Advice Wanted: Going to try and repair an HP3478A from eBay :-( - Page 1
Forum section: electronics / repair

author: madshaman
author_rank: Frequent Contributor
timestamp: March 13, 2013, 06:57:39 am
content: Hello everyone, this is my first post.After literally 25 years or so of not messing with electronics I've avidly picked up the hobby again and with great pleasure. Dave, your blog is just amazing and I can't absorb enough of it (and many other online electronics blogs), so thank-you for sharing your knowledge and enthusiasm with the world!One thing I've been doing is beefing up my ever growing home lab and right now am trying to fill it up with some decent bench and handheld meters which have either USB or GPIB connectivity (I have a GPIB to USB converter). The idea is that I want to rig up some hands-off automated testing that I can monitor remotely if I want.Moving forwards, I've been purchasing some old bench meters based on reviews on this blog's forum and other sources.I've had some bad luck; I've purchased a number of HP3478A's from eBay and the first to arrive is flakey. The VDC ramps up constantly with auto-zero on and down with it off (climbs into whole volts and beyond, shorting the leads seems to have no effect). With a voltage applied (a 9V battery), it just seems to ramp up similarly to with no voltage applied but will go further, when it reaches the first auto-range level, it beeps, zeros, and seems to start the whole process again without switching ranges. I haven't tested VAC yet and two-wire resistance measuring seems to show an overload even with the leads shorted. This is all just a quick look I've done, I haven't even brought it up to the lab yet. Before testing the VDC with a voltage, I left it to sit powered on for about an hour, (the non-voltage-applied behaviour didn't change at all after this).After reading Jim William's inspiring "The importance of fixing" at the beginning of The Art and Science of Analog Circuit Design, I figure the best favour I can do for myself is try to fix it (which seems fairly challenging to me), and the hassle of packing the thing up, communicating with the eBay vendor, going to the post office and shipping it back really isn't worth my time versus the cost of the meter (despite being $200 CAD :-( ).My skill level is that of an absolute amateur at this point. When I started back up again, I "got my feet wet" by building a little circuit to PWM drive a bunch of LEDs using an ATMega328P using almost every available IO pin and increasing the PWM resolution by bit-banging it from an interrupt rather than use the built-in PWM; later driving a string of leds via a darlington array. Did everything from the ground up my self, which I know is kid's stuff, but it was a big step for me getting back into this (as a kid, what little I did was all analog).Since then I've built some simple things like an adjustable constant current source and a batery-based driver circuit for an electroluminescent wire using a some opamps and a MOSFET driving a transformer (the latter wasn't a total success, and I have to redesign it completely, the opamps were set up as a trim-able analog PWM generator which I manually tuned to pulse power into the circuit at it's resonant frequency, next step is to use a sine-wave oscillator which is auto-tuned using a PLL (something else I just learned ^^). So I'm a complete newb to analog electronics and electronics in general but love it (so "amateur" is very appropriate).I am fairly comfortable designing any kind of digital circuitry driven from a micro controller. I find this rather easy (which is what makes the analog stuff exciting for me) because I'm a software developer by trade and I've been an assembly language hacker since I was 10 (started on 6809E and 6502), but I lack a *lot* of experience and about a hundred books worth of knowledge. Anyway, just trying to give an overall background. I have a number of half-way decent meters and a 400Mhz dual-trace analog scope (and also a 200Mhz two analog channel and 16-channel logic analyser combo digital scope) and know how to use them. I know my basic circuit theory, know how opamps behave, and can analyse a circuit, that's about it.So, I'm determined to try and trouble-shoot this meter and repair it if possible. What I lack in experience I can hopefully make up for in enthusiasm and the ever-dangerous hubris. I have a tremendous respect for you folk who've been doing this for many years, and all the knowledge and experience you posses, but I want to join you ASAP(even if it takes the rest of my life, which isn't long 'cause I'm pushing forty.Some questions I have about this repair (which I'll probably start this weekend):1) Has anyone seen this kind of behaviour before? What can it point to?2) I read in the service manual that one may need to perform tests on the meter's digital components using a "signature analyser", I actually don't know what that is, my *guess* is that it drives a baked in self-test of sorts and records a series of bits on one pin which generates the "signature". Is that what it does? Is the information available for me to build one using a uController? Is it worth it? (I don't want to try and buy a used one and end up in the unfortunate situation of trying to fix one broken thing with another broken thing I don't know is broken, maybe later, but that's too hardcore for me at the moment).3) Any advice at where to start? I do plan to go over the schematic, measure voltages where I can and try and figure out what could be wrong based on the overall block diagram, but guidance is much appreciated.Sorry for the long post, it's kind of a half introduction and half question. Thanks for reading!

