Sus(Silicon unilateral switch) VS scr or thirystor

Hi. Not even close -- there's no way an SCR will function the same as a 4-layer diode, or SUS.

These parts are still available at American Microsemi. This outfit is actually a broker. If you're uncomfortable with the price, you may want to try a 2N4990 (might work, less rigorous specs) at this website:

I dealt with them on larger orders some time back. They now apparently accept smaller orders, and take credit cards. Don't know about that, but they have an office in the States, give them a call. They used to be very helpful.

If you find a sample of this part does work for you, it may be helpful to have a test rig with a socket for the SUS, and just try each part before you use it. Seeing as these are brokered parts, that might be a good idea, anyway. Be safe with line voltage, though, and make sure power's off before plugging the Device Under Test.

The thing is, these old thyristor triggers are all basically obsolete. I'd consider a "lifetime purchase" for your repair needs, and start looking at what it's going to take to replace the equipment if you're in-house, or notifying your customers that due to part unavailability, you will soon be discontinuing support for this equipment if it's service.

Sometimes you've just gotta accept it -- "It's dead, Jim".

Cheers Chris