Is anymore specifically using the ARB-704 buffer with their 6300 in keying their amp, I would like to know how to connect this to my 6300 to be used with my SB-220 amp.
Based on the ARB-704 manual, it would be connected as follows:
1) ARB-704 TX-SEND jumpers set for LOW SYSTEM;
2) ARB-704 J2 (Radio) connects to the Flex 6300 TX KEY line;
3) ARB-704 J1 (AMP) line connects to SB-220's key line.
Unless your SB-220 has a Soft-Key modification, you definitely need this or other amp buffering device. The Flex TX RELAY key line is rated at +40V/140mA. An unmodified SB-220 key line is more than +120V. The ARB-704 would not be my first choice, but you've already got it on-hand.
You should still use the ARB-704 even if the SB-220 has the Harbach Soft-Key mod. Here's why:
If the Soft-Key's Q1 fails with an open base, +120V from the amplifier appears back at the Flex's amp key line. Since the Flex TX Relay line is only rated for +40V, the +120V key line from the amp will damage the Flex TX Relay output circuit.
Without the ARB-704, one solution is the inclusion of a +2.5V Zener diode at the junction of the Soft-Key's R1 and Q1 (base). The added Zener is current-limited through R1 and will clamp the transceiver's key line to no more than +2.5V during a fault condition.