[SignalOne] CX7A CW Sidetone

[jerry]

On to the next problem.  "Peeling the onion" somebody said.

In CW modes, the transceiver constantly outputs audio sidetone.  It's 
just on all the time.  Not transmitting all the time, just sideton-ing 
all the time.

I have reviewed the schematics, and the most likely suspect seems to be 
a 40468A MOSFET that acts as
an analog switch in the sidetone line, turning it on & off as you hit 
the key.  I'll verify that with
a scope.

Now, 40468A's are out there.  They ain't cheap.  What's cheap is modern 
MOSFETs.  The 40468A is an
N channel depletion-mode device.  Unfortunately, the MOSFETs I have in 
my junkbox are all enhancement
mode, which seems to be more popular nowadays.

I wonder if an enhancement mode FET could be made to work?  The signals 
go the same way - it's just that
a depletion mode device is normally ON if the gate is disconnected, 
whereas an enhancement mode one is normally OFF.  I've got BS170's and 
2N7000's in the junkbox.  I think a modern MOSFET would work with
the addition of one resistor to bias it on.

   I had a Sideband Engineers SB-34 I was working on.  There was this 
germanium ( rhymes with unobtainium ) transistor that had died.  After 
due analysis of the circuit, I guessed that a modern silicon transistor 
would work.  And so it did!

                    - Jerry, KF6VB