IM-5217 FET Replacement Suggestions

[R. L. Blaney]

Spoken like a tried & true home brewer.  Keep up the good work.  That's the
only way to do it.  You may end up with it working better than before.  73
es gud hamming, de
Dick, WB8MHE
wb8mhe at bright.net

-----Original Message-----
From: John H. Klingelhoeffer <WB4LNM at AOL.COM>
To: HEATH at LISTSERV.TEMPE.GOV <HEATH at LISTSERV.TEMPE.GOV>
Date: Sunday, July 19, 1998 8:47 PM
Subject: IM-5217 FET Replacement Suggestions


>I have a well-worn IM-5217 Utility Solid-State Voltmeter that has been
having
>some zero drift problems.
>
>On looking into this, I'm finding that the input N channel JFET is
EXTREMELY
>temperature sensitive - enough that the meter needle ungrounded with no
input
>goes from 0.3V to > 1.5V when I walk from the air conditioned house into
the
>outside warmth when on the 1.5VDC scale!
>
>I have a bunch of JFETs in the junk box, but none of the original 417-241
>Heath parts.  The instruction manual says this crosses to an "EL131".  That
>proprietary part number does not show up in any of my 1970s or 80s cross
>reference guides nor has an internet search been productive.
>
>I suppose I will go ahead and use one of my junk box 2N3819s on this
>application and see if they will work.  They only cost a buck apiece at
Radio
>Shack to replace them.  However, I'm wondering if an RF JFET is really
>appropriate in this application, or if I need a low-noise, low-frequency
>device?  I also have several National Semiconductor dual NJFETs that are
rated
>more for instrumentation than HF/VHF use.   I figure since this is a
balanced
>circuit, that I will probably replace both JFET's to minimize any Idss
>differential changes over temperature.
>
>Any comments on replacements and potential problems?  Thanks in advance.
>
>John...
>WB4LNM
>
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