Given the benefit of clickless T/R switching and the fact that most ops
who have tried this mod since I introduced it last year prefer to
control their side tone with the AF control, it should at least be given
a try. Many who don't notice the clicks before the mod, notice the
absence of them AFTER they try the mod. In those tough instances where
the clicks and thumps remain, I have reposted a set of fairly invasive
instructions on how I made my Omni's clicks and thumps disappear,
regardless of any power or AF control setting.
The root-cause of the subject matter is a highly click-prone CMOS 4066
quad bilateral switch in the AF circuitry. In order to completely
eliminate the clicks, the circuit must be buffered at the input and
output with FETs or precise R/C component matching, adding considerable
circuitry and cost. Ask any broadcast engineer about the trail of
correspondence generated over the use of this chip (and its predecessor:
the 4016) in high-end audio mixers in BM/E, Broadcast Engineering, and
Radio World magazines over the course of the last two decades.
Some additional notes on the subject matter from the Ten-Tec archives
appear below:
----- Original Message -----
From: Paul Christensen <paulc@mediaone.net>
To: <tentec@contesting.com>
Sent: Sunday, February 14, 1999 11:11 AM
Subject: Fw: Omni Six "Clicks"
Several weeks ago, someone on the list indicated that they could not rid
their Omni Six of audible "clicks" while listening with headphones. I
suggested removing plug 52 on the AF/IF Circuit board, but apparently,
this
did not help.
After making yet another mod to my Omni Six last weekend, I noticed the
same
problem for the first time: I was hearing audible clicks, to the point
of
being annoying regardless of any front panel setting, including power
and AF
loudness.
This weekend, I jumped in to find the cause. On my particular Omni Six,
several areas were contributing to the clicks. Through a combination of
regrounding, using shielded audio cable, and rerouting cables, I have my
Omni back to the point where no or clicks thumps are heard whatsoever.
The
following steps may be of help to those who wish to minimize clicks in
their
Omnis:
1) I'm a firm believer in removing plug 52 from the IF/AF circuit board.
Pull it, and tie-wrap it out of harms way.
2) Ground the heatsink tab of U8, the audio power amplifier. By pushing
U8
against the side chassis, a hole can be drilled for placement of a #4
screw
which now grounds and heat-sinks U8 to the chassis.
3) Many audio cables use ribbon cable. I replaced several of these with
single-pair shielded cable to the AF pot, FIXED AUDIO Plug 57, and cut
two
audio circuit traces on the AF/IF Board that were running too close to
other
witching circuitry.
4) Cut the nylon tie-wrap that holds the cable bundle along side the
chassis
near U8, separate and twist the wires, finding the right combination
that
minimizes the clicks.
>From a schematic perspective, turning the AF control fully
counter-clockwise, should not produce any clicks, only absolute silence
as
the pot's wiper arm becomes grounded. Only, the wires that run to the
pot
are unshielded and the routing of the wires makes for a poor circuit
ground.
-Paul, W9AC
----------------
Paul Christensen (paulc@mediaone.net)
Mon, 7 Sep 1998 08:28:01 -0400
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There were far too many replies to answer each one individually.
The mod consists of unplugging connector 52 on the I.F./A.F. Board. =
That's it. Plug 52 should have only one wire attached to it. Those of
=
you with more recent Omni Sixes should verify this as some of the =
circuit boards have been changed to SMD types. Once connector 52 is =
unplugged, use a nylon tie-wrap to keep it safely out of harm's way.
The theory behind the change: Plug 52 activates a 4066 CMOS quad =
bilateral switch which is configured as a sidetone wrap-around circuit.
=
The audio click and thumps one hears is generated by the CMOS chip. =
These chips are used in the broadcast audio world and are known to cause
=
the same effect when switching audio on complicated audio mixers and =
routers.
You will note after the change that you still have sidetone control and
=
the ability to control its mix relative to incoming received audio =
through the front panel's "ST" button.
Use a set of headphones to listen to your QSK audio before the mod and =
make a mental note of what you're hearing. Then, using your headphones,
=
critically listen to the QSK after the mod. There should be a marked =
difference.
Disclaimer:
This is not a Ten-Tec approved modification. I discovered this while =
making other modifications to my Omni Six Plus. Use care in making this
=
change if you attempt to make it.
73,
-Paul, W9AC
Steve Ellington wrote:
>
> For the life of me, I can't see why anyone would NOT want to control the
> sidetone's volume with the AF gain control. Can someone explain this? All
> other brands of rigs I've had all control the sidetone from the volume
> control and also have an internal adjustment to set the relative gain of
> the sidetone. This only seems natural to me. So, if I had an Omni VI, I
> wouldn't hesitate to rip off Connector 52!
>
> > cure for all OMNI's for this problem, except to remove connector #52 (and
> > live with the consequences I noted below).
>
> N4LQ@IGLOU.COM
>
> --
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