[SignalOne] [Amateur-repairs] CX7A CW Sidetone

[David Eckhardt]

Have a look at the attachment for a "replacement" for unobtanium N-Channel
MOSFETS.  I ran into a problem in restoring a Swan SS-100 with a blown
40673 in the front end of the receiver.  There are OS 40673 MOSFETS
available on the internet, but the price is a tad over $30 !!!  So a www
search turned up with the circuit I'll attach.  It's working in spectular
manner in the Swan receiver front end.

Dave - WØLEV

On Sun, Nov 17, 2024 at 5:14 PM Jerry KF6VB via groups.io <jerry=
tr2.com at groups.io> wrote:

> 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
>
>
>
>
> -=-=-=-=-=-=-=-=-=-=-=-
> Groups.io Links: You receive all messages sent to this group.
> View/Reply Online (#93151):
> https://groups.io/g/Amateur-repairs/message/93151
> Mute This Topic: https://groups.io/mt/109629309/361958
> Group Owner: Amateur-repairs+owner at groups.io
> Unsubscribe:
> https://groups.io/g/Amateur-repairs/leave/4529539/361958/656154073/xyzzy [
> davearea51a at gmail.com]
> -=-=-=-=-=-=-=-=-=-=-=-
>
>
>

-- 

*Dave - WØLEV*