Re: 75 HP Chrysler
In order to generate the necessary 50-60,000 volts to fire the plugs, there is a God-awful amount of power going through that power pack. They normally run warm to the touch. That's why the front is finned. I have seen some of those power packs actually melt-out the whole rear potting (Sealing compound).
I never opened one up without destroying it (it is not possible) but it is capacitor discharge ignition controlled by transistor. The distributor signals the transistor at the appropriate time, using either points or an electric eye. The transistor switches the capacitor to dump into the coil. Since the points or electric eye are only used for signal, not power, you get a relatively long dwell time and higher power charge in the capacitor. When the capacitor dumps into the coil, you get a very high short duration pulse. This generates an extremely high voltage when the pulse generated magnetic field collapses inside the coil. This results in an extremely hot spark at the plug. Gives you a thrill if you happen to get jolted. Runs right up the arm and through the heart. ZZZZZT!
Mode of failure is usually either the capacitor shorts or wire insulation melts and you get no spark, or the transistor fails. If the transistor fails, you get a constant spark rapidly cycling consistant with the charge time of the capacitor.
They can be repaired but since they are potted, the repair costs as much as buying a replacement.