This is an external soft key for your Heathkit amplifier. Why risk going inside your amplifier to install a raw circuit board? Why risk damage to your amplifier if you didn't do it correctly?
Why risk opening up a high voltage amplifier to install such a mod? 
Why waste money on an interface that is gone when you sell your amplifier?
Our RBI-1A will provide isolated safety to your radio keying circuit ALL FROM OUTSIDE THE AMPLIFIER! 
Be aware, the SB-200 (and some other amplifiers) has 120 volts present on the RELAY jack that will damage your radio keying circuit if you do you use a protective buffered interface

For safety, turn off your amplifier and follow the directions below.
We provide an RCA cable to go from the output of our RBI-1A to the "RELAY" jack of your amplifier.
Connect 12 vdc (center of DC plug is positive) to your station 12 vdc power supply.
Connect your radio keying line to the RBI-1A input, included RCA-RCA cable  (We make cables for almost every Ham transceiver made in separate listings if needed).

We also make a "BYPASS" or "STANDBY" switch for any amplifier that does not have one or if you want a convenient under shelf mounted switch in separate listings. 

What is included? 
RBI-1A with DC power cable and DC plug 
High braid quality RCA cable x2

Also note: THE RBI-1A IS UNIVERSAL YOU CAN USE IT WITH ANY RADIO* TO ANY AMPLIFIER**, so when you change radio or amplifier, you do not waste money since you can use the RBI-1A as described below.

* -  ANY radio that can provide a ground on transmit 
** -  ANY amplifier requiring a closure of contacts

73, Dan W7RF 
QRZ good