Hi John,

Thanks for the info. Also thanks to Richard for the link to sphere.
They told me that people that have replaced those diodes used 1N5711.
They are schottky diodes. Sphere wants $3.00 each for them and digi
key has them for 39 cents each.

I drug my probes out that I hadn't looked at in several years. I did
find my manual and it does have a schematic of the probes but gives no
part numbers for the diodes or the FET.

In looking at my probe boards one has diodes that have numbers 392
717. They are the input diodes. The other set at the FET input have
numbers 393 717. I don't know if they are original or replaced.

The other probe has diodes marked HP on them and color bands. The
input ones have yellow, green, gray. The ones at the FET input have
yellow, violet, blue. Don't know if original or replaced.

I see the input capacitor on one probe has been replaced and the 100k
resistor.

I checked the diodes with a meter and they all seem to have proper
forward drop (around .3 volts or so) and high reverse reading. Maybe
they are not bad? The problem may be the FET?

Haven't worked on the thing in years. Sure would like to get it going.


See a.b.s.e., subject HP 8405 Probe Schematic.
What is a.b.s.e. ?

How did you check the FET to find out it was bad?

The sphere site has some HP house number cross references if you have
some HP numbers.

Wonder how you found the FET part number 2N3379 ?

I have run across several people with 8405,s with bad probes. Would be
nice to figure out how to get them going.

To email me direct make address garyschafer.

Thanks for your help
Gary K4FMX


On Fri, 14 May 2004 02:58:00 GMT, "The other John Smith"
wrote:


"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.