Update: it wasn’t the hybrid! The ohms range is now repaired: th-cam.com/video/Ko1Dukv1Byo/w-d-xo.html . Thanks to the astute commenters that pointed out other possible failure modes and encouraged me to give it another go. Special award to @DrFrank-xj9bc who actually looked at the schematics and guessed it right.
@@CuriousMarc Believe me, you get the same stomach knot when you do something stupid on a live server and you accidentaly drop the wrong database. And now you have to explain to the customer (and your boss) why a simple cleanup job to free some disk space caused a three hour downtime...
@@dodaexploda I see some machinists working with various clips and other springed/tensioned items doing that work under a cloth so it will bounce off the cloth (and stay in the area) when it inevitably springs away. The camera can’t see it of course, unless you want to use a plastic sheet which might rustle and mess up your audio! Though I suppose a cheesecloth might offer the best of both worlds… up to a certain size of spring 😅
Those “Shadow” switches were pretty popular back in the 70’s and 80’s. I designed a number of production test and repair stations using them. I liked working with them. Fortunately the equipment was redesigned over time and automated using HPIB systems so we never ran into the intermittent switch problems.
Thanks for the video, again! Restored my 3466A last spring, new power supply caps, checked out the all other components. Had the same problem with my switches so physically cleaned all the switch contacts and applied deoxit gold to protect the contacts. New set of rechargeable batteries installed and fully calibrated. Love the LED display. Fortunately, no hybrid chip fault. Geoff
An odd problem with gold finishes used for soldering and electrical contacts is the adsorbtion of organic contaminates. This happens on circuit boards with electroplated gold finishes which pick up contamination during the subsequent manufacturing operations. Sometimes a light plasma cleaning is used to remove this and clean the gold without damaging the finish. Your theory that the plastic may have outgassed is probably correct. This is generally not an issue with connectors (which should make gass tight connections) or low usage contacts that have a strong wiping force (like gold fingers). On switchs that are specified for high cycle counts the contact wiping force must be relatively low in order prevent damage to the relatively soft and thin gold surface. As a result of the contacts are in an environment that exposed them to some types of vapors and contaminates they may become contaminated and exhibit poor surface. resistance
Metallurgist here. Another issue with soldering gold contacts is the reaction of the tin with the gold plating. Tin forms a brittle intermetallic (AuSn alloy) amalgam that can cause joints to crack. You normally need to apply and remove solder several times in order to basically dissolve and get rid of thebgold plating so you can get a good contact with the nickel plating or base copper. Especially with gold cup connections.
@@marshalltjones fascinating. I’ve seen some people say corroded gold solder pads must not really be gold since gold doesn’t corrode. I’ve previously suggested they might’ve used brass, since it’s conductive and was used quite a lot in the 19th century electrical. Reading this, since tin is happy to corrode, maybe it was the gold-tin amalgam which corrodes in those cases?
I've been wondering if those hybrids can be repaired with modern surface mount devices - or if it'd be possible to replace one with SMDs + quality pcb material. Or has someone already tried?
For a 4 digit instrument it might work. The resistors in the hybrid are laser trimmed, and thermally locked by their proximity. But again, 4 digits, Ohms scale only, worth a try.
Working for a competitor in my youth, I was more familiar with this dual ramp integrator. 😊 Please note that these voltmeters are very sensitive to contact quality. A few milliohms resistance in the switch may cause a false reading. Though it is rarely constant : playing with the switch makes the reading going all over the place, not stabilize on a value.
Wow, at the first shot of this meter a kind of 'sensory memory' from 35 years ago kicked in. At university these were in the lab, and I remembered how it felt to press those buttons.
Glad you were able to get it somewhat functional. Too bad about the hybrid being damaged. Students accidently sending the wrong signal into an input is a constant danger in an educational setting. One of the HP 3476As I got in the same lot as your HP 3438A had a damaged hybrid as well. A few years ago, a couple of grad students vaporized (slight exaggeration) a nice SMU. The push button gang switches on that era of equipment are always a pain as well. If I happen to come across another 3448A that might be a suitable parts doner I'll send it your way.
How do you do that, use the SMU as a sink, past it's limits? Not sure, but shouldn't a SMU simply go into overrange, unless input voltage is exceeded? Unless you use some good amount of RF/HF
@@hinz1 I don't recall the specifics. The carnage was impressive though. I have an old Fluke signal generator on my bench at work which had the output current resistor smoked from a student feeding a signal into it. Luckily that was an easy fix.
To be fair, it's not only students who mess up instruments. I'm making circuits as a hobby and i've burned up a few (cheap) scopes in late-night debugging sessions. And one at mid-day, it's really astonishing how much amps a couple of solar panels can force into a small hand-held multimeter... 😕
My first job in electronics was as a tech in a university lab. The EET students had practical knowledge and were easy, but the EE students were all theory and were very hard on the test equipment. When EEs came into our lab, it was time to call Newark and order more fuses! 😂
Nice meters those, readable AND auto range. Mine also had problems with the switches, but I do not have your patience, sprayed them with contact cleaner, massaged the switches and blew out the residue with compressed air.
