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

[David Stinson]

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