Hi,
I have been following this thread. Are you sure that your HP4274A is broken?
Let me show you what happens with a perfectly healthy 4274A.
If I press the ZERO open button with no fixture connected to the HP4274A Err 1 is displayed:
If I connect an HP16047A fixture to the HP4274A and press ZERO open, the HP4274A displays CAL until the ZERO open is complete:
If I connect all the shield of the BNC connectors together and connect Lcur to Lpot and Hcur to Hpot and I press the ZERO cal the test is performed the same way as with the 16047A fixture:
Here is a close up of the wires:
The BK Precision Test fixture may not be wired the same way as the HP fixture. I would check it.
Regards,
Jay_Diddy_B