author: psycho0815
author_rank: Regular Contributor
timestamp: March 13, 2013, 07:49:56 am
content: I'm not very experienced with fixing thing, so this is little more than guessing, but first thing i would do is check the voltages, The service manual should tell you where you're supposed to have what voltage. Also from Experience with other digital stuff, I would have a close look at the caps, maybe check the capacitance if you can do so in circuit. I had the weirdest failures caused by noisy voltages. Also have you tried googling the problem? maybe it's a known error.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 13, 2013, 08:02:42 am
content: Thanks! That's pretty good advice. I'd be ecstatic if it was a bad cap. Since you mentioned noisy voltages it sounds like a good idea to probe the voltages with a scope after I test with the meter; that should have occurred to me, but I now realise it's possible the meter will show the right voltage but not some oscillation riding on it. Weird thing is, the self test passes fine, and I don't think it should given the state of the thing. Haven't looked inside yet, hope I don't see a half-corroded mess.I'm really hoping it's not the ADC, because it looks like I'm screwed if it is. Hopefully I'll learn at least one thing doing this.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 13, 2013, 08:09:00 am
content: (P.S. I did google around, maybe I'm not using the right searches, but nothing relevant was found so far)

author: grumpydoc
author_rank: Super Contributor
timestamp: March 13, 2013, 09:23:06 am
content: Not fixed one of these myself - though do have a (little used) one on the bench.Have you got a service manual?What test equipment do you have available?As above check all the supply voltages first - nothing's going to work if there's a volt of ripple on one of the power rails.Also check the lithium battery - should be 3V, if not the unit will have lost its calibration data. Not sure whether this would cause the symptoms that you describe though.Does it respond to the front panel buttons, set ranges etc as expected - if so at least the microprocessor is probably OK.For fault finding I'd turn auto ranging off, and get the DC volts range working first.Then read the theory of operation - these things basically time various voltages charging and discharging a capacitor to work out what the unknown voltage applied to the terminals is. One of the more knowledgable regulars (free_electron, I think) also wrote up a good description in a recent thread (can't remember which forum topic I'm afraid).Then (if you have a 'scope) start looking at the waveforms and voltages in the measuring cicuits.

author: vtl
author_rank: Regular Contributor
timestamp: March 13, 2013, 11:15:09 am
content: Does the thing pass self test when you turn it on? Any error messages can be helpful in determining the problem

author: madshaman
author_rank: Frequent Contributor
timestamp: March 13, 2013, 03:45:43 pm
content: Quote from: grumpydoc on March 13, 2013, 09:23:06 amYes, in PDF.Quote from: grumpydoc on March 13, 2013, 09:23:06 amAnalog scope (400 MHz), DSO (200 Mhz), Logic analyser, LCR meter, Function Generator, Various multimeters, Variac, loads of parts.Quote from: grumpydoc on March 13, 2013, 09:23:06 amWill do, although the service manual says self test does a checksum test against the calibration data RAM.Quote from: grumpydoc on March 13, 2013, 09:23:06 amYes, all user input seems to be respected, the LCD panel doesn't show any signs other than being driven properly. A couple strange things: the self test seems to complete *really* quickly. It's supposed to cycle "SELF TEST" "SELF TEST OK" "GPIB ADDR: xxxx", but "SELF TEST" flashes by in 1/60th of a second or so and so does the GPIB address display. only the "SELF TEST OK" is displayed long enough to be read; the other strange thing is that the display update occasionally jitters, that is, seems to hang then resume (not sure if I remember if this happens with auto-ranging off, so that might just be the auto-ranging being flakey). I don't have a working one yet to compare this to, still, maybe something's going wrong with the main clock.Quote from: grumpydoc on March 13, 2013, 09:23:06 amThank-you, sounds reasonable, I'll probably start from there.Quote from: grumpydoc on March 13, 2013, 09:23:06 amThanks for the advice, might just start this tonight no matter how tired I am. Bench is a ruin though, need to clean first...

