[Heath] SB-104/A low tx audio fix

[ChrisIwata at aol.com]

I pulled out the schematic for the SB104 after your last post. In the audio 
 chain between the MIC and the Tx mixer, the caps that determine the bass  
response are C201, C202, C203, and C204.  Of these the one that has the  
highest corner frequency (meaning cuts off Bass at the highest frequency) is  
C204 at about 1KHz, way too high.  That's the one that was previously  
suggested to change to a higher value to get better Bass response. 
 
C202 has a corner frequency much lower, about 30Hz. Changing it  shouldn't 
make much difference unless; (1) the value of the cap was not 0.1uF as  
specified, or (2) somebody replaced the MIC GAIN pot or the VOX GAIN pot and put 
 in one with a much lower value.
 
Anyway, glad you got the rig to work better.  When I had a 104 I  too 
changed C204 even though I seldom worked Phone.
 
BTW did you really find a 2.2uF Mylar cap at Radio Shack?  Maybe  you meant 
0.22uF? 
 
Chris, KL7DM     
 
--------------------------- 
Recently, I changed C204 on the B Board (TX Audio) from  the original .01 
disc ceramic to a .1 Mylar (RadioShack). Did help some, but  still had the 
mic gain 3/4 open to get enough TX  audio. 
I checked the schematic. C202, a .1 disc ceramic, is the  coupling cap 
between the two sections of the dual op-amp audio amplifier, just  before the 
mic gain control. I changed C202 to a 2.2 Mylar (again, plain ol'  RS), and 
now I have tons of audio with the audio gain about 1/3 open. Now,  that's more 
like  it!
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