Excellent video! Electronic test equipment with those snap action gang type switches are the biggest repair tease. I remember my first foray into electronics repair with a Valhalla 2101 power meter displaying very odd symptoms and thought 'oh this should be a fun repair!'... just the gang switches. Also love how you meticulously disassemble the switch to clean it - I just douse them in Deoxit and exercise them a bunch. I like your way better!
Thank you for another interest video and the switch cleaning tutorial. I have started using "clean room" swabs for this type of work as they don't leave any stray fibers behind. I'm just finishing up repair & refurbish of two Guildline 9577 bench meters with beautiful Panaplex displays that I can still read from across the room. Regards, David
Those switches are notorious for being a pain. Ive learned that it was the flux in the wave soldering process that leached into these switches. Many time she shoulder breaks as well. Im happy to see That form factor of switch gone in modern devices. I use LPS 1 then LPS 2 on them in droves when fixing ward beck mixing consoles ❤
Your next challenge is to create your own stand-in hybrid using precision 0402 resistors and modern electronics fabrication. Bet it wouldn't be too out of spec for a 3.5 digit DMM. You can do it!
Hello Marc, thank you for showing this beautiful DMM! I did not download the Service Manual yet, so have to speculate a little bit, what's your real OHM fault. I got some experience with these switches, i.e the front/rear switches on the later 34401A, and on the HP3458A, very similar construction, as well showing similar faults in 2W OHM, subsequentially, i.e. a contaminated switch shows a residual OHM reading of several Ohms. I guess, that this 6 Ohm zero Offset probably will not be caused by a fault of the hybrid, but more probably from a residual resistance in the OHM input path. The range and mode switches (the ones you've cleaned so far) might still show these 6 Ohms contact resistance, maybe due to bad contact force, or you might have made an assembly fault, when you had to re-attach the spring and the contacts.. pls. have a closer look, if these contact plates are somehow 'polarized': break-before-make.. Please measure this said zero resistance with another DMM, starting from either Input + and Input - jack to the belonging current source GND and Hi nodes. Your external 'short' does not look very well, maybe you might tighten its screws, or measure its resistance with your other DMM. I don't overlook the whole story fully yet (as I didn't study the schematics), but your maybe erroneous zero alignment and then consecutively faulty gain adjustment (which then of course does not reach full scale as well) is caused by such a residual resistance, and then flaws the whole measurement scheme. Frank
Gee Marc, I expected you to launch into an effort (complete with hen-scratched lined paper theory of operation accompanied by your favorite elevator music) to pad the resistor array to bring the meter back in spec at least at lab temp.
Is it at all possible to replace the defective part of the circuit? I don't mean the whole device, but to build a discrete circuit to bypass the damaged portion.
I had problem with HP 3466A and similar switches, where the range indicators would be stuck even when the range was not selected. Seemingly some contacts would short together, and pushing the affected switch repeatedly would fix it for a while, but fault always came back, and there was tin whiskers growing at least on the metal bar holding the banks together. I cleaned switches best I could from outside, but even that did not help. Finally I took 1,5V battery and connected it between every switch contacts, and after that so far all of the switches have been working correctly. I was too afraid to remove the plungers due old brittle plastics. There was similar switches still available on digikey, but price for single one was high and some needed configurations just not were stocked anymore.
Had MANY tin whiskers on the U-bar that hold these switches too (not a HP, but other brand DMM). Thousands of them, shorting several contacts out. Looked like a woolly stick, whiskers up to 6mm. De-soldered all switches, sand papered and spray painted the U-bar after dismounting the switches. And then I put a electrical tape barrier between the bar and any contacts, so new tin whiskers would brake before penetrating.
Contact cleaner is great stuff. I had an LG washer that would not turn on, I thought maybe the control board was bad. A quick blast with contact cleaner on all the tactile switches and it was good to go. I am sure many of these modern appliances get dumped out due to switch oxidisation. I used HP test gear during my years as an Electronics undergrad, memories.
WOW! I had to do a double-take when you flipped the unit over at 4:19... That UMR-EE engraving means it came from my alma mater, University of Missouri - Rolla! I may or may not have items in my collection with similar engravings. The university used to have AMAZING surplus sales, I once bought a working gas chromatograph that was missing an interface card for $10 because it was cheaper than a microwave. I used it to heat up frozen burritos.
I have its portal version . It also had a dead switch hybrid. Back in the 1970's I was quoted over £600 for a replacement from hp. I made my own from a set of cmos switches and a few diodes (not exactly accurate but at least it worked sort of ;-)
Hello Marc, after briefly studying the schematics, I strongly assume, that the hybrid is fine, but one or several switches are still fishy, especially the 20 Ohm switch is suspect, S7 A/B on page 8-17 / 8-18 . The OHM circuitry should be protected enough, to avoid any damage of the current source (which is partly in the hybrid). At first, pls. check, if this fault, or 6 Ohm offset (when you reset the offset to its original 6 Ohm reading) is visible in the 200 Ohm, 2kOhm ranges as well, and if all these ranges read correctly full scale, if you simply subtract this offset. Please check all OHM ranges for F.S. reading with your resistor box. Your external short must be low ohm, of course, as mentioned in the manual.
