You should be able to find a complete data sheet on the original devices.
Make up a chart of the principal factors that need to be similar. You
may find similar devices in the spec sheets for the original no. On the
web find out if there are applications notes that state any critical
parameters.
You want to match Beta, cut off frequency, and input and output capacity
if possible. Make sure the housing is the same or else you have to
fabricate a holder for the new transistors. Dissipation rating of equal
or higher is pretty much a necessity for a driver.
Before you spend a lot of time, be sure to scour the legitimate
distributors, (Mouser etc.) There are specialty distributors that carry
military semiconductors which may be equals to what you seek.
Look up the original manufacturer of the devices, and see if they can
point you to a source (if the original vendor is still in business)
Will spare final transistors work as drivers? If you can fit them into
the board or above it, that may be a solution, and carries the advantage
of the drivers can run cooler.
Stuart Rohre
K5KVH
Buy plenty of spares!.
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