HP3478A AC ranges failure - Page 1
Forum section: electronics / testgear

author: AMR Labs
author_rank: Frequent Contributor
timestamp: October 01, 2025, 02:31:32 pm
content: Had this instrument for about 10 years now and back then replaced the internal battery, so cal constants where never lost. As it is now when put into any VAC range the display will usually show a noisy random quite high reading (20-50V or sometimes higher) jumping around even with nothing connected (not even probes), or often the display will just immediately show OVL. This happens on all AC ranges including the higher 30 and 300V (I manually switched ranges to test them individually). If I input say 120VAC, no meaningful reading is shown, way off and not even close. Power up self test passes OK, and all DC volt ranges work fine, as does 2W ohms.One thing that I am not sure about ever seeing it, and have so far not been able to find in the service/operation manual, is that the most right hand decimal point (just to the right side of the VAC label) on the LCD display is constantly flashing about twice a second. Can't remember that happening way back, but again no mention in the manual I could find. I also have a fully working HP3468 which incidentally does not do the flashing decimal, and that also allowed me to some extent to swap one critical part to rule it out. More on that later.This DMM got relatively little use over that 10 year time period as one usually tends to grab the multimeter that is more convenient to use for the task and right in front within hands reach, so usually end up using one of my HP974A's, unless more digits are required. In any event AC ranges where working last time I used them (can't remember when that was) and only noticed the failure recently when I moved my lab to a new location and started testing everything. Main PCB was thoroughly checked on both for any obvious damage or corrosion, but none was found. The only other thing that catch my eye was that the U302 metal can (TO-5) was a bit crusty on top, but after wiping it off with some alcohol it looks fine now.On the service manual went to chapter 7-B-7 (a) of Service Group B AC Volts Troubleshoot, and as indicated applied 3VAC 1KHz stable sine signal to the input, but I do not get any +3VDC at test point JM302, nor do I get 0.12VAC at jumper test point JM303. Both these test points show a reading of basically zero, or a few (noisy) millivolts at best.Then I traced the 3VAC 1Khz signal from the input terminals, then on the front/rear switch, relay K104, and then C301, and finally pin 42 of the input hybrid IC. The signal reads just fine at all previously mentioned test points, and makes it into the hybrid at pin 42. The signal is supposed to come back out from the hybrid at pin 37 after being properly scaled, and into pin 2 of U301, the first amplifier of the AC to DC converter stage. However the measured voltage at pin 37 is zero (or something really too low to be measured).Given these results the service manual suggest that IC102 the input hybrid might be bad, which of course at this point is pretty much unobtanium. But also having on hand a fully working HP3468A which uses the exact same PN input hybrid, I swapped the supposedly defective 3478 hybrid into the 3468 and it still worked fine on all ranges and functions. So that was a relief. So input hybrid failure definitively ruled out. I did not test the known good 3468A input hybrid in the 3478A, as the service manual states that a faulty A/D driving logic could damage the hybrid, so out of an abundance of caution did not perform that test.Testing the A/D logic chip (chapter 7-B-16) requires a signature analyzer, which I don't have, so not sure if U462 the A/D controller chip might be the problem, or possibly one of the adjacent chips in that section. basically this chip is what drives the input hybrid and sets the proper scaling and attenuation of the input by sending data to the input hybrid U102.Then I started measuring around the AC-DC converter stage, and while feeding the prescribed 3VAC 1KHz AC signal into the input terminals, found that I get a reading of exactly 30mV AC at pin 6 (output) of U302, even though no measurable level seems to be present at U302 pin 3 (input). In any event this 30mV AC reading seems to be somehow scaled quite well against the 3V input signal, as when the input signal was increased to 3.5VAC, the reading at pin 6 increased accordingly to 35mV AC.A few days later wanted to individually test each stage of the AC-DC converter by removing the U102 input hybrid which would then basically isolate the inputs and outputs of each stage and provide easily accessible test point for measurements. But then I though let me first just go over the previous measurements to make sure nothing had changed since last time, so once again put 3VAC 1Khz at the input, and to my surprise now the expected measurements as per the troubleshooting section where correct!. I had 3.0VDC at JM302, and also the 0.12VAC at JM303 where present. Then wanted to check that the signal was also reaching and going through capacitors C306 and C304 that couple the signal into the RMS converter stage but then suddenly everything dropped once again to zero in less than a minute. I did not even have a chance to look at the LCD while those first voltage readings where correct.Then checked the 3 address pins (clock data, mode lines) between U462 A/D controller and the input hybrid U102, and while the DMM is in VDC or ADC, or Ohms function I can see what appears to be normal activity. But once VAC or AAC is selected, all activity ceases on those lines.Also tested the input pins to U403 the A/D converter hybrid chip, and I can see activity on the DC IN (pin 25) while in VDC mode (a square wave which is shaped by the input circuitry in relation to the amplitude of the DC input being fed into the DMM). But absolutely no activity was detected on the AC IN (pin 27) while in ACV or AAC mode, which should originate at the RMS converter at the end of the AC-DC converter section. I am assuming that no signal will be present as the U102 input hybrid not being addressed properly will result in no output coming out of it.So that is where I am for now, not sure what else to try to pinpoint the faulty area. Anybody has any suggestions?Oh and BTW already had posted at the IO HP?Agilent group some time ago, but got nothing meaningful out of it. And so far my thread there seems to have gone cold without any additional postings. Very disappointed with how that group that used to be such a great resource of knowledgeable HP people, seems to have evolved over the past few years into what feels to me like nobody there is either interested or they just simply don't care. Maybe now they are more into Agilent/Keysight than good old HP. Oh well.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 01, 2025, 06:34:31 pm
content: Does it work from fully cold, and only stop working once there's some heat in it? That's what it sounds like you're describing here:Quote from: AMR Labs on October 01, 2025, 02:31:32 pmIf you have access to a thermal imager, having a look at U102 and U301 might be worthwhileEDIT: You could try a freezer spray on the hybrid and op-amp to see if it comes back right, as well

