To All:
The CIRCUIT used in the generic CDI modules was quite simple and robust because of it. Failures in shipped, manufactured units was rare beause the circuit test was relatively simple: Power it up and you could hear the inverter singing about three octaves above middle C. That said the input-side of the circuit was working. To test the output you simply unclipped the trigger lead from ground. That would cause the SCR to dischrage the capacitor through the lead to the coil or other load.
Failures in use were usually caused by over-heating or operator error. I fried a unit crossing Laugna Salida, a (usually) dry lake about 70 miles long. The dust got packed into the fins to such an extent than the you couldn't even see the unit. (You can't believe the dust... finer than flour.) Sometimes the heating failures were caused by catching too much air-time. An the cheaply built units, such as the ones from Universal, the heat sinks on the power transistors are not attached to the case (which serves as the ultimate heat sink) but coupled to it by spring-clips wedged between the case and the transistor. Depending on the orientation of the clips inside the case, and how the case is mounted to the chassis, and how much shock-protection the mounting provides, a hard landing could knock the springs out of position. If they didn't short-out the circuit board, the lack of thermal coupling would eventually cause the transistors to fail... often hours after and miles from, the butt-buster that caused the problem.
But the most frequent cause of problems was the guy who put it in wrong. I've seen them tucked up next to the muffler (so as not to clutter-up their All-Original engine compartment), polished to a blinding shine... to match the acres of chrome (and retain lots of heat) and connected in a bewildering variety of ways, none of which matched the relatively simple instructions that come with the unit.
But during the heyday of CDI modules all were offered as kits (Universal did so until they were acquired) and the hands-down favorite reason for failure was improper assembly. ( For some reason I've never understood, most Americans think they are very handy with tools and things.)
Full-flow oil filter, swivel-foot adjusters, vacuum-advance distributor, CDI module, platinum-tipped plugs -- you can LITERALLY double the useful life of your engine, save money, reduce the amount of required maintenance and end up with a vehicle that drives like a real car :-)
Too bad being smart isn't the same thing as being kewl.
-Bob Hoover
PS - 'Singing' -- Low cost CDI modules use inexpensive bipolar transistors. When used as the switching transistors in a DC-to-DC inverter their maximum frequency was about 4kHz. The singing came from the bipolar-wound step-up transformer and was exactly half the running frequency of the oscillator, or about 2000cps... which is about three octaves above middle C.
Modern high-tech CDI modules typically use MOSFET's instead of bipolar transistors, driven by an oscillator chip at about 40kHz. The thing still sings but you can't hear it unless you're a bat.