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I have heard from several sources the powermates have a weak regulation system in general. I dont work on them so I dont have a schematic of how they are designed.
I have worked on a few generic off brand ones that were likely the low end no name version of the powermates. Those used a badly designed regulator. It was under built and poorly protected in all aspects.
Its likely yours has a similar regulator that is either failing to the high output state or has a bad calibration setting from one of the filter capacitors on the regulator board having failed.
Assuming yours is of similar design to the ones I have worked with.
The 200 volt capacitor likely failed because when rectified 160 volts gives a peak of around 226 volts. Cheap low grade capacitors dont have much over voltage tolerance.
I hope you used the correct type of capacitor too! 138 -150uf 250 volt sounds like a motor start capacitor. They burn up if ran for more than a moment at a time on full AC and dont take working in DC electrolytic aplications for very long as well.
I am not familar with Powermate circuit. Is this for field D.C. or is this the filter on the rectified three phase alternator output on an inverter-generator?
If so the alternator output voltage is a function of engine speed and load. My Yamaha EF3000i has an unloaded, at 3800 rpm engine speed, D.C. voltage into the inverter of 335 vdc. At full load it drops to just under 200 vdc. EF3000i is 120 vac sinewave inverter-generator.
I am guessing this cap in for the field coil supply. If it blew out because something else failed in the field regulator then just putting a higher voltage cap won' fix the problem. But it may at least let you further trouble shoot without blowing another cap.
You seem to have full drive on the field winding. Perhaps a shorted switching transistor that feeds the field winding.