HP 3455A AC or DC same reading

[ebaker]

I have three 3455A's that I recently ebay'd.  They all seem to read fine DC wise, but with a DC supply on the front or rear terminals (2-wire), one reads almost the same on the AC setting while the others converge to nearly zero as they should.  Anyone encountered this before?

I have checked the supply voltages and they look to read within the tolerances.  I did notice this one has a darker display than the others but it also looks darker when off like it has a darker tint to the filter in front of the LEDs.

[Krytron]

Its been a long time - I donated my 3455A - but I believe it had AC and DC coupling that could be selected in the AC mode.  A real True RMS meter will read DC on the AC input.  I think it had a switch that allowed this.

On one of my hand held meters, a Beckman true RMS unit, this is also a check for the True RMS converter, to apply DC instead on AC and verify it reads the same.

I think the 3456A was a close made unit to the 3455A, which I ended up with several of them.

Check the manual on the 3455A and see what the coupling options are on the AC mode.

[ManateeMafia]

On the rear of the meter there is a hidden switch to choose between AC or AC+DC modes. I bet the AC+DC mode switch is enabled on that meter. The info is located in the manual inside section 3.

[ebaker]

I wanted to swing back around and let you guys know that you nailed it.  So why would you want to bypass the ac coupling cap on the true RMS circuitry - LF AC?  It was interesting that when measuring DC with this switch enabled did not quite yield the same value as when it was open.  Many thanks for the help.