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...).

I removed all of the electrolytics, and all but one tested
good on my scope. I replaced the dubious one, but the flash
still doesn't work. The oscillator whine doesn't start up.

But I did find one thing that gets really hot when the power
switch is turned on. It is a pair of three-pin parts
(triange pattern footprint) that look like cylinders, and
they are enclsed in a U-shaped heatsink. They are each
marked with the Mitsubishi triagle logo, and "B324ST", with
"12" on the second line. I assume they are transistors or
thyristors, but searching for them comes up with nothing.
This is all from the 1980s, so I guess it's not surprising
that the parts aren't active anymore, but it would be nice
to at least find a datasheet.

By the way, the dubious cap was right next to the mystery
pair.

Any ideas?

https://www.eevblog.com/forum/repair...0-speedlight/?
action=dlattach;attach=1070818;image