Would be good if it were true! Switch 7 is just a digital level switch, so it should not matter how perfect it is. Maybe switch 6 which is in the analog path. But then the V range would not work either. Maybe the protection circuit itself died (Q101?). I had not thought about that one. I’ll investigate further when I’m back after new year.
A different offset for the 20 ohms and 2 K range is suspect. The offset in the 2 K and higher ranges is more a thing of amplifier voltage offset. The 20 ohm range can also have series resistance from the switches ( SW6E and SW6F) and maybe the terminals. Another possibilty could be that the ohms converter circuit is oscillating.
Discombobulating the plunger switches, that's pretty interesting to watch. They come in different designs, I'm very familiar with Polish made "Isostat" licence switches and while they're functionally similar (interlocked or independent, latching or not, and fast action wide gap for mains applications), they're far simpler inside than the ones used by HP. I wonder how switches used by Tektronix compare to both; maybe I'll see Atkelar take one apart someday.
Ohhh, those switches. I have a DEC 3100 motherboard sitting around somewhere that's likely operational except for one of those switches splitting down the sides and all the wee bits disappearing into the night....
I had similarly sticking switches on a Pioneer amp. On disassembly a tiny shard of white plastic had come off. Against my expectations, CA glue held it in place to give mostly reliable latching.
When I worked for HP they issued us with Fluke multimeters if you were field not bench😂 We if course callibrated them by comparison with proper expensive and accurate meters from HP.
Hello Marc, there's also a strange 'input switch', S15 on that schematic.., I could not identify, where this is sitting.. that could affect all ranges, so this one is most suspect. I did not see you cleaning that switch. Seems to be that rotary switch '21'.. check internal cables, connections, solder joints as well.
And you guessed right! It **was** the rotary switch, see update video: th-cam.com/video/Ko1Dukv1Byo/w-d-xo.html . Free vintage pat in the back for you.
Grundig satellit 2100s also have this type of switches and suffer from the same problem. It's quite tedious to clean them since they have many more contacts as well. Still, we're talking 50 year old switches so i guess it's acceptable.
I couldn't quite see how the switches were linked, is there a plate underneath so that they are all effectively touching each other in a clockwork like mechanism. Also another one for me I will have to go back and have a look at how the individual switches are released so that you can take the shaft out. Thank you for another great video.
Hello Mark Have you thought of making a new push swìtch shaft by three d printing. I like the way HP, Tektronics and Marconi Intruments, technical hand books are written. The only thing was the US circuit diagrams were usually on several pages, witch made fault finding and calibration difficult. As I worked at Marconi Instruments (part of English Electric) in St Albans, UK, for severr years during the changover from valves to ic's.
I have two 3466A meters -- I love them for their clear and bright displays. I had a 3rd unit, but it had the problem with deteriorating switches. Their plastic suffered from serious melt-down. I think a former owner used alcohol or a bad solvent in an attempt to clean them. That meter is now my spare-parts unit.
Have you tried superglue and baking powder as a filling material? It forms a kind of plastic that will bond to any surface - including plastics that super glue will not ordinarily adhere to. It sets as hard as a ceramic, and leaves materials like epoxy dead in the water!
It's an old HP probe system that's much, much better than the new ones (though fragile, you have to take care of them). They are hard to find, usually pricey, usually incomplete. Mine come from years of gathering bits and bobs on eBay. Search for HP-Agilent series 1160 probes. The dual lead adapter itself is 5063-2147. The mini grabbers are also HP, much more common.
I'm a UMR (now MUST) alum. In the early 80's, I spent a lot of time with the tech. (Dick Schroeder) who repaired and calibrated all our instruments in the EE department - he was the one who first exposed me to the glorious Tektronix 555 dual beam oscilloscope. I also spent some time in charge of the Hobby Club. Having been an electronic hobbyist since my early teens, I was appalled at the number of people in the program that didn't know which end of a soldering iron to pick up. 😕 I even repaired a TV for an EE prof. there...
Right next to my lap-top I so happened to have one of those switches in an ultra cheep car graphic equalizer. So I test it and the contacts work fine...go figure.
More switch issues. HP really hated switches; in my 8350b more than 80% had lost the metal part and was a pain to refit them. @curiosmarc look at positives side... The spring from the switch did not jump to a random location of the lab
No such luck. The hybrid is a laser trimmed, carefully thermally balanced analog resistive circuit. Modern digital electronics won’t help at all for that one. HP (and Fluke) still make the hybrids for their higher end voltmeters.
Might be an interesting experiment to duplicate the hybrid as a pcb with smd components. Would still be small, so might work reasonably well. Do it as a 'flip chip' with the components on the bottom and a shielding plane on top and you might have reasonable thermal stability.
Hi, I would always like to see what kind of voltage reference these DMMs use and where it is. Always interesting to know what is their "beating heart" (if the A/D hybrid is the brain) to compare. One can only imagine how stable they might be nowadays if its been run for 40+ years.
