Agilent 34401A repair help request (showing voltage without input) (FIXED !) - Page 1
Forum section: electronics / repair

author: HighVoltage
author_rank: Super Contributor
timestamp: May 12, 2014, 07:27:21 pm
content: Hello eevblog community,I recently bought an Agilent 34401A on ebay and it arrived in good physical condition but has a big problem.This is the newest version, with round knobs.Serial Number is MY47012232No input wires connected:After turning it on, it shows a DC Voltage of 2V to 5V and is changing slowly to different values.After a few minutes, it settles to 4.9593 Volts and it sits there very stable.If I shorten the input terminals, the instruments reboots sometimes.But sometimes the value shown, goes to almost perfect 0 mV, as it should be.If I hook up a small battery with 0.886 Volts, it shows the value correctly or the instrument reboots.The 2W Ohm setting first shows a 100Ohm resistor to be 130 Ohms and then slowly settles to 100 OhmsShorten the input in a 2W setting shows either 20 Ohm, 2 Ohm or 0.3 OhmThe Self Test is passed without any problems.Normally I do not repair test gear and this will be my first attempt to repair an Agilent instrument.May be someone can walk me through a repair procedure on this 34401AI would really appreciate any help.Where to start?Thanks for any help

author: KJDS
author_rank: Super Contributor
timestamp: May 12, 2014, 07:50:05 pm
content: There's an old Irish tale, of a man who stopped and asked for directions to a small town, and the reply was, "If I were you, I wouldn't start from here."I similarly believe, that if you want to start out fixing test equipment, a precision 6.5 digit meter isn't the place to start.Having said that, can you do a full set of tests, with various AC and DC voltages, currents and resistor measurements and let us know what it says. I'm not sure if the service manual is available for this, and if it is whether it has schematics, but check the power supply rails, both with a meter and with a scope to make sure they are ok and not noisy. If that all checks out, undo and reseat every connector in the instrument, check electrolytics for swelling and let us know how you get on.

author: tszaboo
author_rank: Super Contributor
timestamp: May 12, 2014, 07:57:07 pm
content: Quote from: KJDS on May 12, 2014, 07:50:05 pmThe 34401 has a full schematic flying around on the internet.I think I would first open it up, look at it for any obvious issues. This includes components without the smoke still inside. Then, you can start poking around the meter on the analog side, with an other multimeter in one hand, and the schematic in the other, looking for a not conducting transistor or a wrong value resistor etc... I think this instrument is actually easy to repair, well if the ASIC still works.

author: plesa
author_rank: Frequent Contributor
timestamp: May 12, 2014, 08:57:23 pm
content: Measure the current flow in the 2wire Ohm measuremet on all ranges, and check for changes in time, and check the current measurement if it works correctly.

author: linux-works
author_rank: Super Contributor
timestamp: May 12, 2014, 09:30:32 pm
content: check the caps. always check the caps first. to restore a box, re-capping is one of the first things to be done.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 12, 2014, 10:53:06 pm
content: Ok, thanks for the replies.I poked around and I think I found a problem alreadyThe service manual can be found here:http://cp.literature.agilent.com/litweb/pdf/34401-90013.pdfPage 162 shows the Power Supplies SchematicI tested those voltages and wrote them down on the attached Schematic excerpt.Obviously the +18 Volts are out of range, by showing 19.01 Volts.When I looked closer, it looks like the LM317 (U551) has been replaced.Or do you think it is still the original LM317 on the board?What would be the next step?Replacing the LM317?What could have been the reason for this regulator to fail in the first place?The big electrolytic capacitors look all good.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 12, 2014, 10:54:47 pm
content: And here are the voltages I measured

author: tautech
author_rank: Super Contributor
timestamp: May 12, 2014, 11:33:38 pm
content: You will have to check R551/552 as their values may have drifted or been "hurt" resulting from the apparent previous V-reg failure.If all looks OK then refer to a 317 datasheet and recalculate the resistor values for 18V out.Editalso check ripple & waveforms both sides of regulator to make sure the bridge, e-cap and reg are each doing their job

author: free_electron
author_rank: Super Contributor
timestamp: May 12, 2014, 11:39:18 pm
content: Quote from: linux-works on May 12, 2014, 09:30:32 pmthere ain't any in a 34401.... it has a few big ones after the rectifier but no electrolytics anywhere else...the two tantalums around the ad637 are prone to explodingnow, the machine rebooting is not normal.

author: linux-works
author_rank: Super Contributor
timestamp: May 12, 2014, 11:44:09 pm
content: Quote from: free_electron on May 12, 2014, 11:39:18 pmwell, there's your problem, right there.(sorry, lol)ok, next is solder connections and offboard connections (go ahead and tell me there aren't any of those, either!)

author: VK5RC
author_rank: Unknown
timestamp: May 13, 2014, 01:27:45 am
content: My 34401 had an ac v reading problem due to a power supply issue, the tant caps can fail, mine was a zener shorting close to open, there is a HP old gear group (yahoo I think), the documentation is good but heavy going, these are high performance machines. The error message I got (on advanced self test) was of no help.As someone told me, go for the "low hanging fruit" ie power supply, caps etc

author: echen1024
author_rank: Super Contributor
timestamp: May 13, 2014, 01:46:05 am
content: Isn't this just High-Z? Resistance seems screwy though, but isn't High-Z on voltage rails like that? To the OP: Put it in manual range and take it up to the highest DCV range. Then see what happens.

author: Vgkid
author_rank: Super Contributor
timestamp: May 13, 2014, 05:48:39 am
content: That seems a bit high for an open high impedance dmm to me.Can you put a 1meg resistor between the volt input terminals?

author: HighVoltage
author_rank: Super Contributor
timestamp: October 08, 2014, 12:22:33 pm
content: After a few month, I finally got around to this repair job again.For the longest time I could not find the problem on this meter.I exchanged the LM317 and the capacitors next to it and the rail voltagegot stable at +18.5V.But every time I turned the instrument on, I got different readings.Now I replaced one of the relays and the instrument works perfectly.Calibration worked on all ranges in all settings.Instrument is repaired and performs like new.Thanks for all the valuable inputs on this forum.

author: nukie
author_rank: Frequent Contributor
timestamp: October 11, 2014, 03:36:09 am
content: The 34401a is a great example of a device that you could learn to repair with not only the freely available schematics but also detailed explanation of how each part of the circuits works. It's the pinnacle of a fantastic piece of instrument coupled with excellent documentation. This will never happen if it was today and if there is it will be hard to obtain.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 11, 2014, 11:32:00 am
content: Quote from: nukie on October 11, 2014, 03:36:09 amSo true!I just talked to a calibration lab here in Germany and they also do AGILENT repair work on high endequipment down to the component level. And guess what, Keysight is becoming one of their bestcustomers, because even "Agilent / Keysight Service" can not obtain the service manuals of theirown instruments anymore.That is why they have to exchange complete PCB's or just exchange the whole instrument.

author: tszaboo
author_rank: Super Contributor
timestamp: October 13, 2014, 03:54:06 pm
content: Quote from: HighVoltage on October 11, 2014, 11:32:00 amWhat? Did corporate BLLSHT go that far? Dont they have a big database for all the manuals ever made, catagorised, made it avaliable on corporate internal network? Are they so afraid someone steals their precious information?This doesnt sound right. Board level swap is usually made to keep the appearance of quality of the instrument (because some repair made by a skilled person can be as good as the original one), or reduce the servicing cost.