Κυριακή, 31 Αυγούστου 2014

ICOM SM8 GAIN REDUCTION MODIFICATION

 
 
 
http://www.db0anf.de/app/bbs/messages/show-30395_VK3KSD
 
From: VK3ALM@VK3KSD.#MEL.VIC.AUS.OC
To  : MODS@WW
ICOM SM8 GAIN REDUCTION MODIFICATION
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By Adam Maurer, VK3ALM
July 23, 1995

The Icom SM8 desk microphone has "bags" of gain - way too much!

Generally, when feeding a typical transceiver the SM8 will be switched
to the LOW output switch setting and the gain trimpot will be set at 
the low end, usually just off the end stop.

I have reduced the gain of my SM8 by a factor of two.

This is done by increasing the value of the bias stabilisation resistor, 
R6, in the emitter of the 1st stage amplifier.

R6 was 10R 0.25W resistor and now becomes 33R 0.25W resistor.

R6 (marked on the silkscreen) is located near the "OUT2" header connector.

MODIFICATION PROCEDURE:
1) Remove the four philips head screws at the bottom of the mic.
2) Remove the three small philips head screws that secure the PCB.
3) Locate and remove R6.
4) Fit 33R resistor to R6.
5) Reassemble mic.
6) Re-adjust O/P level trimpots to suit.
By connecting an AF signal generator to the I/P of the 1st stage and 
measuring the output, the following results were obtained;

   INPUT      O/P BEFORE MOD       O/P AFTER MOD
  40mV P/P       4V P/P               2V P/P
   1kHz

You can further reduce the gain of the 1st stage if you so desire by 
simply increasing the value of R6, say between 33R - 56R.

Incidentally, the frequency response of my SM8 was virtually flat from 
100Hz to beyond 23kHz.


73, Adam, VK3ALM  (UKSMG #843) 

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