I sadly don’t have a hybrid laying, myself are missing a MRC for a HP5334B. I wonder if master Kent has some hours, he maybe can create a replacement chip for it, I really hate to see it just standing there.
At glorious 3 1/2 digits, it shouldn't be too difficult to replace the hybrid with some selected/adjusted 0.1% low tempco resistors. And make it more foolproof for future generations ;-)
With this model, you can only read. Function and Range have to be selected manually. I bought one from eBay before I found this out. I should have read the manual first.
Why not replace the bad parts off the hybrid with some discrete components? Or is it built in such a way that doesn't make that feasible. Meaning there is more in there than a resistor network and some such.
Shame about the hybrid being damaged, I wonder if later revisions had some kind of protection for it since it seems like something that's asking to be damaged by accident.
Actually, you are right. Looking at the schematics, there are no less than 3 protection circuits. I’m now wondering if it’s the protection circuit that gave up the ghost to save the hybrid. That would be mighty good news if that’s the case.
flying spring: I can't tell you how many times I've had that happen. After it (minuscule spring) vanishes from sight, and after a few choice words, a long protracted search ensues! :/
if the plastic is off-gassing, they will probably keep doing that (all of them). ideal solution is to machine new parts out of PTFE. or just 3D print some stuff out of resin etc and replace the plastic switch stems
"UMR" could stand for University of Missouri, Rolla, in Rolla, Missouri. This is the engineering campus for the Missouri schools. They have a new name for it, but it was UMR when I got my masters from them. 😁
Gteat video. It's just that automatedbtranslation which as a user I cann't seem to disable for good. Oh the satisfying click of these mechanical switches - and broken unobtainium chips more headache-catalytic than the finest moonshine fusel 😢
Hey, I have the same meter with a very similar problem ! It's waaaay off calibration, but the switches seems to work fine (nothing changes when I wiggle them). Thanks for the service manual, I'll try to fix it. Some day...
Pensare di ricreare quel ASIC danneggiato, anche con l'aiuto di un piccolo uP "moderno" è follia? Oppure, ancora meglio, con una FPGA che il tuo giovane aiutante che lavora in SpaceX sa usare benissimo 🙂
Update: it wasn’t the hybrid! The ohms range is now repaired: th-cam.com/video/Ko1Dukv1Byo/w-d-xo.html . Thanks to the astute commenters that pointed out other possible failure modes and encouraged me to give it another go. Special award to @DrFrank-xj9bc who actually looked at the schematics and guessed it right.
CuriousMarco Reps
I wish!
United in their dislike for failing irreplaceable HP era ASICs 😶🌫
@@CuriousMarcmaybe team up with @SamZeloof
@@reps LOL !
I swear my brain does this mixup every time.
When the spring went boing and disappeared, I felt that.
I know. That sinking feeling when our are trying to repair something and potentially just made it worse instead…
I totally saw it coming. I feel like someone needs to make an acrylic box for doing these sorts of operations.
@@CuriousMarc Believe me, you get the same stomach knot when you do something stupid on a live server and you accidentaly drop the wrong database. And now you have to explain to the customer (and your boss) why a simple cleanup job to free some disk space caused a three hour downtime...
@@dodaexploda I see some machinists working with various clips and other springed/tensioned items doing that work under a cloth so it will bounce off the cloth (and stay in the area) when it inevitably springs away. The camera can’t see it of course, unless you want to use a plastic sheet which might rustle and mess up your audio! Though I suppose a cheesecloth might offer the best of both worlds… up to a certain size of spring 😅
Those “Shadow” switches were pretty popular back in the 70’s and 80’s. I designed a number of production test and repair stations using them. I liked working with them. Fortunately the equipment was redesigned over time and automated using HPIB systems so we never ran into the intermittent switch problems.
I also have the 3438a because I can see it. Thank you Marc.
That logic board is just a Work Of Art. Such beautiful engineering there.
Thanks for the video, again! Restored my 3466A last spring, new power supply caps, checked out the all other components. Had the same problem with my switches so physically cleaned all the switch contacts and applied deoxit gold to protect the contacts. New set of rechargeable batteries installed and fully calibrated. Love the LED display.
Fortunately, no hybrid chip fault.
Geoff
Congrats! One more instrument saved.
An odd problem with gold finishes used for soldering and electrical contacts is the adsorbtion of organic contaminates. This happens on circuit boards with electroplated gold finishes which pick up contamination during the subsequent manufacturing operations. Sometimes a light plasma cleaning is used to remove this and clean the gold without damaging the finish. Your theory that the plastic may have outgassed is probably correct. This is generally not an issue with connectors (which should make gass tight connections) or low usage contacts that have a strong wiping force (like gold fingers). On switchs that are specified for high cycle counts the contact wiping force must be relatively low in order prevent damage to the relatively soft and thin gold surface. As a result of the contacts are in an environment that exposed them to some types of vapors and contaminates they may become contaminated and exhibit poor surface. resistance
Metallurgist here. Another issue with soldering gold contacts is the reaction of the tin with the gold plating. Tin forms a brittle intermetallic (AuSn alloy) amalgam that can cause joints to crack.
