Author Topic: Agilent 34401A repair help request (showing voltage without input) (FIXED !)  (Read 9182 times)

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Offline HighVoltage

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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 MY47012232

No 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 Ohms
Shorten the input in a 2W setting shows either 20 Ohm, 2 Ohm or 0.3 Ohm

The 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 34401A
I would really appreciate any help.

Where to start?
Thanks for any help
« Last Edit: October 08, 2014, 12:23:04 pm by HighVoltage »
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Offline KJDS

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #1 on: May 12, 2014, 07:50:05 pm »
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.


Offline NANDBlog

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #2 on: May 12, 2014, 07:57:07 pm »
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.
The 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.
 

Offline plesa

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #3 on: May 12, 2014, 08:57:23 pm »
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.
 

Online linux-works

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #4 on: May 12, 2014, 09:30:32 pm »
check the caps.  always check the caps first.  to restore a box, re-capping is one of the first things to be done.
 

Offline HighVoltage

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #5 on: May 12, 2014, 10:53:06 pm »
Ok, thanks for the replies.
I poked around and I think I found a problem already

The service manual can be found here:
http://cp.literature.agilent.com/litweb/pdf/34401-90013.pdf

Page 162 shows the Power Supplies Schematic
I 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.
« Last Edit: May 12, 2014, 10:55:59 pm by HighVoltage »
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Offline HighVoltage

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #6 on: May 12, 2014, 10:54:47 pm »
And here are the voltages I measured
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Online tautech

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #7 on: May 12, 2014, 11:33:38 pm »
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.

Edit
also check ripple & waveforms both sides of regulator to make sure the bridge, e-cap and reg are each doing their job
« Last Edit: May 12, 2014, 11:39:13 pm by tautech »
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Offline free_electron

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #8 on: May 12, 2014, 11:39:18 pm »
check the caps.  always check the caps first.  to restore a box, re-capping is one of the first things to be done.
there 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 exploding

now, the machine rebooting is not normal.
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Online linux-works

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #9 on: May 12, 2014, 11:44:09 pm »
check the caps.  always check the caps first.  to restore a box, re-capping is one of the first things to be done.
there ain't any in a 34401....

well, 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!)
 

Online VK5RC

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #10 on: May 13, 2014, 01:27:45 am »
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
Whoah! Watch where that landed we might need it later.
 

Offline echen1024

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #11 on: May 13, 2014, 01:46:05 am »
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.
« Last Edit: May 13, 2014, 03:37:33 am by echen1024 »
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Offline Vgkid

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #12 on: May 13, 2014, 05:48:39 am »
That seems a bit high for an open high impedance dmm to me.
Can you put a 1meg resistor between the volt input terminals?
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Offline HighVoltage

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Re: Agilent 34401A repair help request (showing voltage without input)
« Reply #13 on: October 08, 2014, 12:22:33 pm »
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 voltage
got 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.

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Offline nukie

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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.
 

Offline HighVoltage

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This will never happen if it was today and if there is it will be hard to obtain.

So true!
I just talked to a calibration lab here in Germany and they also do AGILENT repair work on high end
equipment down to the component level. And guess what, Keysight is becoming one of their best
customers, because even "Agilent / Keysight Service"  can not obtain the service manuals of their
own instruments anymore.
That is why they have to exchange complete PCB's or just exchange the whole instrument.

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Offline NANDBlog

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This will never happen if it was today and if there is it will be hard to obtain.

So true!
I just talked to a calibration lab here in Germany and they also do AGILENT repair work on high end
equipment down to the component level. And guess what, Keysight is becoming one of their best
customers, because even "Agilent / Keysight Service"  can not obtain the service manuals of their
own instruments anymore.
That is why they have to exchange complete PCB's or just exchange the whole instrument.

What? 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.
 


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