DRU-3 in the TS870 recording RX-Audio
My TS870 had a DRU-3 (Digital Recording Unit) installed when I purchased it.
Strange thing is, Kenwood designed it to playback recordings from the microphone and not from the RX. Since I am not the hard boiled contester and really don't need to record my own voice, rather to record my fellow hams for modulation tests etc., I went through the circuit diagrams and found the solution.
First of all we need to change the record audio path from mike to rx-audio:
On the solder side of the tx-rx-unit (the big board housing the DRU-3) the capacitor C121 (1 µF, 50V) needs to be unsoldered at the plus-side only. From the plus-pin of the capacitor we wire a shielded cable (shield to ground both sides) to the ANO-Pin of the ACC2-Jack (J5) or, which is the same, to the minus-pin of C205 (10µF, 16V). This Mod leads the AF from the Analog out (ANO) to the Record in (DRM) of the DRU-3. The ANO Signal is independent from the AF-Gain setting on the Front Panel.
Second, a few thoughts about the DRU-3:
The Digital Recording Unit is using th ISD2560 Voice Recorder chip. It is able to record 60 Seconds of sound with a sample rate of 8 KHz. It has a couple of neat features which we really don't need for our RX-recorder: MIC Preamp and MIC-AGC.
In the unmodified DRU-3, the signal (Microphone) enters the DRU-3 through the DRM-Pin, which is Pin 17 at the ISD2560. from there it gets preamplified and an AGC-signal is derived. The time constant of the AGC (Pin 19)is determined by R905 and C908 onboard the DRU-3. At Pin 20 of the ISD-Chip the preampified Voice leaves the ISD2560 and is coupled back across a 1µF 16V capacitor (C906) into pin 21. Now, we neither want the Mic Preamp nor the AGC to work! AGC makes the Audio pump and at low level speech phases makes the noise go up, this we want to control with RX AGC on the front panel. So we simply bypass the whole preamp stage and go directly from DRM into pin 20 (ANAin). To do this, we lift off (solder iron, SMD-Tip) pins 17 (Mic in) 19 (AGC) and 21 (ANAout) of the ISD2560. Now we solder a little piece of wire from the C906 + (was connected to pin 21) to R904 (was connected to Pin 17, DRM)
Recording is levelled with VR3 on the TX-RX-Unit.