"Gary Schafer" wrote in message
...
Can anyone tell me what type of diodes I should try as replacements in
the HP 8405 vector voltmeter probes?


I took the probe apart and looked at the bands around the diodes, wrote them
down, and have now lost the note. In about two days, I'll have a chance to
look again and I can either email you or post my findings here. OTOH, maybe
it would be better to move this to the science.electronics.equipment forum?


Anyone have a schematic of the boards in the probes?
I think I have a broken capacitor in one as well as bad diodes.


See a.b.s.e., subject HP 8405 Probe Schematic. I posted a portion of the
schematic I have. Unfortunately, the diode types are not indicated. The
B.O.M. I have shows HP in-house numbers, I guess, so I didn't learn anything
from it.

The SMT transistor in the probe is a 2N3379. So far, I have found one bad
transistor and a bad diode in one probe and two bad diodes in the other
probe. As I recall, searches on Mouser and DigiKey suggested replacements
were 1N270 silicon equivalents.

I probably don't have an ice cube's chance in hell of getting my 8405A
fixed.

Or, would anyone have replacement boards for the probes?


I think the probes are the weak link in this fine instruments heritage. The
manual profusely warns about applying voltages in excess of 2VAC or 50VDC.
I'd bet excess AC is responsible for blowing the FET while excess DC blows
the input capacitor *then* the FET.

I'd wager you won't find any replacement boards. I hope I'm wrong. And, if
you do, please let me know.


Thanks for any help.
Gary K4FMX

I'll also watch the s.e.e. forum for additional posts by you.

Good luck.

John

my email address is jocjo underscore john at yooha dot com. But spell it
yahoo instead.