[Boatanchors] TCS Receiver: No More CW "Ear-Popping"

[Jacques Fortin]

Hi Dave,

Excuse me if I'm wrong, but if you are actually using two 5.6V zeners IN
PARALLEL, the effect is the same than using, say, two 1N4004 in parallel.
Your clipping level will be about 0.6-0.7V across the headphones.
However, using two zeners IN SERIES and opposite directions will create a
limiter having the Zener rating plus about 0.6V (6.2V in your case).
Is that what you meant in the first place ?

73 de Jacques, VE2JFE

-----Message d'origine-----
De : Boatanchors [mailto:boatanchors-bounces at mailman.qth.net] De la part de
David Stinson
Envoyé : 10 février 2016 07:31
À : milsurplus at mailman.qth.net; boatanchors at mailman.qth.net
Objet : [Boatanchors] TCS Receiver: No More CW "Ear-Popping"

In the quest to make the TCS into a good CW rig without "ham-mering" it,
there are several challenges.
One is the terrible "POP" in the headphones and speaker when keying.  The
keying relay interrupts receiver screen voltage and mutes receiver audio on
transmit. 
When keyed, there is a nasty transient on the audio.

I've never understood the grousing about the TCS "keying relay clacking."
It just ain't that loud when the rig is buttoned-up.  Oh, I know... 
"Barbie and Ken's" $8000 plastic toy radios don't make any noise at all, but
ya know what?  
I didn't pay $8000 for this sweet little rig. 
And I can fix it when it breaks.
That said, the switching transient both in the phones and in the remote
speaker is brutal.  It makes using the rig on CW annoying on the speaker and
impossible on headphones.  Attempts to suppress the transients on the screen
buss were not satisfactory.

The fix is simple, if one is using properly-matched earphones and/or a
matched speaker.
Zener diodes in parallel, one pointing in each direction, across the
headphone jack fixed the problem in the phones.  Used 1N4734A, 5.6 V 1W I
happened to have in "the Cave."
The TCS headphone jack is isolated from
the speaker audio buss by an 1800 Ohm resistor, so this doesn't affect the
speaker audio.
5 Volts of audio across matched headphones is a big plenty unless one is
stone deaf.

I can't say what Zeners to will work with improperly matched headphones or
"ham-mered" audio stages, but every Zenering voltage from 1.4 to 500 Volts
is available for a few cents these day.  Check with Digikey.  I've been very
happy with them for years.

Another pair of the same type Zeners across the speaker voice coil (not the
audio buss in the receiver, but at the speaker voice coil terminals) cured
it in the speaker audio.  Clipping distortion in the speaker audio is only a
problem at very high volume levels but if one finds it objectionable, adjust
the Zener clipping voltage upward until you find a balance between clipping
and keying transient you can accept.  The rig still sounds sweet on AM to my
ears because, with all the volume I can stand, the diodes aren't clipping.
Working CW with headphones and/or speaker is now very nice.  Between the
filter module in the IF and this, I be a happy boy.

Caveat asinus calcitrando:
This fix will not work on rigs that have a sidetone winding on the
Modulation transformer which feeds  back to the receiver audio, like the
SCR-274N, ATA and AN/ARC-5.  The cause is reflected impedance.  When you
transmit on AM, the modulation waveform is impressed both on the transmitter
plate/screen circuits and on the sidetone winding.  If there is clipping on
the sidetone winding, that will be reflected back into the modulation
winding (and stepped-up by it).  
The result is clipped and hosed modulation.  
Been here done this.  Unless you disconnect the sidetone winding (and I have
on occasion), solving keying transients on these rigs will have to wait for
more bench time.

GL OM ES 73 DE Dave AB5S




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