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Re: [TenTec] New Omni VII....first impressions

To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Subject: Re: [TenTec] New Omni VII....first impressions
From: "Bob McGraw - K4TAX" <RMcGraw@Blomand.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Thu, 20 Dec 2007 15:10:49 -0600
List-post: <mailto:tentec@contesting.com>
Stuart has good points on using the D-104 with Tentec rigs.  My experience 
falls in two categories.

A good D-104 is not one that has output vs. one that does not, it is a mike 
with a crystal element that has not been exposed to heat or moisture  Both 
cases, heat and moisture, will cause the element to become brittle and sound 
bad, although it still may have output.  So good is not necessarily good.

The basic D-104 head as used on a desk stand or G stand needs to see a load 
Z greater than 1 megohm.  Even better, something in the order of 3 megohms. 
This is typically provided by tube type input circuits.  Most radios today 
provide a 10K to 50K input impedance which is much too low for the D-104 
element.  Although some have been successful, I do not advocate the use of a 
transformer on a mike input due to the susceptibility of hum being 
introduced from nearby AC fields.  One can use the D-104 head on the Astatic 
TG or TUG stand, the one with the transistor amp in the base and this will 
work, as the FET amp is basically serving as an impedance converter to 
provide the correct load to the mike and still be able to drive the lower Z 
inputs of most solid state radios.  A word of caution when using the TG or 
TUG stand.  It can easily overdrive the audio input stage in the radio. 
Once this happens, no amount of reduction of mike gain on the radio will 
clear the reported distortion.

It is also reported that, depending on wiring of the TG or TUG stand 
switching, the PTT activation also turns on power from the internal 9V 
battery to the amp.  Often the result is a DC transition resulting in a big 
whump or pop or thump that is fed to the radio input.  This can saturate the 
input stage which may require several seconds to recover depending on the RC 
constants in the input stage.

All of these "ills" have been encountered or observed by me and often heard 
on the air.  The typical result leaves the operator with disgust saying the 
mike is bad or the radio is bad or a host of other ills when the general 
fault is poor application of the elements, otherwise not understanding the 
needs of the mike or the radio.

Here is a website that covers most of the wiring conventions of the D-104 
and TG or TUG stands.  http://www.qsl.net/wa2mzf/d104.html

Bob, K4TAX

----- Original Message ----- 
From: "Stuart Rohre" <rohre@arlut.utexas.edu>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Thursday, December 20, 2007 2:08 PM
Subject: Re: [TenTec] New Omni VII....first impressions

> Mike, if you have the very oldest D 104, the bad audio reports you 
> describe are typical of an impedance mismatched D104.   It is a high 
> impedance mike. To use it for modern low to medium impedance SSB mike 
> inputs, you need a matching transformer, or an emitter follower preamp to 
> step down the impedance from the 100,000 ohm range to say 600 ohms.  Then, 
> the mike will sound great.  I use the D104 very successfully with Ten Tec 
> rigs with attention to matching.
> 73,
> Stuart Rohre
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