author: madshaman
author_rank: Frequent Contributor
timestamp: March 13, 2013, 03:46:10 pm
content: Quote from: vtl on March 13, 2013, 11:15:09 amUnfortunately, it passes.

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: March 13, 2013, 04:10:45 pm
content: I haven't worked on one of these myself or seen the service manual but historically, HP has produced the most wonderful service manuals on much of their test gear. As stated above, check the caps, check voltages and once you have the simple stuff out of the way (assuming you've not found the issue), go through the test procedures in the manual step-by-step and you should find your issue (from what you've described, you should have the gear and the basic knowledge to succeed - just keep at it and don't give up). GL

author: vtl
author_rank: Regular Contributor
timestamp: March 14, 2013, 01:24:15 am
content: Does the meter respond at all to any inputs at the terminals? This might sound dumb but are the rear inputs currently selected by the switch at the front?

author: PaulAm
author_rank: Frequent Contributor
timestamp: March 14, 2013, 01:57:49 am
content: For a while around that time period signature analysis was really big. You'd hook these things up, they would look at the data over some timeframe and produce a signature which you could compare with the manual. The good news is these tend to go cheap since nobody uses them anymore. A decent logic analyzer will give you the same info, just not in a compact form that is in the manual.There's a couple hp 5004 signature analyzers on ebay now. One sold recently on a buyitnow for $40 will all the accessories. If you go for one of these, make sure it's the model required by your manual.I haven't looked at the manual, but if it passes the self test the problem is likely in the analog side somewhere.Good luck. HP made awesome gear and it's amazingly affordable now.

author: SteveH
author_rank: Contributor
timestamp: March 14, 2013, 04:56:42 am
content: madshaman, check that the front/rear terminal switch is set to use the front terminals. I got caught out by this on my 3478.Cheers,Steve.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 14, 2013, 05:26:52 am
content: Quote from: PaulAm on March 14, 2013, 01:57:49 amThanks Paul.Yeah, I *might* buy one, I'd prefer to have the functional specs so I could build one though :-), hopefully someone'll read this and can provide; might be nice to have an all in one signature analyser. Although iirc the service manual simply says "acquire a signature analyser", leads me to believe they all did the same thing?Didn't even finish clearing my workspace tonight and it's band night tomorrow night, looks like I won't be able to start until Friday or Sunday :-(, Wish I didn't have a day job.. Still plan to film myself, maybe do this from now on instead of keeping a logbook.I *really* hope it's something simple like a bad cap, but then again, I'll learn more if it's worse (so long as it's fixable).On the chance I'm mad enough to attempt it and the problem is a custom IC, what are the chances of ever acquiring the info required to reproduce an equivalent using a uC or CPLD or something?

author: casinada
author_rank: Frequent Contributor
timestamp: March 14, 2013, 06:33:57 am
content: I would recommend you to download the HP Service manuals:http://www.ko4bb.com/manuals/index.php?dir=HP_Agilent/HP_3478A_MultimeterAlso join the HP_Agilent group at Yahoohttp://tech.groups.yahoo.com/group/hp_agilent_equipment/I also have an HP3478A that works perfectly except for the OHMS part but I'm too lazy to open it as I have other Bench meters with 2 and 4 terminal OHM capability.Between the manual, this forum, the yahoo group and some sweat equity you should be able to find what's wrong with your meter.Good luck.

