Can anyone tell me where I can get a schematic of the probe for the HP
8405 Vector Voltmeter?

It can be found in the scanned copy of the ops-and-service manual for
the 8405, which is in the HP section of the Boat Anchor Manual Archive
site and its mirror.

Look for the 8405_service_6.pdf manual. The quality isn't great (a
somewhat fuzzy scan of the Army-depot version of the HP manual,
partially optical-character-converted into DejaVu and then turned into
PDF) but it's probably usable.

From what I've read (I just bought one of these and am starting the
test-and-refurbish process) the matched diodes are the most vulnerable
components in the probes. The original parts are HP house-numbered
and I assume they're no longer available. Some people have
substituted 1N5711 Schottky diodes (easily available), others suggest
1N270 (gold-doped RF germanium diodes, still available), one gent
suggested the HP/Agilent 5082-2815 which seems to be a matched-quad
set of 5082-2811 Schottky diodes. The latter are probably the most
sensitive but I'm a bit concerned about their low breakdown voltage.
The 1N5711s have a higher threshold voltage but are a lot more rugged,
I expect... don't know if they'll perform as well at high frequencies
as the originals (whatever they are) or the alternatives. Other
high-speed, low-threshold diodes (e.g. those intended for RF mixer and
detector applications) could probably be used. No matter what you
use, hand-matching pairs or quads would probably be a good idea.

The P-channel JFET in the probe was reported by one writer to be a
2N3379 but I don't know if that's correct. Possibly something like a
MMBF546x part might be substituted?

The input capacitor is also sometimes damaged by overvoltage, I
gather, and may need to be replaced as well.

There's a guy on eBay who sells a probe-repair kit... 8 diodes, 2
FETs, and instructions on doing the repair.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!