HP/Agilent 34401A DMM startup problem

[dundee]

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 Regards
Steffen

[free_electron]

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 ?

[dundee]

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)...

[free_electron]

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

[dundee]

No socket's

[free_electron]

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...

[dundee]

That's not the mistake... I resoldered the CPU with a heatgun, then the blister apperars.

The failure was the same before the blister.

[free_electron]

 

so you fried the board now too ...

[razberik]

Hello. Sorry for zombie-thread-digging, but I wonder - did you resolve the problem with the DMM ?
Thanks.

[SPAZIONICA]

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 quartz
see attached jpg

Bye Bye

Francesco