Bypassing the AGC on the DVR2
                           September 29, 1997

The original DVR2 for the RLC-2 and RLC-3 used an AGC (Automatic Gain
Control) to automatically adjust the record level.  Unfortunately, it
also introduced quite a bit of noise into the signal.  Cleaner
recordings can be made by bypassing the AGC and setting the record level
manually.  Your DVR may have already been modified when you got it.

#1:  Find the trace that goes from pin 21 of the ISD chip to a via.  It
     is on the back of the board.

#2:  Cut that trace in the middle.  That disconnects the output of the
     AGC from the line level record input of the ISD chip.

#3:  Solder a wire from the via mentioned in step #1 (the one that was
     connected to pin 21) to the closer end of R7.  The wire should only
     need to be about 3/8" long.  That end of R7 is right next to pin 20
     of the ISD chip.  This connects the output of the op amp to the
     line level input of the ISD chip.

#4:  Clip one end of C3.  It normally feeds the audio to the AGC.

#5:  Change R4 from 300K to 22K.  This increases the level coming out of
     the input buffer op-amp so it is high enough to drive the line
     level record input directly (the AGC uses really low microphone
     levels).

Note:  the following components are unused after doing the above
modifications:  R3, C3, C4, C5.


                             Reducing Noise

#6:  If there is high-frequency noise going to the DVR, it can be
     filtered out by putting a 100pF capacitor between pins 6 and 7 of
     the 8-pin op-amp (U5).


                            Increasing Gain

#7:  You can get more playback gain if needed by reducing the value of
     R1.  It is normally 5K.  I used 2.5K and had to turn the playback
     pot most of the way down.  I think about 3.5K would let you center
     the output level pot.

                  Making Sure Messages Play Completely

In some configurations, the DVR2 will record correctly, but will not
play back.  Instead it stops almost immediately when the message is
played.  This is most common on the RLC-2 controller, especially when
the autopatch and front-panel LED display are installed.  The solution
follows:

#8:  RLC-2 Only:  Make sure that Q1 (it is in the top left corner of the
     circuit board) is installed with the flat side DOWN.  The
     silk-screen printing on the board shows it with the flat side up,
     which is correct for the RLC-3.

#9:  RLC-3 Only:  Remove R12 (it is in the top left corner of the
     circuit board).  If you continue to have problems, install a 10K
     resistor between the left hole that R12 used to use and ground (the
     edge of C8 nearest the edge of the edge of the board).

#10: If there is a 1K resistor between pins 25 and 26 of the ISD2575A
     (on the back of the circuit board), you can remove it as it should
     no longer be needed.