"Gary Schafer" wrote in message
...
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.


The color bands you mention sound familiar to me. I think they may be
matched pairs.


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?


Yours May not be. I verified three of mine had reverse leakage.


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. ?


alt.binaries.schematics.electronic


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

With a DVM. The schematic shows an N-channel junction FET. It should read
like a diode with the + lead on the gate and the - lead on the source. It
should show open with the leads reversed. It should show open with the +
lead on the drain and - lead on the source with the gate connected to
source. Mine had reverse gate leakage.


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


They're not available to me right now.


Wonder how you found the FET part number 2N3379 ?

The part number was stamped on the FET. I used a magnifying glass. Keep in
mind that the FET may have been replaced by the previous owner.


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


Yes, it would. You do know that the little probe boards unplug from the
probe body, right? If you have a working probe on a working meter, you could
try repairing the broken probe and try it on the working meter. I'm going to
do that as soon as I can get the time.


To email me direct make address garyschafer.

Thanks for your help
Gary K4FMX


And thank you for your help, too, Gary. Maybe together we can get something
going eventually.

John KD5YI