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@psci.net>
To: "'Discussion of Ten-Tec Equipment'" <tentec@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@contesting.com [mailto:tentec-bounces@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@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@contesting.com
>> http://lists.contesting.com/mailman/listinfo/tentec
>>
>>
>
>
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