You normally need to apply and remove solder several times in order to basically dissolve and get rid of thebgold plating so you can get a good contact with the nickel plating or base copper. Especially with gold cup connections.
@@marshalltjones fascinating. I’ve seen some people say corroded gold solder pads must not really be gold since gold doesn’t corrode. I’ve previously suggested they might’ve used brass, since it’s conductive and was used quite a lot in the 19th century electrical. Reading this, since tin is happy to corrode, maybe it was the gold-tin amalgam which corrodes in those cases?
Nice. Looking forward to Marc sourcing and replacing that defective hybrid to bring it back to full functionality. 😁👍🏻
I've been wondering if those hybrids can be repaired with modern surface mount devices - or if it'd be possible to replace one with SMDs + quality pcb material. Or has someone already tried?
For a 4 digit instrument it might work. The resistors in the hybrid are laser trimmed, and thermally locked by their proximity. But again, 4 digits, Ohms scale only, worth a try.
Working for a competitor in my youth, I was more familiar with this dual ramp integrator. 😊
Please note that these voltmeters are very sensitive to contact quality. A few milliohms resistance in the switch may cause a false reading. Though it is rarely constant : playing with the switch makes the reading going all over the place, not stabilize on a value.
Nice to see someone else struggling. You are still my hero!
Wow, at the first shot of this meter a kind of 'sensory memory' from 35 years ago kicked in. At university these were in the lab, and I remembered how it felt to press those buttons.
I got my very pretty and immaculate HP 3455A because of watching Marcs content and I love this meter, love his troubleshooting videos
Congrats! The 3455 is a beast, one of my favorite HP DVM.
Glad you were able to get it somewhat functional. Too bad about the hybrid being damaged. Students accidently sending the wrong signal into an input is a constant danger in an educational setting. One of the HP 3476As I got in the same lot as your HP 3438A had a damaged hybrid as well. A few years ago, a couple of grad students vaporized (slight exaggeration) a nice SMU.
The push button gang switches on that era of equipment are always a pain as well.
If I happen to come across another 3448A that might be a suitable parts doner I'll send it your way.
How do you do that, use the SMU as a sink, past it's limits?
Not sure, but shouldn't a SMU simply go into overrange, unless input voltage is exceeded?
Unless you use some good amount of RF/HF
Thanks Birt for the donation!
@@hinz1 I don't recall the specifics. The carnage was impressive though. I have an old Fluke signal generator on my bench at work which had the output current resistor smoked from a student feeding a signal into it. Luckily that was an easy fix.
To be fair, it's not only students who mess up instruments. I'm making circuits as a hobby and i've burned up a few (cheap) scopes in late-night debugging sessions. And one at mid-day, it's really astonishing how much amps a couple of solar panels can force into a small hand-held multimeter... 😕
My first job in electronics was as a tech in a university lab. The EET students had practical knowledge and were easy, but the EE students were all theory and were very hard on the test equipment. When EEs came into our lab, it was time to call Newark and order more fuses! 😂
Nice meters those, readable AND auto range.
Mine also had problems with the switches, but I do not have your patience, sprayed them with contact cleaner, massaged the switches and blew out the residue with compressed air.
Excellent video! Electronic test equipment with those snap action gang type switches are the biggest repair tease. I remember my first foray into electronics repair with a Valhalla 2101 power meter displaying very odd symptoms and thought 'oh this should be a fun repair!'... just the gang switches. Also love how you meticulously disassemble the switch to clean it - I just douse them in Deoxit and exercise them a bunch. I like your way better!
Thank you for another interest video and the switch cleaning tutorial. I have started using "clean room" swabs for this type of work as they don't leave any stray fibers behind. I'm just finishing up repair & refurbish of two Guildline 9577 bench meters with beautiful Panaplex displays that I can still read from across the room. Regards, David
So when can we expect the workaround circuit video where you design a board that plugs into the slot to replace the hybrid? Happy New Year!!
Thanks for the Video and: for the scanned manual 👍
Ah, that’s a nice way to end a year filled with wonderful videos Marc. Thanks for that, Bonne année 🎇and we’ll see you soon
Those switches are notorious for being a pain. Ive learned that it was the flux in the wave soldering process that leached into these switches. Many time she shoulder breaks as well. Im happy to see That form factor of switch gone in modern devices. I use LPS 1 then LPS 2 on them in droves when fixing ward beck mixing consoles ❤
Ooo I remember using these things in the lab at the Uni I worked in. Standard at the time. Amazing how things moved on in just a few years.
Your next challenge is to create your own stand-in hybrid using precision 0402 resistors and modern electronics fabrication. Bet it wouldn't be too out of spec for a 3.5 digit DMM. You can do it!
Agreed. Someone else does it for me pleaaaase…
I love the strip capacitors on the top of the board - both power distribution and spike suppression.
Hello Marc, thank you for showing this beautiful DMM!
I did not download the Service Manual yet, so have to speculate a little bit, what's your real OHM fault.
I got some experience with these switches, i.e the front/rear switches on the later 34401A, and on the HP3458A, very similar construction, as well showing similar faults in 2W OHM, subsequentially, i.e. a contaminated switch shows a residual OHM reading of several Ohms.