author: grumpydoc
author_rank: Super Contributor
timestamp: March 14, 2013, 08:23:28 am
content: Given that it passes the self-test it might not be all that serious.QuoteConsider "bad caps" and they can cause just plain weird problems sometime - of which I was reminded last night.I have a Racal 1991 on the bench at the mo. When I got it the fault was that it just sat there displaying a random frequency even with no input. Looking at the 10MHz out there was a huge amount of jitter on the clock trailing edge and, opening it up, about 200mV ripple on the 5V supply to the oscillator (which is separate to the main 5V rail but taken from the same filter cap).So I changed the main filter caps - except the big 4-pin 10,000uF one as the cheapest I've found these is nearly 15quid so that stays unless it's broke. It measured OK, the ripple waveform looked OK and temporarily swapping it for another, new, cap didn't change anything. I could set the 'scope and sig-gen up to measure its ESR but can't be bothered at present. The bottom line is that it doesn't look to be the culprit.There was no change on replacing the big caps so I turned to the small 47uF post-regulator caps. Swapping the one on the oscillator supply banished the ripple and cleaned up the clock signal but didn't change the random numbers. Swapping the ones on the +/- 5V rails stopped the random display -even though the rails looked OK for ripple and noise on the 'scope.The remaining fault is now "low sensitivity" - it needs 100mV for a stable count but it should be (a lot) less than 25mV - but I'll bet if I replace all the other 47uF caps that will go away. Even if it doesn't I have to be sure the decoupling/filtering on the rails is good before going further.So, always be prepared for a wholesale cap swap in older test equipment.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 18, 2013, 05:22:43 pm
content: Okay, so last night I finally got around to troubleshooting this meter. I went to some trouble to mount cameras facing the oscilloscope, the test meters and pointing down onto the HP3478A because I wanted to record a video and share with others. Well...First, a little side-note, there's an error in the service manual, but not a big one. At the beginning of "Service Group A, DC Volts and DC Current Troubleshooting", section 7-A-4, you find the following text:QuoteMaybe there's another way to interpret the text, but U405 pin 6 (U405 is an opamp with output at pin 6) should be at-10V,not+10V. As I was happily measuring voltages and writing down the results in my logbook, when I came to U405, I paused... Then I went and looked at the schematic and it's very clear that it should be at -10V, not withstanding that the supply rails for U405 are Ground and -15V! I downloaded the service manual directly from Agilent's website.Okay,that'snot the interesting part. After I'd measured nine planned voltages with my meter (and numerous others I thought would be useful), and then probed each point with my oscilloscope to ensure there wasn't any noise; apart from something I saw on the scope which I'll add at the end of this post, I saw nothing unusual.So, since I figured I'd have surely seen *something* amiss by this point, I decided to hook the 3478A to my bench supply at 12V through the back panel, and you know what, it !&@&$ worked and matched exactly the reading on my Fluke 27/FM (which is fairly recently calibrated). Disconnecting the power supply, I looked to see if the weird upward/downward voltage drift with no input was still happening; NOPE!Stunned, I reconnected the bench supply to the front panel to see if maybe the front/back-panel switching mechanism was flakey. Nope, it !@%#$ worked there too! (to those who'd asked before, yes, I did test both the front and back panels when I originally tested the meter; see my first post)To see if auto-ranging was working as well as the millivolt range, I switched back to the back panel (more convenient the way I had things on the bench), hooked up the bench supply at 12V and toggled off the output of the bench supply to let it's output caps slowly drain (I know, maybe not the best behaviour for this bench meter, but at almost no current drain it takes a while for the voltage to drop to zero after toggling off the output, and hell, it makes for a convenient test), the HP3478A and the Fluke 27 metering this in parallel.They tracked each other perfectly all the way down to 0.1 millivolts (as low as the Fluke goes). So, it seems my meter "magically" repaired itself. Well, there's no such thing as magic, so here's my best explanation of what happened, please let me know if this sounds reasonable:During shipping, some kind of shock or vibration caused an already dubious connection between some component and the PCB to become flakey. Because I was probing many points, and of course pressing the probe firmly either against or down on the pins, I probably restored a flakey connection somewhere, thus restoring the meter to working condition.Does that make any sense? Can anyone think of an alternate explanation?P.S. some additional notes/questions:1) Apart from a faint 5-15 mV ripple at roughly 100Mhz which appeared everywhere, I saw nothing unusual, I'm not sure where this noise came from, I put all my wireless devices into airplane mode and shut down the lab computer, but I figured the noise was some oscillation either in the probe, the scope, or the combination of all driven by the environment. My best guess is that what I was seeing was the whole combo filtering environmental RF noise, as when I had my probe out of the semi-shielded enclosure of the meter, the scope showed high frequency broader-band noise (at 2mv/div and the intensity cranked up) which attenuated when I dipped the probe into the semi-shielded 3478A's enclosure. The 100Mhz signal (which looked like it had a third harmonic on top of it) would appear when probing any point within the meter. I'm still learning RF electronics and am still lacking a lot of experience, but does my thought that the whole setup was simply resonating at ~= 100Mhz driven by external RF noise make any sense at all? If not, what else might I have been seeing?2) The voltage reference of the meter was actually 10.18V instead of 10.00V, but I believe this is fine as long as it's stable and the meter has good calibration data. Is that correct?3) When I was setting up my camera to film the scope (Tektronix 475, 400Mhz), I noticed that channel 2 *just* started acting flakey, it was fine earlier in the week :-(, it seems to overshoot on the internal calibration signal and jump around in amplitude, i.e. flakey, this only occurs at more sensitive inputs, iirc 0.2V/div or less, at larger attenuations, it's perfectly fine :-(. Why does one piece of equipment have to break when I'm trying to fix another... I'll probably be posting for help trying to fix the scope now, which, for me is kind of intimidating, but I have been reading about scope front-end design, so hopefully I'll be up for the challenge.

