RE: Clark electronics TK20
I have 2 of the first batch TK20-T1 boards & 4 of the TK20E boards, & nothing had to be done to have the IR work on any of them but plugging in working IR LEDs.
I think Dan already gave the best advice to troubleshoot the issue. Plug in the flash LED or another known working LED to the Clark board IR port & fire. If the LED lights, the IRport is working & should work with an IR LED too. I would suspect as Dan did for an IR that didn't light up, that maybe the IR leads are either reversed, or there was something wrong with the IR used, so I would then try some other IR LEDs.
If the regular LED doesn't light up when connected to the IR port & firing, then re-connect the same LED to the board's flash port & fire again just to make 100% sure the LED used for the testing lights up & is still working. If it does light up connected to the flash port, then you know something is up with the Clark board IR port because it wouldn't light the working known LED. At that point I would use a multi-meter to check the voltage output of the IR port to see if it is working at all.
~ Craig ~