Seeking HP 3456A multimeter troubleshooting advice (repaired!)

[SharpEars]

Yes this is turning into a high risk repair job. I got the unit for $80 though including shipping, so if I'm willing to spend maybe another $20 before calling it a day and selling off the parts (or the unit as a whole to someone with a scope)...

[MadTux]

In which continent do you live? In case it's Europe, I could repair it for you for some cash or would buy it before it ends up as scrap

[SharpEars]

U28 removal update. I have successfully removed the chip. Thanks to everyone who suggested to clip the pins prior to suction desoldering them.

Pics or it didn't happen:

Front:

Full size image

Rear:

Full size image

I hope I did a reasonable job and didn't damage any attached pads too much.

Update: Replacements have been ordered: eBay item link

[timb]

Hey SharpEars, I've got an Xprotolab I can send you, if it would help. It's not much (2x200kHz), but it's something!


Sent from my Smartphone

[MarkPalmer]

Your desoldering looks very clean.  Should be easy to drop and solder the new DIP in.  Will keep fingers crossed for it to work 

-Mark-

[MarkL]

For whoever asked about the unit specifics, it's a 2201A1xxxx that apparently went dead before the electrolytics gave out (since the voltages are all great - ripple aside, since I have no way to test that).

I realize the debugging focus is elsewhere at the moment, but I wanted to point out that you can measure ripple with your DMM set on AC.  It will measure just the AC component in the presence of DC.  The limitation is that you won't see any readings if the ripple is out of its frequency range (like with a switcher), but it will at least show you any line frequency components from bad filter caps in a linear supply like this one.

[SharpEars]

I realize the debugging focus is elsewhere at the moment, but I wanted to point out that you can measure ripple with your DMM set on AC.  It will measure just the AC component in the presence of DC.  The limitation is that you won't see any readings if the ripple is out of its frequency range (like with a switcher), but it will at least show you any line frequency components from bad filter caps in a linear supply like this one.

Interesting, I'll have to try that. I've always thought that if you set the setting to AC for a DC circuit, you'll just get some bogus value that assumes your DC is a sine wave when it isn't. I didn't know that there would be a capacitor in series with the circuit (or something) to filter out the DC. Speaking of which, maybe a capacitor in series with the DC (and some alligator clips for connecting it) is a good way to measure for ripple - not sure if that would work though or what value of capacitance to use if it would.

[SharpEars]

Your desoldering looks very clean.  Should be easy to drop and solder the new DIP in.  Will keep fingers crossed for it to work 

-Mark-

Mark, thanks for the kind words. I literally did this in about 15 minutes total. The idea to clip the leads of the original IC made the job so much easier. I used a special very fine nose diagonal clipper/cutter to get in between the pins. I think just the (careful) clipping took a quarter of the total time. Then it was all desoldering iron with bulb and technique to get me through the rest. Most of the time was spent on pins that were rotated/wedged in the through holes (i.e., difficult to suck out, requiring multiple attempts and unfortunately damage to unconnected pads).

I tried to apply heat to the side with unconnected pads, but sadly many of the pins could only be sucked out from the top due to being thicker and wider on the front as compared to the back. Sometimes I had to apply heat to both sides to get all of the solder out. Sadly, if you look at the bottom right pin in the (from the bottom) picture, I partially damaged the trace going to it. I will have to fix that up with solder when I put in the new chip and check for a proper low resistance connection that leverages the full width of that (power?) trace.

[MarkL]

I realize the debugging focus is elsewhere at the moment, but I wanted to point out that you can measure ripple with your DMM set on AC.  It will measure just the AC component in the presence of DC.  The limitation is that you won't see any readings if the ripple is out of its frequency range (like with a switcher), but it will at least show you any line frequency components from bad filter caps in a linear supply like this one.

Interesting, I'll have to try that. I've always thought that if you set the setting to AC for a DC circuit, you'll just get some bogus value that assumes your DC is a sine wave when it isn't. I didn't know that there would be a capacitor in series with the circuit (or something) to filter out the DC. Speaking of which, maybe a capacitor in series with the DC (and some alligator clips for connecting it) is a good way to measure for ripple - not sure if that would work though or what value of capacitance to use if it would.

Well, I'll update my statement to "should".  On old analog meters what you say is the true about displaying a bogus value.  But on a modern DMM (at least all the ones I've worked with), there's a blocking capacitor or filtering circuit as you describe.

