Amplifier Interface for Flex 
An External "Relay in the Cable" interface fits the Flex 6300 6400 6500 6600 6700

by Bob W2ENY 

MANY amplifier relays draw more AC or DC than what your Flex may support and that's why you need this cable (insurance policy).   It simply plugs into the 15 pin accessory rear jack (like VGA) on the radio and uses pins 9 and 11. Over five feet of shielded cable to an RCA/Phono type plug that fits to your amplifier T/R relay connector.

This package is complete, compact and streamline with all components located inside the connector.  Unlike other costly solutions, there is no klunky external box (like ARB-704) or wall wart (psu)

Yes there's a relay inside the connector, but looks like a regular cable.

So, here's why I offer this...  Yes I'll admit it, I smoked my Flex relay with my Ameritron AL-811 amp and had to send it back.  This is the solution I came up with, wish I did this before.

Tech Details: The circuit uses voltage available on two pins of the radio's accessory jack for switching, operate delay is 2ms (relay device only). Contacts are rated for 2A DC 0.6A AC, much more than the T/R relay has in your Yaesu, Heathkit, Alpha, Ameritron, Dentron, Collins, Drake, Henry, Amp Supply, QRO, etc. For Yaesu FL-2000, 2100 (strip off the connector and wire direct to terminal strip)...

THIS is the original relay in the cable, thousands in the field with Icom's since 1997.

If you have a Flex 65xx radio and an amp, this is what you need, before you send it back for repair