On Sunday, September 13, 2020 at 2:01:59 PM UTC-7, Peabody wrote:
I have three of these flashes that I've converted to manual flashes. One
works ok, The second i have to turn on and off several times, but it
finally starts whining, and works perfectly thereafter so long as it stays
on.

The third used to do that too, but now it won't whine at all. I've taken
it apart, and the main capacitor checks out good, as does the main power
switch.

As others have mentioned, that usually is an oscillator-startup problem, and
the entire circuit is ONE feedback loop oscillator; every component is a suspect.
With age, high voltage parts are most susceptible to failure; the HV rectifier
and its associated switch (SCR or transistor) that drive the coil are inexpensive
to replace (except you'll have to identify them, and match to modern available
parts, and pay shipping for onesies...).