[ARC5] Some questions...transistors...

[Kenneth G. Gordon]

OK. I don't know what other forum to address these questions to, so I am going to address 
them here for advice from folks here who know more than I do...which isn't hard...

I have two questions:

1) I am trying to help a friend resurrect a power supply for an Icom IC-2KL. The power supply 
uses a pair of 200 watt NPN SilllyCon power transistors which are no longer available, 
Toshiba 2SD797s. The only "source" for these, an outfit in the UK, turned out to only be 
Chinese fakes,at $25 each, which blew up the second voltage was applied.

I have found what I consider to be a possible replacement which exceed the original Toshiba 
specifications in all parameters except one. Vebo for the Toshibas is 7 V while Vebo for the 
replacements is 4 V.

Must I "adjust" the parameters in the original circuit so that no more than 4 V is applied Vebo, 
or can I safely ignore this?

2) I am resurrecting an Astron RS-35A power supply for a "newbie" which looks as though it 
received a spike from a remote lightning strike. The over-voltage crowbar circuit and its SCR 
were blown up and the board was "toasted". All other components appear OK.

I bought a replacement circuit board from Astron ($20 + $7 shipping) which installed with no 
problems, and I also bought 4 Ea exact replacement transistors for it from Mouser ($2.79 
each). 

My only issue is that the Hfe for the original transistors test out at between 50 and about 79 
(none were shorted) while the new ones test Hfe at between 103 and 155.

I am reluctant to reinstall the old original transistors, but am also concerned that the much 
greater Hfe of the new ones might cause problems.

Advice?

Ken W7EKB

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