Feeding the audio through the ACC in back of the 870 is an excellent way to achieve that great, clean audio. It is a straight line to line input, so no need for a mic to line converter such as a dB pad. However, by doing this, you'll loose a lot of the low frequency response (the bass).
So you will need to either re-EQ your low filters or perform this mod in your radio.
The C131 mod was originally thought out and designed by Paul (W9AC). Pictures as seen here are from courtesy of LA4WAA who has successfully done this mod.
On the back of the TS-870, the 13 pin DIN accessory plug has line level audio input for data. This is a low level and the low frequency response is very limited.
The C131 mod entails an additional 1uF electrolytic or better yet, a ceramic SMD type capacitor to be placed in parallel with the existing 0.1 uF chip capacitor, the IC-23 (see picture below).
To get access and solder the cap, you'll need to completely remove the TX-RX circuit board as C131 is located on the bottom of this board.
This is where the IC-23 is located on the TX-RX Unit. The C131 is directly on the opposite side of the IC-23
This is where the C131 is located on the under side
Finished product with the new chip capacitor added to the C131
This mod is relatively simple to do, just a little time consuming and you gotta have some knowledge in working with SMD components. Sorry, there's no easy way around it.
You'll need anti-static mat, clean and fine soldering iron, good lighting, steady hand and possibly magnifying glasses.
The C131 mod extends the low frequency response by a factor of approximately 10X (as compare to no mod), plus you'll gain the cleanliness of not having to have wire to the front mic input and completely by-pass the mic-preamp in the radio.
This produces cleaner, smoother sound. No adjustment to the radio is needed for this modification other than may be just touch up on your EQ settings. Now you're ready for "Dat VooDoo Audio"!
Warning: This mod is not for anyone to perform as it deals with very small and delicate SMD components.
NA1A assumes no responsibility if your radio broke or warrantee voided due to this modification.
Go To Rear Input wiring instruction
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