If you have a true RMS meter, you'll get a reading of the actual AC ripple.  If the meter has no such claim, you'll probably get some reading computed from the peak with the assumption it's a sine wave.  In either case, you'll get some kind of a reading and can determine if there's a possible problem with a filter cap.

If you're unsure of your meter try measuring AC voltage on a battery.  You should get 0V.

If you use your own blocking cap beware of DC leakage through the cap.  It may be significant given the high impedance of modern meters.  This can be minimized by shunting the meter with a lower resistance (say 10k or 100k), but then you've got a frequency-dependent RC circuit to take into account.

[SharpEars]

Raise your hand if you bet on U28 on board A4  :



And it's calibrated (at least for low volt DC, 100 mV - 10 V)! Now, what do I do with the other nine 74ls377 chips?

Virtual beers to all...

Update: Just checked full DC calibration and it's friggin' perfect (I'm talking <5 ppm for all ranges save the 100 millivolts range that's off by around 10 uV and jumps quite a bit at 6 1/2 digits - it's a hard range to test). This was with the voltage standard I got last week from voltagestandard.com (within 1 week of calibration via a NIST certified Agilent 3458A). The 3456A matched all settings but the 100 millivolt one to within 5 ppm, the 5 and 10 V ones to within 2 ppm. The 5-digit Data Precision voltage standard in the pic is a tiny bit off. I can use the 3456A to calibrate it - the irony!

So, best $87 ($80 for the HP 3456A and $7 for the ten replacement 74ls377 chips of which I used one) I ever spent! OK, I guess the 8-10 hours I spent on the troubleshooting and repair have to be mentioned, but it was a good learning experience for me.

Update 2: Just measured a 480 ohm 1% metal film resistor to get a feel for the state of the resistance section and the reading came within 4 ppm of my Fluke 8506A. (476.404 ohms on one and 476.408 ohms on the other).

I am beginning to get the feeling that this unit was calibrated in the not too distant past and that that 74ls377 blew on the main board and it wasn't used since. I just wish it was in better cosmetic condition, but hey beggars can't be choosers  .

Update 3: Found the "calibration on the unit's" Achilles Heel, the AC. It's showing about 1.5 V too high at 120 V US line voltage, compared to three other meters I have, which are all within a quarter volt of each other. I guess that can use some calibration in the future, but I don't measure AC all that much and have no way to really calibrate it either.

[Vgkid]

good job

[saturation]

   Most excellent.  Also a new reference for any such failure in the future.  I haven't found a similar issue searching the net in the past many years, i.e., failure caused by that flip flop.   

[HighVoltage]

Congratulations on the repair and thanks for the details.
Sounds like a real bargain.

[SharpEars]

  Most excellent.  Also a new reference for any such failure in the future.  I haven't found a similar issue searching the net in the past many years, i.e., failure caused by that flip flop.   

I am very happy that in this thread a fairly thorough troubleshooting/repair guide has been created for others with display issues. Almost all of the other HP 3456A repair info on the net seems to deal with bad capacitors and PROMs and most of the info is rather vague (i.e., "I replaced the caps on the power supply board and now it works!"). Surely, that's not the only thing that can go wrong on these units after 25-30 years.

[MarkL]

Your initial characterization of the problem was key to narrowing down the possibilities immediately.  Getting a complete understanding of what works and what doesn't, and how it behaves with various stimuli is time well spent.  Nicely done.

I never take anything off the "possibilities list", but your description put power supply related issues very low on the list.  Absolutely check all power supply levels before proceeding with more in-depth debugging, but the knee-jerk reaction to replace all caps just because it's old equipment is not the right approach.

I really like these old 3456A meters.  In their prime in the 80's, they were touted by HP as transfer standards and sold for US$4100 (1987 catalog).  They're built like tanks and they don't have any NVRAM to go bad.  You just have to have the bench space for them.

Enjoy!

[MarkPalmer]

Nice work   It's always satisfying when you get something working again.  It can and does happen occasionally, but it's very rare if electrolytics in 1980's HP equipment need to be replaced, and I would never do it just as a precautionary measure.  I will remove and test them if they raise suspicion, and only if they fail for being out of tolerance or high ESR or current leakage do they get replaced. If they test OK they go right back in. For the bunches of 1980's HP equipment I have, only a few have ever needed replacement.  In fact a piece of HP gear I'm working on now from 1962 has only needed about 50% of its electrolytics replaced, I think that from the piece of equipment sitting dormant for so long. 

-Mark-