A-B 1336 VFDs do indded have SCRs in the front end. They use them instead of diodes because they don't use a pre-charge current limiting resistor (and associated contactor) to protect the DC bus from inrush when you first power them on. Instead, they ramp the incoming voltage into the DC bus using phase control firing of the SCRs configured for rectification. It's a convoluted way of attaining the desired effect, but it saves them from having to design a pre-charge circuit. Several VFD manufacturers do this on larger HP drives now, especially those like A-B who also make DC drives and Soft Starters where their buying power for SCRs makes them competitive to diodes.
Still, IMHO the firing circuit necessary for them just makes for another thing to fail and if it does, they misfire, causing the SCRs to short. I have had dozens of 1336 front ends go bad on me in areas where line power is less than perfect, i.e. generator power in Alaska. The generator AVR can interact with the SCR firing circuit and cause both of them to start oscillating, which ends up in the SCRs failing. Once I figured out what A-B was doing, I gutted the SCR front ends and built my own diode front end and a pre-charge circuit, problems went away. Incidentally, A-B insisted all along that I was getting hit by lightning strikes, yet THEIR DRIVES were the only thing failing!