I guess, that this 6 Ohm zero Offset probably will not be caused by a fault of the hybrid, but more probably from a residual resistance in the OHM input path.
The range and mode switches (the ones you've cleaned so far) might still show these 6 Ohms contact resistance, maybe due to bad contact force, or you might have made an assembly fault, when you had to re-attach the spring and the contacts.. pls. have a closer look, if these contact plates are somehow 'polarized': break-before-make..
Please measure this said zero resistance with another DMM, starting from either Input + and Input - jack to the belonging current source GND and Hi nodes.
Your external 'short' does not look very well, maybe you might tighten its screws, or measure its resistance with your other DMM.
I don't overlook the whole story fully yet (as I didn't study the schematics), but your maybe erroneous zero alignment and then consecutively faulty gain adjustment (which then of course does not reach full scale as well) is caused by such a residual resistance, and then flaws the whole measurement scheme.
Frank
I recently bought a 3438A. I also have a 3478A.
Approved. Excellent purchases!
Happy New Year, Marc 👍
Either its Christmast, or its Easter Sunday.
Or Marc has a new video! ❤
I see the engraving "UMR-EE" on the top.
I may have used this exact meter when I was a student.
Gee Marc, I expected you to launch into an effort (complete with hen-scratched lined paper theory of operation accompanied by your favorite elevator music) to pad the resistor array to bring the meter back in spec at least at lab temp.
I know! I chickened out.
Is it at all possible to replace the defective part of the circuit? I don't mean the whole device, but to build a discrete circuit to bypass the damaged portion.
I had problem with HP 3466A and similar switches, where the range indicators would be stuck even when the range was not selected. Seemingly some contacts would short together, and pushing the affected switch repeatedly would fix it for a while, but fault always came back, and there was tin whiskers growing at least on the metal bar holding the banks together. I cleaned switches best I could from outside, but even that did not help. Finally I took 1,5V battery and connected it between every switch contacts, and after that so far all of the switches have been working correctly. I was too afraid to remove the plungers due old brittle plastics. There was similar switches still available on digikey, but price for single one was high and some needed configurations just not were stocked anymore.
Had MANY tin whiskers on the U-bar that hold these switches too (not a HP, but other brand DMM). Thousands of them, shorting several contacts out. Looked like a woolly stick, whiskers up to 6mm. De-soldered all switches, sand papered and spray painted the U-bar after dismounting the switches. And then I put a electrical tape barrier between the bar and any contacts, so new tin whiskers would brake before penetrating.
I have an HP3435A, which I was given 30 years ago. It's still a workhorse, although the NiCad batteries have long ago expired.
Contact cleaner is great stuff. I had an LG washer that would not turn on, I thought maybe the control board was bad. A quick blast with contact cleaner on all the tactile switches and it was good to go. I am sure many of these modern appliances get dumped out due to switch oxidisation. I used HP test gear during my years as an Electronics undergrad, memories.
WOW! I had to do a double-take when you flipped the unit over at 4:19... That UMR-EE engraving means it came from my alma mater, University of Missouri - Rolla! I may or may not have items in my collection with similar engravings. The university used to have AMAZING surplus sales, I once bought a working gas chromatograph that was missing an interface card for $10 because it was cheaper than a microwave. I used it to heat up frozen burritos.
I have its portal version . It also had a dead switch hybrid. Back in the 1970's I was quoted over £600 for a replacement from hp. I made my own from a set of cmos switches and a few diodes (not exactly accurate but at least it worked sort of ;-)
Portal /portable
Happy new years! Wish you a great 2025 with man more vidoes to come I hope... Your quality is astounding.
Hello Marc, after briefly studying the schematics, I strongly assume, that the hybrid is fine, but one or several switches are still fishy, especially the 20 Ohm switch is suspect, S7 A/B on page 8-17 / 8-18 .
The OHM circuitry should be protected enough, to avoid any damage of the current source (which is partly in the hybrid).
At first, pls. check, if this fault, or 6 Ohm offset (when you reset the offset to its original 6 Ohm reading) is visible in the 200 Ohm, 2kOhm ranges as well, and if all these ranges read correctly full scale, if you simply subtract this offset.
Please check all OHM ranges for F.S. reading with your resistor box.
Your external short must be low ohm, of course, as mentioned in the manual.
I was thinking this as well. that would be best case scenario!
Would be good if it were true! Switch 7 is just a digital level switch, so it should not matter how perfect it is. Maybe switch 6 which is in the analog path. But then the V range would not work either. Maybe the protection circuit itself died (Q101?). I had not thought about that one. I’ll investigate further when I’m back after new year.
A different offset for the 20 ohms and 2 K range is suspect. The offset in the 2 K and higher ranges is more a thing of amplifier voltage offset. The 20 ohm range can also have series resistance from the switches ( SW6E and SW6F) and maybe the terminals. Another possibilty could be that the ohms converter circuit is oscillating.
Discombobulating the plunger switches, that's pretty interesting to watch. They come in different designs, I'm very familiar with Polish made "Isostat" licence switches and while they're functionally similar (interlocked or independent, latching or not, and fast action wide gap for mains applications), they're far simpler inside than the ones used by HP. I wonder how switches used by Tektronix compare to both; maybe I'll see Atkelar take one apart someday.
