Electronics > Repair
Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
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JDubU:

--- Quote from: fzabkar on November 21, 2024, 08:01:29 pm ---
--- Quote from: floobydust on November 21, 2024, 07:42:00 pm ---Look at the IM-2420 Heathkit docs, BoM and schematic ...
--- End quote ---

Are these available anywhere that doesn't want money for them? ISTR that somebody purchased Heathkit's IP and inventory, and then began asserting his copyright over the manuals. Previously, they had been freely available for download.

--- End quote ---

Heathkit Schematic and Manual Archive:
https://www.vintage-radio.info/heathkit/
fzabkar:

--- Quote from: JDubU on November 21, 2024, 08:56:56 pm ---Heathkit Schematic and Manual Archive:
https://www.vintage-radio.info/heathkit/
--- End quote ---

Thanks. I should have searched for them, but I presumed they were gone.
David Hess:
Based on the faded schematic, it looks like 1.7 volt Vgs at 1.7 milliamps Id.

Ft of a JFET source follower is roughly Gm/(2 Pi C), so candidate devices have high transconductance and low total output capacitance.

I would pick up a couple of different available RF JFETs, like the BF256, J109, and J304, and see which works best.  There is not a lot of choice about pinchoff voltage and drain current at zero gate voltage.
 
Kleinstein:
I would not consider the J109 a RF fets. That was likely a typo: a common RF type the the J309.
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