Howdy Rui!
Just want to welcome you to the Voodoo Forum again!
Glad to have you here!!
Can anyone explain the c131 mod to the 870s
Sure.
C131 is a .1uf cap that is in between the ACC2 input connector and the Mic Audio circuitry of the Transmitter.
A cap this low in value will roll off all the low frequencies.
The reason Kenwood put it there was because that is a Digital Audio input.
In order for us to use that for Voodoo Audio, it has to be increased in value.
A 1uf chip cap is a good value to change it too.
The reason to even use this ACC2 Mic Audio input is that it bypasses the Mic Amplifier inside the rig and allows us to be able to input Line level Audio.
The second reason is that since we don't have to go thru this first op amp, we can boost the high frequencies allot more to increase the bandwidth without getting the dreaded '870 SPLAT'.
It is a win win situation.
Here is a website that has pictures and some more information about it.
http://home.online.no/~la4waa/TS-870/C131.htm
We've also made some High Resolution Pictures of it to help you locate it.
These pictures are 1meg in size, but you can blow them up to see whats around C131.
Hope this helped Rui!