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I am saddled with a perfect 8405A with one defective probe. Swapping the
sampling board between probe tips shows that the fault follows the
board. The measured voltages are correct for the good board, but not for
the failing board. The diodes check out OK for F/B ratio, leaving not
much else to change except the FET.

Can anyone on the list suggest a suitable replacement part or
troubleshooting strategy?

Arthur Shulman VE3ZV

--- In hp_agilent_equipment@..., Arthur Shulman
<dentist@n...> wrote:

I am saddled with a perfect 8405A with one defective probe. Swapping the
sampling board between probe tips shows that the fault follows the
board. The measured voltages are correct for the good board, but not for
the failing board. The diodes check out OK for F/B ratio, leaving not
much else to change except the FET.

Can anyone on the list suggest a suitable replacement part or
troubleshooting strategy?

Arthur Shulman VE3ZV

Hmm, surely, the fet could die, too, but "normaly" the probes fail
with shooted diodes! And they could measure nearly OK but some
parameters may have changed and so the bridge is roughly out of
balance...
the forward voltage differences have to be <20mV as far as i knew
and sometimes some diode leaks badly reverse and bring the bridge out
of balance. And sometimes a shooted diode measures fine at low
voltages as a Multimeter apply in the diode test mode but at higher
voltages (as the (reverse)bias from the bridge) the diodes act as a
soft zener and voila the bridge is out of balance and nothing goes.
Have you a chance to check them with a curve tracer?

You can substitute them with HP 5082-2815 Matched Quad.
Obsolete now but sometimes findable at some stores.
Did you have the hp-number for the fet? Maybe i gould give some data.
i have my 8405 manual no longer so i couldn't have a look.

73's

Jorgen
dj0ud

Thank you Jorgen. Your reply made me rethink my troubleshooting strategy. I had failed to consider that the diodes are at greater risk of failure, more so than the FET. The bad voltages measured around the bridge do suggest this. You are right - I should subject them to some reverse bias and see if they are leaky, at say, perhaps -20vdc.
I don't have a curve tracer but I can make a test jig to do this. I will forward the FET part# if I can find it - The board was sold by HP as a unit replacement, so I'm not sure the FET carries a #.
Thank you once again for some fresh insight and the benefit of your experience.
Arthur

Jorgen Lund-Nielsen wrote:

--- In hp_agilent_equipment@..., Arthur Shulman
<dentist@n...> wrote:

Thank you Jorgen. Your reply made me rethink my troubleshooting
strategy. I had failed to consider that the diodes are at greater risk
of failure, more so than the FET. The bad voltages measured around the
bridge do suggest this. You are right - I should subject them to some
reverse bias and see if they are leaky, at say, perhaps -20vdc.

Hello Arthur,

20 Volts is a bit high, do the measurement at 5 to 10 Volts.
The reverse voltage rating of this fast switching shottkys is quite low.


Jorgen

I don't have a curve tracer but I can make a test jig to do this. I will
forward the FET part# if I can find it - The board was sold by HP as a
unit replacement, so I'm not sure the FET carries a #.
Thank you once again for some fresh insight and the benefit of your
experience.
Arthur

Jorgen Lund-Nielsen wrote:

Hmm, surely, the fet could die, too, but "normaly" the probes fail
with shooted diodes! And they could measure nearly OK but some
parameters may have changed and so the bridge is roughly out of
balance...
the forward voltage differences have to be <20mV as far as i knew
and sometimes some diode leaks badly reverse and bring the bridge out
of balance. And sometimes a shooted diode measures fine at low
voltages as a Multimeter apply in the diode test mode but at higher
voltages (as the (reverse)bias from the bridge) the diodes act as a
soft zener and voila the bridge is out of balance and nothing goes.
Have you a chance to check them with a curve tracer?

You can substitute them with HP 5082-2815 Matched Quad.
Obsolete now but sometimes findable at some stores.
Did you have the hp-number for the fet? Maybe i gould give some data.
i have my 8405 manual no longer so i couldn't have a look.

73's

Jorgen
dj0ud


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