(wonders when Marc will 3d-print a jig to make cleaning the plungers easier while preventing contacts from sliding out)
would it be possible to (re)build the integrated hybrid with discrete smd parts?
Such a pretty device built, bummer they had no experience with more reliable switches
With the lost material problem could you just add epoxy wait for a full cure and scrape that to the correct size, or at least better than it was ?
my first bench meter was an hp 3466a, also had dirty switches and a non working Gates battery option
Any chance of adding resistance at the pot? It was headed the right direction.
Would it be possible to add a extra resistor outside the hybrid module to compensate and calibrate the damaged inside resistor?
I think the issue was that the switching transistor is burned.
I always wonder how he gets the equipment manuals
I pay the ransom on eBay so you don’t have to.
Ohhh, those switches. I have a DEC 3100 motherboard sitting around somewhere that's likely operational except for one of those switches splitting down the sides and all the wee bits disappearing into the night....
The space stuff is cool but I enjoy the HP equipment fixes even more. I hope you find a replacement part.
May you can build a hybrid equivalent with modern parts on a small board?
I had similarly sticking switches on a Pioneer amp. On disassembly a tiny shard of white plastic had come off. Against my expectations, CA glue held it in place to give mostly reliable latching.
When I worked for HP they issued us with Fluke multimeters if you were field not bench😂
We if course callibrated them by comparison with proper expensive and accurate meters from HP.
Hello Marc, there's also a strange 'input switch', S15 on that schematic.., I could not identify, where this is sitting.. that could affect all ranges, so this one is most suspect.
I did not see you cleaning that switch. Seems to be that rotary switch '21'.. check internal cables, connections, solder joints as well.
And you guessed right! It **was** the rotary switch, see update video: th-cam.com/video/Ko1Dukv1Byo/w-d-xo.html . Free vintage pat in the back for you.
@@CuriousMarc Thanks! 🥰
Grundig satellit 2100s also have this type of switches and suffer from the same problem. It's quite tedious to clean them since they have many more contacts as well. Still, we're talking 50 year old switches so i guess it's acceptable.
Would be feasable to design a replacement board with different type os switches?
I couldn't quite see how the switches were linked, is there a plate underneath so that they are all effectively touching each other in a clockwork like mechanism. Also another one for me I will have to go back and have a look at how the individual switches are released so that you can take the shaft out. Thank you for another great video.
Yes there is a bail plate way underneath linking all the switches, couldn’t see it well myself.
Hello Mark
Have you thought of making a new push swìtch shaft by three d printing.
I like the way HP, Tektronics and Marconi Intruments, technical hand books are written.
The only thing was the US circuit diagrams were usually on several pages, witch made fault finding and calibration difficult.
As I worked at Marconi Instruments (part of English Electric) in St Albans, UK, for severr years during the changover from valves to ic's.
I have a 3466A which looks identical apart from it's slightly more portable case format and no HPIB. But it's 4 1/2 digit.
I have two 3466A meters -- I love them for their clear and bright displays. I had a 3rd unit, but it had the problem with deteriorating switches. Their plastic suffered from serious melt-down. I think a former owner used alcohol or a bad solvent in an attempt to clean them. That meter is now my spare-parts unit.
Have you tried superglue and baking powder as a filling material? It forms a kind of plastic that will bond to any surface - including plastics that super glue will not ordinarily adhere to. It sets as hard as a ceramic, and leaves materials like epoxy dead in the water!
Find that very hard and brittle. I think the switches use a bit of a soft material.
Great video! your persistence is amazing. Who makes the scope probe to micro grabber adapter?
It's an old HP probe system that's much, much better than the new ones (though fragile, you have to take care of them). They are hard to find, usually pricey, usually incomplete. Mine come from years of gathering bits and bobs on eBay. Search for HP-Agilent series 1160 probes. The dual lead adapter itself is 5063-2147. The mini grabbers are also HP, much more common.
@@CuriousMarc Thank you for taking the time to reply. Your channel is outstanding!
I'm a UMR (now MUST) alum. In the early 80's, I spent a lot of time with the tech. (Dick Schroeder) who repaired and calibrated all our instruments in the EE department - he was the one who first exposed me to the glorious Tektronix 555 dual beam oscilloscope. I also spent some time in charge of the Hobby Club. Having been an electronic hobbyist since my early teens, I was appalled at the number of people in the program that didn't know which end of a soldering iron to pick up. 😕 I even repaired a TV for an EE prof. there...
I was a user of the Hobby Club in the 90s. All kinds of good stuff in there.
@@markrages I wonder if the Hobby Club still exists? Hope so!
Right next to my lap-top I so happened to have one of those switches in an ultra cheep car graphic equalizer. So I test it and the contacts work fine...go figure.
More switch issues. HP really hated switches; in my 8350b more than 80% had lost the metal part and was a pain to refit them. @curiosmarc look at positives side... The spring from the switch did not jump to a random location of the lab
Wonder if it'd be possible to replace the hybrid with a CPLD, FPGA or Arduino.
