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[TenTec] TNC hookup to Omni

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Subject: [TenTec] TNC hookup to Omni
From: pklein@seattleu.edu (Peter A. Klein)
Date: Fri, 7 Nov 1997 00:00:06 -0800 (PST)
> From: ki0er@juno.com (R Ogborn)  

> I just hooked up my new AEA DSP-232
> Multi-mode controller to my TenTec Omni D last night (Audio FSK).  I
> used the RCA patch-in/patch-out and PTT jacks on the back of the rig to
> interface with the TNC. 
> I'm having a problem adjusting the TNC's line out to the rig's patch-in. 
> Seems the line-out of the TNC's level is too high as it is overdriving
> the rig.  There is a jumper in the TNC to set the line-out (they call it
> TX out) to a "low" setting, so I set it from high to low, and
> re-adjusted
> the line-out pot, but didn't seem to notice much difference.  I can get
> it to drive the rig at a reasonable level by turning down the rig's drive
> control at about 1 1/2 setting, but if moved just a hair, I go to
> overdrive (ALC lights bright and watts out goes to 105) to nothing.  I
> want to have full control so I can run QRP if I wish and half power (50
> watts) other times.

On my Omni VI, I noticed the same thing with both RITTY by K6STI (uses a
sound card) and a HAL P38. There is a point on the mic gain control where
the output goes from zero to full and the ALC light lights.  Most TNCs
advise you to adjust the mic gain to the point just below where the ALC
action starts.

On my old Icom 751a, I can do this, and I get full output just below the
point where the ALC meter begins to move.   With the Omni, it seems
like the ALC is almost like a trigger. You get almost no output up to a
point--on my Omni VI it's at about the 9:00 position.  Past the trigger
point, I get full output, which stays that way even if I turn up the gain
further.   And no, the speech processor is not on.

I'm guessing that the ALC "idiot light" (aka annunciator LED :-) begins to
light just as output is at full without any gain foldback.  Remember, the
manual tells us to adjust our mic gain on voice to the point where "the
LED just blinks on peaks."  I wish we had a meter to monitor the ALC
voltage. But I suppose once you learn how the LED works, it's still

If there's no distortion on your signal, I suggest you set the mic gain to
the lowest point at which you get full output, and leave it there.  Use
the PWR control to lower your output if desired.  At that point the 
ALC is probably amplifying the input without compressing it.  Further
increasing the gain would probably cause distortion.

If you can hear hum or squealing on your signal in a second receiver, get
a 600 ohm isolation transformer (available at any Radio Shack), and put it
in the audio line.  I had to do this no matter what I did with my grounds,
and it's needed on both radios.

I don't know the OMNI D, so I'm telling you what I do on my Omni VI.
Hopefully it's applicable.

One other tip--have a little fan blowing on the rig's rear heat sink when
you run digital modes.  I use a fan cribbed from a PC power supply.

Peter - KD7MW
Peter A. Klein  (pklein@seattleu.edu)  :    -----==3==      ---      ---
Network Administrator, LAN/WAN/Novell  :   |    |  |  |    |   |    |   |
Seattle University, 296-5569           :  @|   @| @| @|   @|  @|   @|  @|

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