Help me repair Agilent 34401A

[chihaxinh]

Hi All

I have one Agilent 34401A ( old version - Rectangular button ) can't start

- Pressing power button
- Display on with all segment
- DMM have one beep on speaker

But DMM turn off shortly . I checking power supply everything ok , We have +5v , +-18V and +-15V

I check and replace front display panel and cable but have same error !

Many thank you !
Best regards

Nam Nguyen Hai

[TiN]

Did you check cable between front panel and mainboard?
Is there correct ROM with firmware? Front panel runs own processor, so if there is noone talk to it - it will beep and show no data.
Check if there is digital life there.
Make sure no damaged components, leaked electrolyte, bulged caps elsewhere.

[HighVoltage]

It sounds like a very strange problem.

When the DMM turns off by itself, does the power supply also turn off or just the display?
Did you measure the ripple voltage on the rails?
I had one 34401A that was not working because of high ripple at the regulated power supply outputs.

[chihaxinh]

Many thanks for all reply , today i checking 2 Optocouple , U704 and U506 .

U506 have signal out, U704 no signal .

I checking U500 , U501 ( processor ) don't have reset signal .

Reset signal come from U553 ( LM2925 ) , i read datasheet LM2925 - Reset have 2 state ( Low- High ) with Output High = 5V , Low= 0.3V.

If have any error ( unregular ) reset state will be 0V.

I check Power Supply Schematics , see CR553 (BAV99 high vol switching diode ) and CR555 (6,2V zenner) . I measurement CR555 have 5.xxxV ( not 6,2V) . And check CR553 on board , pin 2 & 3 have open.
I think CR553 have problem !
Does anyone run into the same problem?

Many thank you so much !

Best regards
Nam Nguyen Hai

[chihaxinh]

Update : Now everthing ok ! When i running self-test , i have 620 error ( AC rms zero failed ) 

[HighVoltage]

Did you exchange CR553?
Or how did you fix the problem?

In regards to AC RMS zero failed, you should do a calibration for AC 0 with a short block on all 4 input terminals.
This is described in detail in the service manual.
And then check the rest of the calibration.

I had a similar problem and calibrating the zero value helped to fix it.
Good luck.

[chihaxinh]

Hi HighVoltage

I replace C558 connect to pin 4 U553, Reset signal turnback to 5v ! C559 setting time relay, bad cap and reset signal never in high state

I have another 34401a have 612 613 615 617 619 error ! Like you fix one years ago, i replace  U201 but cant fix bug. i think hybrid resistor divider have fail !

Can you show me howto check hybrid resistor divider alive or died?

Many thank you so much!

Best regards

Nam Nguyen Hai

[HighVoltage]

OK chihaxinh,
Interesting, that you had a broken C558 0.1uF cap
One has to wonder, why that one failed?
I had a few meters with broken CR553 and CR555 but I never had a bad U553.
Since I have lots of these meters in use every day, once in a while, one will fail.

The first time repairing a 34401A I was very hesitant, but then it gets easier every time to fix them.

In regards to the U102 Hybrid Resistor...
You should be able to measure the resistance as indicated by the Service Manual.

But first you can measure the current in the different OHM reading with another multimeter.
Since you have a second 34401A meter, you can just switch through the OHM range manually and note the current in each range.
This way, you should be able to track a faulty resistor, without removing it.
Especially since you have a second meter for comparison.

I think, these errors either have to do with a broken Resistor Divider or the voltage supply to the resistor is broken.

Also, if you fix the problem, make sure to re-calibrate the ranges.
Otherwise, if a bad calibration value is stored, this can cause the error to be persistent.
 

[chihaxinh]

Hi HighVoltage

Back to DMM have error 602 , i follow your guide with short all terminal and running calibration, but not help .

I check AC Circuit Schematics and mearsurement powersupply .

Main power supply have +18,7V and -18,24V , drop through CR306 , CR305 for V+ , CR307, CR304 for V- ( you can see in Page 9-11 service manual )

In CR306 - CR305 have drop 3,7V and 3,55V - 15A and 15B have 15.07V
In CR307 - CR304 have drop 3,6V and 3,7V - -15A and -15B have -14,5V

AC out from AD637 RMS convert in AC range have 1,5V ( Pin 4 Q301 to ACGND - very  large - with service manual must have from -10mV to 70mV )

I checked tantalum cap ( C312 C323 for negative supply and C311 C322 for positive supply ) everything OK, no leak no burn ,short or open

With offset go to high in output AD637 , how can i check and reduce offset . Do you think coupling cap C316 - C312 and Cav - C318 0.22uF have broken ?

( From Datasheet AD637 "The value of C AV  and the 25 k? feedback resistor establish the averaging time constant, and solely determines the magnitude  of the rms-to-dc conversion error "
and
"The ac ripple component of averaging error is greatly reduced by increasing the value of the averaging capacitor. However, the value of the averaging capacitor increases exponentially while the settling time increases directly proportion to the value of the averaging capacitor (T S  = 115 ms/µF of averaging capacitance). " )

Many thank so much !
Best regards
Nam Nguyen Hai

[chihaxinh]

More information : When i switching to frequency , i have 50khz with no input signal .

[HighVoltage]

Hi HighVoltage
Back to DMM have error 602 , i follow your guide with short all terminal and running calibration, but not help .

Error 602 is very different: RAM read/write failed
Are you sure it is Error 602?

May be you can add your measured values to a schematic sheet and attach the picture.

[chihaxinh]

Hi HighVoltage

Sory for my typing mistake , it's 620 bug ( AC rms  zero fail) . I using signal gen with 1V pk-pk - Sin 1khz .

DMM Mearsurement 0.679mV AC , but i switching to Frequency - it's show 2,7Khz .


I check dc offset behind coupling cap , 1st stage i have 1,2mV  , 2nd stage i have 0.3mV . R318 rather hot .

DC offset 1st stage value with 300mV RMS in


DC offset 2nd stage with 300mV RMS in



Check Power Supply


Do you thing Frequency Stage have problem ?

Thank you so much !
Best Regards
Nam Nguyen Hai