author: AMR Labs
author_rank: Frequent Contributor
timestamp: October 01, 2025, 11:17:04 pm
content: I don't think this is merely a thermal issue as the DMM has as of late been kept in a air conditioned room, but just in case I will have a look at the thermal signatures of the ICs in the AC-DC converter section while the instrument is powered up. The Input Hybrid U102 has been confirmed to be good and ruled out as being the issue as it was tested in the HP3468A and worked fine. My feeling is that the A/D logic chip that actually steers U102 the input hybrid is somehow not sending the correct signals, but then again the only way to verify that is with a digital signature analyzer which I don't have. So really would like to rule out other possibilities before embarking in that direction.Thank you for your input.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 02, 2025, 05:15:30 pm
content: I'm assuming you've checked the power rails; a little bit too much might cause a thermal issue.As far as the A/D controller goes, you should be able to sniff the control data going to U102 with a digital storage scope and compare it to the 3468A. Although the 3468A uses a different uP which controls the hybrid directly, the signal words should be the same.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: October 03, 2025, 02:20:14 pm
content: Yes all concerned power rails (+5 +15 and -15) are good and present. I will get back to the DMM for some additional tests around the A/D converter and logic sections on the weekend. But my reasoning since there is absolutely no input into the A/D converter chip while in AC mode, this issue must be related with the A/D logic section that drives U102 the input hybrid. Your suggestion to sniff those control lines and compare between both multimeters is a good idea to se where the differences might start to pop up.BTW the 3468 and 3478 are exactly the same instrument, the only difference is the firmware, and some internal PCB layout due to the different form factor. The 3468 lacks the very lowest ranges, but otherwise both are exactly the same, and this was just done as a marketing decision from HP at the time to be able to offer two separate feature tiers. This information came first hand from someone that used to work at HP and that was directly involved in the design of these two instruments. He even at some point offered to look up among his stash of testing firmware from way back to see if there was one that would allow to restore the missing low ranges on the 3468. Unfortunately he passed away some time afterwards. He was such a great source of information on anything related to the 3468/78, and a very generous and nice guy, so a big knowledge void has since been left at the HP IO group on these two multimeters, and which this person used to frequent.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2025, 03:04:54 pm
content: The 3468 and 3478 are similar, but still a bit different. The 3468 has no GPIB and RS232 instead. In addition it has a battery option. Lacking the lowerst range would indeed be a bad point - the 30 mV DC range is one of the nice points of the 3478.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: October 03, 2025, 04:23:09 pm
content: Ok well yes you are of course right if we go down to all details, but I was really referring to differences in the context of basic functionality and internal circuitry, mainly to address what the other poster suggested that there where some actual differences in the internal circuitry of both DMMs. I did not mention these other differences like the battery option in the 3468, and the external interfaces (HP-IL vs HP-IB) as they are kind of obvious and besides the main point.Thanks for the input.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 06, 2025, 05:52:08 pm
content: sometime it works ... sometime it not ...bad soldering contact connection junction ... ?

