SB-650 input question

[Robert Myers]

The HFO, BFO and LMO inputs on the SB-650 digital frequency display each go
to one gate of a dual gate 407673 MOSFET. Q1 (HFO input) is Heath p/n
417-274; Q3 (BFO input) and Q5 (LMO input) are Heath p/n 417-240.  They are
all biased the same, and are all 40673 type MOSFET's, but the manual warns
to use the only "selected" 40673 (Heath p/n 417-274) for the HFO input.  

What is so special about that particular transistor?   I.e., How do I order
a replacement for the HFO's 40673 MOSFET that meats this 'special'
criterion?

 

Thanks,

--- Rob Myers

VE3JQL

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