is to temporarily remove them.

author: maginnovision
author_rank: Super Contributor
timestamp: September 25, 2020, 12:50:55 am
content: Your best bet is to stop using alkalines and switch to lithium. I've done it with all my meters now after a similar experience with my keysight u1282a that's not even 2 years old.

author: kipp
author_rank: Contributor
timestamp: October 08, 2020, 03:52:58 am
content: Regarding the higher precision versions, the Yokogawa model numbers are 7544-01 and 7544-02, but it's hard to find documentation for them.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: October 08, 2020, 06:23:10 am
content: Quote from: kipp on October 08, 2020, 03:52:58 amMentioned and pictured earlier in this thread:https://www.eevblog.com/forum/testgear/hp-973a-974a-multimeter-service-manual-anyone/msg2690670/#msg2690670

author: Pisami
author_rank: Newbie
timestamp: September 18, 2024, 06:27:10 am
content: I found the 20 kHz model from trash and it was labeled as broken. I regularly fix interesting electronics as a hobby and I grabbed it. Sure, it had green-blue death from leaked batteries and it required a lot of elbow grease to get most of it away. Glad to see that others have also succeeded. The battery connects are a bit flakey still and the Power button does not want to co-operate. I'll need to check the above posted fix if it helps, but I have already tested that everything works when covers open and the DMM is powered on from board directly.Nobody mentioned that there are 4 pinholes on the very top of the PCB. I soldered pin headers to them in hopes they are non-documented serial output, but I haven't had the time to measure what they do. Any ideas?

author: AMR Labs
author_rank: Frequent Contributor
timestamp: September 18, 2024, 08:06:43 pm
content: Unsolder the battery contacts and use vinegar to get the blue-green corrosion off, then wash off with water and leave to dry before reinstalling. Repeat as necessary until all corrosion is gone. Also make sure both PCBs are clean as any slight deposit from the battery acid fumes can create problems. Also make sure no tracks have been damaged by corrosion. There are a few very fine tracks near the battery holder that I had to repair as they had been eaten away by the acid.No idea about the 4 connections you mention, but please let us know what you find out.