34401A DMM freezes in AC mode

[Nevada]

I have a 34401A with a very strange behaviour: when I switch it to AC mode (either in voltage & current or frequency counter), the display freezing after some minutes. If I push any button then it continues measuring, but freezes again after a while. Some similar loop later, the meter freezes completeley (although it shows the shift button press, but I guess it's only a front-panel feedback - so the main processor stop working). It passes on self test.

Do you have any suggestion?

[wn1fju]

I can't help you with your particular problem, but it seems like these 34401A's have a mind of their own.  For instance, I can leave my unit set to DC volts for an hour only to come back and see that it has switched itself to AC volts.  I thoroughly inspected/cleaned the front panel board, I even put a scope across the Vcc rail for an hour to see if there was a power glitch.  Everything checked out fine.  It remains a mystery.

[Swake]

Spooky stuff here :-D
The shift indicator is indeed managed by the local  display controller

[Nevada]

As far as I can see, the AC measurement made by an AC-DC converter (AD637), and is processed by the DC section. So the question, what is the difference between the AC and DC measurements?

[Dr. Frank]

I have a 34401A with a very strange behaviour: when I switch it to AC mode (either in voltage & current or frequency counter), the display freezing after some minutes. If I push any button then it continues measuring, but freezes again after a while. Some similar loop later, the meter freezes completeley (although it shows the shift button press, but I guess it's only a front-panel feedback - so the main processor stop working). It passes on self test.

Do you have any suggestion?

Hello,
please engage TEST command first. That should already come up with an error code, related to this AC malfunction, see service manual, chapter 6, Self Test Procedures.

My first guess is, that the shift registers, which provide the steering signals for the AC circuit, might have a problem.
See AC Circuit Schematic, sheet 4/8, U309, U311.

But let me know first, what the outcome of the TEST is.

Frank

[Nevada]

Unfortunately (?) there is no any error after a Self Test.