HP 34401a error 624 - Page 1
Forum section: electronics / repair

author: MicroEmbedded
author_rank: Contributor
timestamp: May 30, 2019, 02:48:55 am
content: 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?

author: TenderTendon
author_rank: Newbie
timestamp: May 30, 2019, 04:43:42 am
content: You can download the service manual here:https://xdevs.com/doc/HP_Agilent_Keysight/service/34401A/34401A%20service.pdfYou can locate the component on page 9-5. I would start by checking R555, C555 and CR554

author: MicroEmbedded
author_rank: Contributor
timestamp: May 30, 2019, 05:41:03 am
content: 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.

author: MicroEmbedded
author_rank: Contributor
timestamp: June 03, 2019, 08:14:11 pm
content: I ordered and replaced C555 R555 and CR554 still have 624 error.Per manual:• Self-test 624, determines the correct power-line frequency for themeasurement section.Unable to sense line frequency This test checks that the LSENSElogic input to U500 is toggling. If no logic input is detected, the instrumentwill 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?

author: coromonadalix
author_rank: Super Contributor
timestamp: June 03, 2019, 08:50:53 pm
content: 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 manyWhat 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

author: MicroEmbedded
author_rank: Contributor
timestamp: June 04, 2019, 12:46:57 am
content: I'm not getting + and - 18V out of U552 and U551. I might try replacing c553 and c551

author: TenderTendon
author_rank: Newbie
timestamp: June 04, 2019, 03:53:59 pm
content: 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.

author: MicroEmbedded
author_rank: Contributor
timestamp: June 04, 2019, 07:33:55 pm
content: From schematic 9-15:Red P551-8 is .9VACRed P551-6 is 8.2VAC

author: MicroEmbedded
author_rank: Contributor
timestamp: June 04, 2019, 07:36:18 pm
content: Voltages from red wires don't seem right I might need to replace it.

author: coromonadalix
author_rank: Super Contributor
timestamp: June 04, 2019, 08:32:01 pm
content: 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 transformerThey sell around 80$ usd, you have originals or clones on Ebay, i bought one, months ago directly at KeysightYou need to do a good check up of all the main dc voltages lines

author: TenderTendon
author_rank: Newbie
timestamp: June 05, 2019, 03:38:41 am
content: What is the AC voltage between the 2 red wires ( pins 6 & 8 )?

author: MicroEmbedded
author_rank: Contributor
timestamp: June 05, 2019, 02:31:13 pm
content: Quote from: TenderTendon on June 05, 2019, 03:38:41 amFrom GND to pin 8 is .9VACFrom GND to pin 6 is 8.2VACI will measure the Voltage across the 2

author: MicroEmbedded
author_rank: Contributor
timestamp: June 05, 2019, 03:07:31 pm
content: Voltage between pin 6 and 8 is 20.3 VAC

author: coromonadalix
author_rank: Super Contributor
timestamp: June 05, 2019, 08:18:07 pm
content: 34401a transformer voltages and impedancesSecondaries Voltagesreds only = 45vacred and white = 22vac white being a center tap for te redsoranges = 9vacgreens = 4.8vacgreen and blue = 2.4vac blue being a center tap for the greensSecondaries Ohms Valuesgreen to green = 3.83 ohmsoranges = 2.28 ohmsred to red = 25.2 ohmsPrimary Ohms values100 vac = 45.7 ohms120 vac = 54.6 ohms220 vac = 119.5 ohms240 vac = 132.3 ohmsI 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 capsI've done them in my 2 meters, just as a precaution, and put Nichicon caps.

author: MicroEmbedded
author_rank: Contributor
timestamp: June 06, 2019, 12:28:25 am
content: 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 Voltagesreds only = 20VACred and white = 22vac to Pin 8 2mVAC to Pin 6 20.1VACoranges = 8.9VACgreens = 5.1VACgreen and blue = 2.553 and 2.551Secondaries Ohms Valuesgreen to green = 4.2 ohmsoranges = 2.3 ohmsred to red = OPENPrimary Ohms values100 vac = 44.3 ohms120 vac = 52.6 ohms220 vac = 118.8 ohms240 vac = 131.6 ohms

author: coromonadalix
author_rank: Super Contributor
timestamp: June 06, 2019, 01:45:00 am
content: 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 recallAre you able to resolder a wire on the broken x-former wire

author: MicroEmbedded
author_rank: Contributor
timestamp: June 06, 2019, 08:26:26 pm
content: Thanks guys, I went ahead and resolder the wire coming out of the transformer to the other end that got detached. no more errors

author: TenderTendon
author_rank: Newbie
timestamp: June 11, 2019, 04:03:11 pm
content: That's great news. Enjoy the meter!