The transistor is used in a circuit from the transmitter output to a vintage POWER AMP.
Seems like the T/R relay in our rigs is very current sensitive...and...the T/R relay in the amp can actually 'blow out'
the rig relay.
His circuit only puts 350uamps to the rig relay.
The TRX lab gent is a 'genius' when repairing ham gear!!
Here's his video on the relay protector:
https://www.youtube.com/watch?v=j7Aw0Y_Gan4Since you never gave us any history in this thread, this post sheds no information on what the problem was.
After reviewing a portion of the video, I see why you want the transistor. He should offer the video for sleep deprived people
Interface circuits for modern solid-state radios to interface with older power amplifiers that run high keying voltages has been around since the middle 90's if not earlier.
I use the Harbach designed interface between my solid state rigs and my Heath SB-200 amps that have a -130 volt keying voltage. The design uses an Opto-Isolator design and works extremely well. MFJ and several other manufacturers and garage builders also have designs on the market.
Here's the Harbach keying interface for the TL-922/922A:
https://harbachelectronics.com/shop/kenwood-tl-922/sk-922-soft-key-keying-interface/W8JI has also developed an universal relay line buffer:
https://www.ctrengineeringinc.com/universal-relay-line-buffer/The problem with the simple TRX design, if that, if the transistor breaks down for whatever reason, the high keying voltage of the amplifier will appear on the delicate relay contacts of your solid-state rig and, most likely, will fuse the contacts together.
