HP 34401a error 624

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HI everyone, first time posting here.  I recently purchased a 34401a from ebay knowing it had an error message on the front display.

My research through the manual points me to U500 but no reference designator are on the board. Can anybody help me find u500?

[TenderTendon]

You can download the service manual here: https://xdevs.com/doc/HP_Agilent_Keysight/service/34401A/34401A%20service.pdf

You can locate the component on page 9-5. I would start by checking R555, C555 and CR554

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Thanks for the manual and helping me find U500. 

A little bit more background about my troubleshooting and findings.  Once I opened it, I found out the transformer was loose (swinging) and missing one of the mounting bolts which I later found all the way across case so it probably shorted a few things along the way.

Per manual: Code 624 Unable to sense line frequency.

I will check those parts TenderTendon suggested and will report test results.

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I ordered and replaced C555 R555 and CR554 still have 624 error.

Per manual:

• Self-test 624, determines the correct power-line frequency for the
measurement section.

Unable to sense line frequency This test checks that the LSENSE
logic input to U500 is toggling. If no logic input is detected, the instrument
will assume 50 Hz line operation for all future measurements.

I checked the fuses and the found out the input current fuse(3A) in the back was blown.

Anybodyelse has any suggestions?

[coromonadalix]

As  written by other members  you have to follow the sense line up to the u500 chip, and test every component you find on the way ... yeah you dont have many


What do you have on pin 24 of the u500 mcu ??   normally you shoud have a clamped / amplitude limitted 50-60 hz signal because of cr554 (6.2v zenner)

A good trick would be to use a signal generator with 3-4 volts peak to peak signal to do some tests ??

Unless u500 is busted in some ways ??  or the zenner is pulling the signal to the ground ??  or maybe there is some smt component missing or broken, because of the swinging x-former loose screw ?? 

Page 152  schema 9-5  could help ?



On pin 24 of my meter  u500 gives 2.79 vac at 60hz.   And powered off completely, (no ac cord present), i have around 42k ohm of resistance going to the ground

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I'm not getting + and - 18V out of U552 and U551.  I might try replacing c553 and c551

[TenderTendon]

Check for AC voltage coming out of transformer (2 red wires), and each red wire to ground.
If voltage is present on transformer, check if AC voltage is present on pins 3 and 4 of bridge rectifier C551. If not, check for cold solder joint on PCB pug connector or burnt trace between plug connector and rectifier.

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From schematic 9-15:
Red P551-8 is .9VAC
Red P551-6 is 8.2VAC

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Voltages from red wires don't seem right I might need to replace it.

[coromonadalix]

For the red wire you do take pin 7 as a ground for the dc voltage   and measure pin 6 and 8 for the ac voltage ...   disconnect the xformer secondary connector and measure all of them in ohms , you may have a defective transformer

They sell around 80$ usd, you have originals or clones on Ebay, i bought one,  months ago directly at Keysight

You need to do a good check up of all the main dc voltages lines

[TenderTendon]

What is the AC voltage between the 2 red wires ( pins 6 & 8 )?

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What is the AC voltage between the 2 red wires ( pins 6 & 8 )?

From GND to pin 8 is .9VAC
From GND to pin 6 is 8.2VAC
I will measure the Voltage across the 2

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Voltage between pin 6 and 8 is 20.3 VAC

[coromonadalix]

34401a transformer voltages and impedances

Secondaries Voltages
reds  only = 45vac
red and white = 22vac   white being a center tap for te reds
oranges = 9vac
greens = 4.8vac
green and blue = 2.4vac   blue being a center tap for the greens

Secondaries Ohms Values
green to green = 3.83 ohms
oranges = 2.28 ohms
red to red = 25.2 ohms

Primary Ohms values
100 vac = 45.7 ohms
120 vac = 54.6 ohms
220 vac = 119.5 ohms
240 vac = 132.3 ohms


I strongly suggest hot air station to removes any smt tantalums, the pcb traces may lift, remove the big electrolytics first, change the smt and put in last the big caps
I've done them in my 2 meters, just as a precaution, and put Nichicon caps.

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Thanks coromonadalix! measured values confirms it's the transformer since there's an open between reds.  Makes sense since once I removed some of the tape you can see a disconnected winding cable.  I'll call keysight once funds are available and order a new one.

Secondaries Voltages
reds  only = 20VAC
red and white = 22vac  to Pin 8 2mVAC to Pin 6 20.1VAC
oranges = 8.9VAC
greens = 5.1VAC
green and blue = 2.553 and 2.551

Secondaries Ohms Values
green to green = 4.2 ohms
oranges = 2.3 ohms
red to red = OPEN

Primary Ohms values
100 vac = 44.3 ohms
120 vac = 52.6 ohms
220 vac = 118.8 ohms
240 vac = 131.6 ohms

[coromonadalix]

You have them too on ebay, but they are clones of the originals Keysight and they sell for the same price, and with keysight you have free shipping over 100$  if i recall

Are you able to resolder a wire on the broken x-former wire

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Thanks guys, I went ahead and resolder the wire coming out of the transformer to the other end that got detached. no more errors

[TenderTendon]

That's great news. Enjoy the meter!