[VoIP] 4th Col. DTMF tones

[Steph Kerman]

In response to a couple of direct inquiries:

The pot core oscillator coils have the 4th column coil tap terminated on a pin but it's not connected anywhere.

Second generation dials such as 35Y3A (8 lead standard dial) or 35AF3A (11 lead speakerphone version) have the frequency selection contacts spot welded to the PCB and are probably difficult to 
disassemble.   I'm not sure how difficult it is to get to the tap on these 2nd gen. dials.  I've never examined one from the viewpoint of doing this since I have enough 16 button dials.

However the 1st generation dials (35A3A and 35C3A, 8 & 11 leads) have wire leads from the F-S contacts to the oscillator board.  On these it is not that difficult to lift the board to connect to the 
unused tap.

It may in fact be possible to connect from the top side with either type of dial however if you attempt to do so you need to be extremely careful not to break the very fine enamel wires connecting the 
oscillator coil windings to the pins.  Also, without lifting the board you don't know which pin is the unused tap.

Once the unused C4 tap is identified, you disconnect the C3 contact lead from the board and connect it to the center terminal of a SPDT mini toggle switch.  You connect the switch terminal for one 
position to the C3 board connection point from which the keypad lead was disconnected.  You run the other switch position to
the C4 tap on the high group oscillator coil.

The switch allows the 3rd keypad column to generate either C3 or C4 tones.

Steph