Auto Thyristor Flash problem

[John Reflecx]

I bought a used Sunpak 433D auto thyristor flash, and it has 3 auto
apertures and variable power in manual mode (full to 1/16). The
problem is that the flash always fires at full power no matter whether
it is in auto or manual mode. Do I have a broken thyristor? Would it
be hard finding a replacement?

[Jerry G.]

You would have to test the thyristor first to know if it is the fault.
There are also some electronic control circuits that read back the
intensity in most of these, and then adjust the flash to match. They
change the firing duration rather than the amplitude to adjust the
effect of intensity. If it is a simple adjustable flash, then there
would be a simpler time constant circuit to set the flash duration for
the effective power.

You would have to figure out the specs required for the thyristor, and
research the spec publications to select one that would work. The
manufactures of these flashes usually will not sell individual parts, or
support outside servicing. In house they change the complete
electronics rather than service it. The labour cost to do the service
including the parts, would be higher than the cost of the board
including the installation.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
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