Crazy HP 3478A multimeter

[ik5mic]

Hello,

my working HP 3478A multimeter has become crazy.

At first, last week, it started with U.C. ROM FAILED message at boot, immediately recovered (functional multimeter). After a few minutes of warm-up, and cycling again off-on, the error disappeared.

Now it always displays SELF TEST, U.C. ROM FAILED, and several other messages (e.g.: A.D. LINK FAIL, U.C. RAM FAILED), then after a few seconds the display shows garbage. While the display changes, the input relays switch in a random sequence.

I have checked the power supply lines and ESR for capacitors, but everything is O.K..

Any suggestions for further investigation? Similar experiences?

Thank you and regards.

Alessandro

[MarkL]

Have you worked through any of section 7-D on processor troubleshooting in the service manual?

  http://bama.edebris.com/manuals/hp/3478a/

If not, start there.

What equipment do you have available for debugging this?  Scope?  Logic analyzer?

[yo0]

mine show u.c. rom fail with every power up but go straight to measurement mode after that, measurements seems ok and on par with other dmms, other than that a this time no other reported fail at boot.



no idea where to look. cold solders, battery perhaps, relays? 

any ideas or opinions will be really appreciated.

best regards.


Pio

[coromonadalix]

The start line is :  check all voltages and ripple on them if yo have the right equipment,  maybe your need some recapping if its not done ... check every supply lines going to the ic's

Check the battery voltage, once the battery die you loose calibration

There is a soft made to backup your calibration  before loosing everything
https://www.eevblog.com/forum/testgear/free-hp3478a-multimeter-control-program/

You can build a vey cheap arduino gpib adapter to work with your meter
https://www.eevblog.com/forum/projects/ar488-arduino-based-gpib-adapter/

The other problems maybe contacts / solder related, check for any  crack on solder, you may have bad contacts on the front flat cable, be careful this connector is kinda special, the pins are specially formed to snap.

Maybe you have corruption on an "aged" rom chip, they have life expectancy ...  but the problem is, they are soldered on the main board, you have to use propers tools to remove the chip and put them on quality sockets. 

Maybe you can or could work with freeze spray to see if you can replicate the problem or stop it ??
 

[yo0]

thank you very much!, i will check by your recommendations,  hope the rom is ok (crossfingers).


best regards

Pio