author: Smokey
author_rank: Super Contributor
timestamp: March 18, 2013, 05:44:17 pm
content: Welcome to the wonderful world of ebay gear and troubleshooting intermittent problems. You can just hear the time bomb ticking?? ... tick... tick... tick... tick...Good job digging in. I'm surprised Free_Electron hasn't posted a solution to your exact problem off the top of his head yet

author: madshaman
author_rank: Frequent Contributor
timestamp: March 18, 2013, 06:17:54 pm
content: Quote from: Smokey on March 18, 2013, 05:44:17 pmThanks! :-). I've been reading "The Art and Science of Analog Circuit Design", and wow, this is just an *awesome* book, I can't suck it down fast enough, so, as painful as it is, and as many nights of no sleep I'll get before my day job, I'm actually looking forward to some broken stuff, just so long as I always have a functional alternate lying around. Oscilloscope repair kind of scares me at the moment, but I'll have to get my feet wet at some point.I have to say, since getting back into electronics, I'm quickly becoming a stone cold addict.

author: ddavidebor
author_rank: Super Contributor
timestamp: March 18, 2013, 06:18:26 pm
content: Quote from: madshaman on March 13, 2013, 06:57:39 amFirst, read the service manual.Second, read it another time.Third, just for be sure, read it again.

author: SteveH
author_rank: Contributor
timestamp: March 19, 2013, 04:52:08 am
content: madshaman, please direct me to that part of your first post that says you checked both sets of terminals and the switch setting. Thanks.Regards,Steve.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 19, 2013, 10:18:06 pm
content: Quote from: SteveH on March 19, 2013, 04:52:08 amYou're right. I actually thought I'd included that detail, looks like I didn't. But yes, when it wasn't working initially I went through every possible permutation to see if I could get any normal behaviour out of either end of it, all except for AC volts.