I see that wa2ise mentioned that CR stands for crystal rectifier. I've just scanned through the 3 pages so I hope I haven't missed anything. There's a pretty good chance that you're looking at unidirectional transient voltage suppressors, or TVSs, although those don't look like any that I've used or replaced. 6.8V TVSs, also called transorbs, are very commonly used to clamp inputs from the outside world and have less capacitance and faster response than a zener. It should look like a normal silicon diode forward biased, and you can test it with your power supply and a series resistor, for its clamping voltage. They aren't supposed to be conducting unless there's a surge coming in that needs clamped, so the voltages don't tell you a lot unless they're above 6.8V. The ones I use are black and are marked 1.5KE6.8A(1500W) or P6KE6.8A(600W). That is the wattage that they can withstand for a millisecond or so. A 1.5KE6.8CA has no band and is a 1500W, bi-directional TVS. The 600W transorbs look about the same size as a 1N400x, 1A diode, while the 1500W parts look about the same as the 1N540x, series 3A diodes.
