Four PL tones mod for the Icom IC25A
This is an old rig, no PL (CTCSS) tone capability, so you need to install a com-spec tone board in it. Otherwise you will have a hard time working repeaters, as almost all want a PL tone. You could program the com-spec board to some selected tone for your favorite repeater, but that's rather limiting. I found a way to do one tone for one repeater, and another tone for another favorite repeater. And two more repeaters. Without having to have external switches, I used the VFO 12345 switch which has an unused section, which I put to use here. This switch's unused wiper I (after cutting a trace that connected it to a neighboring wiper) I connected to ground. Then connected wires to the 5 poles, which mean "VFO, 1, 2, 3, 4". The 5th position has no pole. The VFO position does have a pole, if you want a default PL on all VFO frequencies. I then removed the inverter I had driving the com-spec PL board. I connected 5 wires for each dip-switch position. And between these and the 4 wires (positions 1 2 3 4) off the VFO 12345 switch, I used diodes to assert a low on various com-spec dip switch inputs. Cathodes towards the VFO 12345 switch. Looking at the com-spec chart, you use a diode where a 0 is indicated, and nothing for a 1. Each VFO 12345 switch position gets its own set of diodes. The com-spec board essentially uses TTL high or low signal levels, so a diode to ground will make an input a low, at 0.7VDC. Open is pulled up to a high. If it turns out that an input to the PL tone generator chip U1 is always a low, don't bother with diodes to it, just set the dip-switch for it to low.

Only downside to this mod is when the VFO 12345 swirch is in position 5, the com-spec board will default to all highs (except for any input hard set to a low via the dip switch), making the com-spec board produce a default PL tone. All highs will produce a PL of 203.6Hz. If you really need a 5th PL tone you could connect a 4 input NAND gate to wires 1234 to produce a low when 5 is selected. Same if the VFO position has no diodes on it. You could use the VFO line from the VFO 12345 switch to short the com-spec board's output to ground, via a 100uF cap to avoid any upset to a DC bias, thus silencing it.

Another and probably better method to silence the PL tone board is to make use of the tone burst circuit of U2b. This op-amp has a RC time constant that charges up (when the com-spec is powered up in transmit mode, if this power is keyed by the PPT) to disable after the time is up the tone output of U1. This RC circuit is on the inverting pin of this op-amp, and the non-inverting pin is connected to a reference voltage. We will connect the "VFO" wire from the VFO 12345 switch to the inverting pin (to the RC circuit) after being inverted by an inverting NPN transistor. And we need to install if missing diode D2 so its cathode connects to the op-amp pin 7. Remove JP1 if it's there, and check to see if C9 and R11 are present. If missing, use 10K and 0.1uF for these, not critical. When the VFO wire is shorted to ground by the VFO 12345 switch, U2b will disable the tone.

You then have to program the corresponding repeater frequencies into each VFO position. You must use VFO A, VFO B doesn't let you do this for some strange reason. This rig uses a strange method to do this, you dial up the desired repeater frequency in VFO mode, then set this VFO 12345 switch to the desired position (to match the repeater to its PL tone) and then press "write". Then the repeater frequency is then stored into that position, and should now show on the display.

If you disconnect the power from this rig, this programming disappears. Turning it off via the rig's volume/power control keeps it, as long as the power is still connected.

I used the "airspace" above the microprocessor chip to mount, with double sided sticky foam tape, the com-spec board, inverter transistor and the diode natrix. I used circuit board edge card connector material cut off old junk boards to mount the diodes and the inverter transistor circuit. There's very little headroom elsewhere in this rig.