[TenTec] 705 Mic Low Output Help Request

[David & Chris Drake]

This is very interesting, and may deserve more research on my part.  I do 
know that I have ran several TT radios over the last 7-8 years with the same 
705 mic.  Omni C (with battery), Corsair (original, which had no pin 4 
connection until modified), Corsair II which always had a punchy signal, 
Omni V, much the same, and now an Omni VI+ which operates with very little 
mic gain at all, and less than any of the aforementioned radios.  As Bob 
points out, we may have some radios that have less or more voltage at pin 4, 
and the result is variation from radio to radio as to how the 705 performs, 
and how much mic gain is needed to good alc action.  It is interesting why 
TT would design the 705 to be "held back".   Possibly this is due to use on 
earlier TT radios that had higher pin 4 voltage??

I will also say this.....I have seen quite a bit of difference from TT radio 
to radio in the same model series as to how the same mic will react to each 
radio.  Not sure why this is, but may be due to production component 
variation, the ageing of the circuits or both.  For example, I've heard 
several TT owners using heil mics that have no problem with gain.  Others 
won't work at all without outboard amplification or processing.

I would be interested in knowing mic experiences with the Jupiter and Orion, 
since I am leaning toward going for an Orion II this year.

Mic selection and use with most any radio may be more of an art than 
science!!

David
Wd9cmd


----- Original Message ----- 
From: "Bob McGraw - K4TAX" <RMcGraw at Blomand.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Monday, February 04, 2008 9:24 PM
Subject: Re: [TenTec] 705 Mic Low Output Help Request


> The Primo EM-60 ( EM-100 in the 1201 kit ) electret cartridge used in the
> 705 requires 1.5 volts DC minimum to bias the JFET inside the electret 
> well
> into the constant current region. Actually, 3 to 4 volts is much better.
>
> The existing TT design has way too much resistance (24K) in line with the
> electret such that it's bias voltage never rises much above .5 ~.6 volts 
> DC;
> and the JFET never gets out of the constant resistance region, and
> consequently cannot develop adequate gain.
>
> To fix your 705 mic, tack solder a 3300 ohm resistor across R11 ( which is
> 22K now ).
>
> You will find that you will have plenty of output after this mod .
>
> If you are interested in enhancing the low end freq response as well,
> increase C2 to .47 uf or larger.
>
> Also you should verify that the 705 is being run on at least 7 volts by
> measuring its operating voltage at the M+ pad on the 705's printed circuit
> board. Some older Corsairs have 10K behind the mic connector pin 4, which
> should be reduced to less than 68 ohms.
>
>
>
>
>
> Bob, K4TAX
>
> ----- Original Message ----- 
> From: "David & Chris Drake" <drakes at psci.net>
> To: "'Discussion of Ten-Tec Equipment'" <tentec at contesting.com>
> Sent: Monday, February 04, 2008 12:03 PM
> Subject: Re: [TenTec] 705 Mic Low Output Help Request
>
>
>> Don't remember for sure without looking it up.  But I think 9-12v.  The
>> mic
>> runs off a 9v battery if not getting power directly, so it would make
>> sense
>> it needs around 9v to work.
>> David
>>
>> -----Original Message-----
>> From: tentec-bounces at contesting.com 
>> [mailto:tentec-bounces at contesting.com]
>> On Behalf Of Greg Kuczek
>> Sent: Monday, February 04, 2008 9:15 AM
>> To: Discussion of Ten-Tec Equipment
>> Subject: Re: [TenTec] 705 Mic Low Output Help Request
>>
>>
>> I'm running it thru the jack. I haven't checked the voltage at the jack
>> yet,
>> thanks.  Do you know what the voltage should be?
>>
>>  73
>>  Greg W9VOX
>>
>> David & Chris Drake <drakes at psci.net> wrote:
>>  Are you running the 705 by battery or through the Omni V? If through the
>> Omni V, check the voltage at the jack first. If by battery, check the
>> battery. Then check the plug before going inside it.
>> Good luck,
>> David
>>
>> ----- Original Message ----- 
>> From: "Greg Kuczek"
>> To:
>> Sent: Sunday, February 03, 2008 10:23 PM
>> Subject: [TenTec] 705 Mic Low Output Help Request
>>
>>
>>>I have a 705 mic that I am having low output problems when used with an
>>>Omni V. I've read that you can change out the R11 resisitor to 3300 ohms
>>>to
>>
>>>help correct this. Unfortunately I don't have a schematic or any
>>>documentation to help me locate this resistor. Any help identifying R11
>>>would be appreciated.
>>>
>>> 73 de Greg W9VOX _______________________________________________
>>> TenTec mailing list
>>> TenTec at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/tentec
>>>
>>>
>>
>>
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