Borrow Q108 (or any of the like devices) to test your hypothesis. It is just
used as a low leakage protection diode. You can replace Q108 with almost any
n-ch fet.
Tom
----- Original Message -----
From: "Tony Greene" <[email protected]>
To: <[email protected]>
Sent: Tuesday, April 01, 2014 10:13 PM
Subject: [volt-nuts] FET HP3456A
I have been troubleshooting a HP3456A voltmeter.
Its giving a stange fault, but I beleive I have nailed it down. The unit
is reading +10.00000 volts input at +9.99661 volts. If you reverse the
input, it reads correct at -10.00000 volts. All other ranges read correct
on either polarity.
What I need is a source for the FET A20 Q109 or a suitable substitute.
Its HP part number 1855-0298, it was made originally by USA mfg 02883 -
Siliconix, Inc, Santa Clara, CA. It crosses to NSN 5961-01-135-1324. And
Vishay has bought them and the part number is now obselete.
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