OK, we have replaced the diode in our 8471A, and learned a few things along the way. In my preceding descriptions, I had Anode and Cathode reversed (for the standard Negative Output model). It is interesting, as in looking at the diode, you can clearly see the 'cat whisker' connection to the die, and that (I believe) is what begat the schematic symbol. Later measurement of the forward voltage of the new diodes led to further confusion, until it was discovered that the DVM leads were reversed (never happened to any of you, right..?). In this particular diode, 1N831, the whisker is the cathode. So we have something on the order of 0.268 Vf, and a reverse breakdown around 2.9ish.... luckily the Fluke test current is 1mA. So yes, any RF or DC over +3V will toast your diode (well, depends on what is connected on the output end, but be careful). Also of note is the difference between the original diode, and the replacement. In the original, the 'whisker' is well formed into a very sharp point, and contacts the die in the center. The replacement looks like it was banged out by Thor's hammer. A very blunt contact point, nowhere near the center. I don't know how much difference this makes in performance, but I suspect it matters... Anyway, we made some measurements on the 8753D, more for our own edification than to offer any sort of performance spec. In the process, we discover our VNA needs some attention in the flatness department. The realm below 3MHz can't be trusted at present. What is noteworthy is looking at the performance of the input termination resistor in the BNC mount without the diode. Pretty much demonstrates that BNC is garbage at GHz. If any of you have a well calibrated VNA and an 8471A, I'd be curious to see how it looks on your machine...
As far as the replacement process, the photo captions should be enough, but if not I will elaborate.?
/g/HP-Agilent-Keysight-equipment/album?id=272489
Onward up the learning curve...
_Dave KC6UPS