No such luck. The hybrid is a laser trimmed, carefully thermally balanced analog resistive circuit. Modern digital electronics won’t help at all for that one. HP (and Fluke) still make the hybrids for their higher end voltmeters.
@@CuriousMarcsurely you could build /sonething/, albeit not very accurately functioning?
Might be an interesting experiment to duplicate the hybrid as a pcb with smd components. Would still be small, so might work reasonably well. Do it as a 'flip chip' with the components on the bottom and a shielding plane on top and you might have reasonable thermal stability.
I suspect standard smd would be sufficient to emulate the custom hybrid.
@@darrenconway8117 The main concerns would be temperature tracking and whether the original resistors were laser trimmed to match each other.
@@lwilton This is only a 3.5 digit meter. Modern components would be a better option than a broken hybrid device.
If I follow the channel? Of course I follow the channel!
Hi, I would always like to see what kind of voltage reference these DMMs use and where it is. Always interesting to know what is their "beating heart" (if the A/D hybrid is the brain) to compare. One can only imagine how stable they might be nowadays if its been run for 40+ years.
I sadly don’t have a hybrid laying, myself are missing a MRC for a HP5334B. I wonder if master Kent has some hours, he maybe can create a replacement chip for it, I really hate to see it just standing there.
At glorious 3 1/2 digits, it shouldn't be too difficult to replace the hybrid with some selected/adjusted 0.1% low tempco resistors.
And make it more foolproof for future generations ;-)
👍🏻👍🏻👍🏻
Hmm. Considering the diode protection and the symptoms, the problem may be somewhere else.
Although the current source is easy to check.
Do you have a list of codes for the HPIB interface?
With this model, you can only read. Function and Range have to be selected manually. I bought one from eBay before I found this out. I should have read the manual first.
Why not replace the bad parts off the hybrid with some discrete components?
Or is it built in such a way that doesn't make that feasible. Meaning there is more in there than a resistor network and some such.
Shame about the hybrid being damaged, I wonder if later revisions had some kind of protection for it since it seems like something that's asking to be damaged by accident.
Actually, you are right. Looking at the schematics, there are no less than 3 protection circuits. I’m now wondering if it’s the protection circuit that gave up the ghost to save the hybrid. That would be mighty good news if that’s the case.
😊gidday from down under
flying spring: I can't tell you how many times I've had that happen. After it (minuscule spring) vanishes from sight, and after a few choice words, a long protracted search ensues! :/
if the plastic is off-gassing, they will probably keep doing that (all of them). ideal solution is to machine new parts out of PTFE. or just 3D print some stuff out of resin etc and replace the plastic switch stems
"UMR" could stand for University of Missouri, Rolla, in Rolla, Missouri. This is the engineering campus for the Missouri schools. They have a new name for it, but it was UMR when I got my masters from them. 😁
Now Missouri University of Science and Technology. That is where the meter came from ;)
@@HeyBirt Yep, that's what's on the envelopes that request donations. 😀 Those college labs are a harsh environment for test gear!
@@SkyhawkSteve I've worked there for 22 years now and you are quite correct.
I'm a current student at S&T, and I knew the "UMR" engraving looked familiar lol
Amazing that like all the switches were bad. Such a poor design but these types of switches are super satisfying to use.
Gteat video. It's just that automatedbtranslation which as a user I cann't seem to disable for good.
Oh the satisfying click of these mechanical switches - and broken unobtainium chips more headache-catalytic than the finest moonshine fusel 😢
Can't you replace the cooked chip?
It’s a HP proprietary hybrid. You can sure change it if you ever can find a replacement chip, usually from a donor instrument.
Oi! My sympathetic frustration kicked-in watching you work your way through those annoying switches.
Hey, I have the same meter with a very similar problem ! It's waaaay off calibration, but the switches seems to work fine (nothing changes when I wiggle them). Thanks for the service manual, I'll try to fix it. Some day...
Next up 7 digits nixie conversation 😂
Hi Marc. How was your christmas?
I feel so guilty for tossing one of these machines in the bin a few decades ago :/
Pensare di ricreare quel ASIC danneggiato, anche con l'aiuto di un piccolo uP "moderno" è follia?
Oppure, ancora meglio, con una FPGA che il tuo giovane aiutante che lavora in SpaceX sa usare benissimo 🙂
Maybe you could just spray contact cleaner into the switch without pulling it out.
Why didnt he fix to resistor pack by either putting a resistor in parallel or cutting the path and putting a new resist?
🇧🇷🇧🇷🇧🇷
Мда уж, американские П2К ничем не лучше советских🤣
Ew, the switch contacts were covered in VOC juice? :(
Those switch plungers look to be thermoplastic; it might have been worth trying some heat first, to see if they would re-form
Thanks Marc for another interesting episode. Always learn something! Happy New Year for you and your loved ones! 🌃🌟🎆
First
First indeed! You get awarded a free vintage ohm range that is oddly off.
4 digits is plenty to get you through an entire career 99% of the time… there’s really not much that requires much over 1/10th volt accuracy…