Been having a issue with a HP437B, HP8484A and cable not zeroing. Been chasing signals for a while and found that the signal ground pin E on the cable is broken off at one end as well as my power sensor diode is bad. Can't find the broken off pin and it's not stuck in the 12 socket DIN connector.
I found a way to verify the HP437B actually reads a signal and zeroes out. Press the Preset button then press Enter. Using 1/8 watts resistors stick a 10K resistor between pin K and J in the DIN connector on the meter. This tells the meter that a pseudo HP8484A power sensor is connected to the meter. Then place a 1K 1/8watt resistor between pin L and C on the meter which is sampled output and feedback return. You should observe a very low -60 to -80dbm. Press the Zero Key and the meter will zero with a reading close to -100dbm when it is complete. Then remove and replace the 1K resistor between Pin C and Pin L which is sampled output and auto zero and you should read about -70dbm. At least you now know the meter can zero.