HP/Agilent 34401A DMM startup problem - Page 1
Forum section: electronics / testgear

author: dundee
author_rank: Regular Contributor
timestamp: November 15, 2013, 06:42:25 pm
content: Hello there!I have a problem with my HP 34401A DMM.It doesn't startup. The display turns on, it beeps and turn off. Sometimes the relais are flipping around.I already checked the Power supplies, everything seems to be ok. Also i checked the Display Unit - works fine (tested it on another 34401).Here you can see a Video on youtube, what happens if i turn on the device:Anybody ideas?Sorry for my bad English. I'm from Germany.Best RegardsSteffen

author: free_electron
author_rank: Super Contributor
timestamp: November 15, 2013, 08:19:10 pm
content: Check the ground referenced circuitry in the back around the gpib connector.It looks like that has trouble starting and it resets the main controller.These machines ( like the 34970 and many of the power supplies with VFDs ) has a unique way of starting.the back 8051 initialises rs232 and gpib and runs a selftest. In the mean time the front 8051 (on the display board) does a segment test.When both are done the main processor starts.IF either one is defective a restart will occur.that is what you are seeing.it looks like something is overheating. after cold start it takes a long time before restart, then restarts occur faster.check the 7805 regulator in that section if it runs hot.have you been playing with the RS232 connector ? fried the MC14568 RS232 levleshifter chip perhaps ?

author: dundee
author_rank: Regular Contributor
timestamp: November 15, 2013, 09:14:00 pm
content: Hello free_electron!Thank you for answering.The 7805 is working fine 5,XXVolt and no overheating.I bought the device with this fault. Don't know if somebody used the RS232 connector.Can't find the MC14568, only a LT1180A (Low Power 5V RS232 Dual Driver).You can hear three "beeps" in the video. This is not always the case. Sometimes I power on, it beeps, display turns on, turns off and that was is.(no second or third beep)...

author: free_electron
author_rank: Super Contributor
timestamp: November 15, 2013, 09:46:09 pm
content: are the main firmware roms in a socket ?pop em out and reinsert them.resolder the socket pins on the board if needed.it is your main cou not starting within the allowed time and the i/o cpu pulls the reset line

author: dundee
author_rank: Regular Contributor
timestamp: November 15, 2013, 09:55:46 pm
content: No socket's

author: free_electron
author_rank: Super Contributor
timestamp: November 15, 2013, 10:50:18 pm
content: whoa. what is that big blister on the circuit board ?between the big intel processor and the white 3 pin connector, just above the little 8 pin fairchild chip.it looks like a round waffle . that is board delamination !check those interconnections in that area. you may have a broken via in there around the capacitor or the pins of the big cpu.especially since a trace goes to the optocoupler that carries signals to the 8751 in the back portion.if that fails you will see those whacky resets...

author: dundee
author_rank: Regular Contributor
timestamp: November 15, 2013, 11:03:47 pm
content: That's not the mistake... I resoldered the CPU with a heatgun, then the blister apperars.The failure was the same before the blister.

author: free_electron
author_rank: Super Contributor
timestamp: November 16, 2013, 01:03:23 am
content: so you fried the board now too ...

author: razberik
author_rank: Frequent Contributor
timestamp: March 06, 2017, 01:48:15 pm
content: Hello. Sorry for zombie-thread-digging, but I wonder - did you resolve the problem with the DMM ?Thanks.

author: SPAZIONICA
author_rank: Newbie
timestamp: December 25, 2017, 09:51:58 am
content: Hello everyone,my HP34401 had the same problem,I solved by re-doing the welds in the back area PCB in particular I think those of the quartzsee attached jpgBye ByeFrancesco