The main problem is that the input capacitance of the probe is too high and the Rp is lower than it should be up at VHF when compared to a good probe. I think this could be caused by the wrong choice of diodes (1N6263). I won't know for sure until I'm able to take the diodes out of the probes. It's not my VVM yet so I can't do any further internal testing.
This VVM is in such good condition it deserves to be repaired properly and if I do buy it I will service/adjust it to try and get as close to the original performance as possible.
I've got lots of the HP accessories to go with these meters plus I made a few of my own plus I have various directional couplers to use with it.
It's all a bit nostalgic because I also have a couple of modern lab VNAs here, but the HP 8405A is a classic instrument and it's also quite educational to use. I usually use the DMM recorder outputs at the back for voltage and phase, but I might also experiment by coupling up an Analog Discovery 2 to the 20kHz outputs and code it up to measure voltage and phase at 20kHz.?