author: AMR Labs
author_rank: Frequent Contributor
timestamp: October 06, 2025, 06:39:32 pm
content: Worth a shot, but I had already checked over the whole PCB and specially around the AC-DC converter, and all solder joints are absolutely perfect in typical HP style, and no sign of corrosion anywhere. But to be clear I never really said it was working again, just that I was briefly getting the correct readings for about a minute or so after a cold start, at the two initial test point readings as per the AC Voltage troubleshooting guide in the service manual. I did not even get a chance to look at the display or try to read some AC voltage. These test point readings previously where at zero, and after a minute or so of reading correctly went back again to zero.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 07, 2025, 03:01:49 pm
content: do you think ,as you have 2 dmm the same, it worth to open both, same source , and see a difference in volage map across ....basically AC or DC input the same in U403 , internally it switching to the same adc , just different dividers. so if AC input to U403 the same for bot dmm , most likely U403 ,jumping and OL on any range , means problem after dividers , usually indicate 2 options , signal too high , or something leak in to high impedance input .. internal switching in U403 can be faulty , if you see equal input on both DMMs for U403 , it can be it ,

author: AMR Labs
author_rank: Frequent Contributor
timestamp: October 08, 2025, 01:00:20 pm
content: Earlier I had already confirmed that there is zero input to the A/D converter chip U403 at the AC IN pin 27 while the DMM is setup to measure VAC and a valid input. So suspect a problem related to the function of U102 the input hybrid, which scales the input by changing the gain of the two amplifiers in the AC-DC converter section, is not being driven correctly. A no signal condition in the AC-DC converter tells me that none of the gates in U102 are being opened, so nothing from the input signal seems to be getting through to the A/D converter. I also have confirmed that U102 is working good by swapping it into my 3468A.Since U403 sends its data into U463 the A/D logic microprocessor which then instructs U102 what fets to open or close, obvious suspect would be U463 or its surrounding logic, or something is messing up that stage. Most troubleshooting procedures in that section require a signature analyzer that I don't have. But some procedures just call for a logic probe and pulser (and a scope might be helpful) which I all have. Perhaps comparing to the fully working 3468A under identical setup and input voltage might be a valid option to look at the data lines and at least see if anything might be missing, wrongly timed or stuck.

author: AMR Labs
author_rank: Frequent Contributor
timestamp: «Reply #11 on:Todayat 03:11:31 pm »
content: I managed to rule out the A/D converter and logic section, mainly by removing JM302 to isolate both the RMS converter from the A/D converter as the manual suggests, and injecting a DC voltage directly into the AC input port of the A/D chip U403 pin 27. I tried with 1.0VDC and 3.0VDC at that pin and got the expected values to show on the 3478 display.Furthermore, with 3.0VAC into the input terminals of the 3478 I could also see a 3.0VAC input into the RMS Converter chip U303 pin 4, with 13.3VDC out (pin 8) on pretty much any VAC range. That would explain why the display is almost always showing OVLD, as the full scale value for the A/D converter is 10V for any range.So that now leaves me with a faulty U303 AD536A RMS Converter chip, which basically converts any AC input into the equivalent DC RMS value to be fed to the A/D converter. The service manual also shows HP 1826-0893 being the internal PN equivalent. The IC is a 10-pin TO-5 case style. BTW I could not find 1826-0893 listed on any of the HP parts lists out there that I came across.Also couldn't find any sources for the chip itself except for the typical insane price "request a quote" sites.Anybody perhaps have one of these gathering up dust? I know, very long shot...So will also keep an eye on any 3468A's / 3478A's selling "for parts only" but I'm guessing the days of the $25-$35 ugly units are over.That is how I got my own 3468A a couple of years back that in the end turned out that it just needed a good cleaning.