Mixer FET in Heath SW-717

[Harvey A. Kader]

Richard Post wrote:

> Heathkids,
>
> It's totally solid-state so it's not my normal boatanchor line of inquiry.
> I suspect Q2, the mixer in this radio.  The oscillator is functioning, and
> I can pass modulated IF beyond the mixer.  The voltage on the source is
> definitely too high (3.2 volts instead of 0.4).  Just a good 100 ohm
> resistor to ground connecting to it so it is drawing more current than it
> is supposed to.  Drain is low and Gates are at and a bit below normal
> voltage.  This is an RCA 417-240K  914 which, in the NTE reference
> database, crosses to an NTE-222 dual gate N-channel MOSFET.
>
> SInce one of the gates is only a tuned circuit and a cap away from an
> antenna input, I assume it may have been damaged by a good static shot.  I
> had also suspected the two ceramic filters that are at the output end of
> this FET.  Any other possibilties here?  Has anyone else had trouble with
> the mixer FET or the ceramic filters in their SW-717?
>
> Rich
>
>

I built my SW-717 back in the early seventies. I never had a problem with a blown FET. I
attached all sorts of external indoor antennas to the receiver with no negative results.
And believe me, some of these indoor antennas ran along carpeting and picked up lots of
static electricity. So, I am not convinced that the front end on the SW-717 is
susceptible to static discharge.

Now, as for my Sony ICF-2010, that unit has an ultra sensitive front end. Its 2SK152 FET
in the front end is such a sensitive FET(which makes the receiver really HOT), that it is
known to blow at the drop of a dime. Mine blew, and I never even used an external antenna
other than the Sony AN-1 active antenna, which is the only one Sony recommends for this
receiver. Other active antennas are known to blow the FET. My Sony went from being a low
sensitivity receiver to a super sensitive snoop with just a single transistor change.

My SW-717 originally had transistor 40673  for Q2(Heath part #417-240). When I sent the
unit in for some unrelated repair, Heath changed Q2 to Heath part #417-274. They never
told me that they did this until I examined the repaired unit, and then inquired about
the change. They said that 417-274 was a hotter transistor with more gain, and would
increase the receiver's sensitivity.

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