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

[Scott Robinson]

HI Ken,

Icom:  I use only two types of power transistor these days: ON Semi 
15024 NPN and 15025 PNP. They have  a very large Safe Operating Area and 
are better in this vital respect than nearly anything else made. Ft is 4 
mHz, not so fast as to be likely to cause instabilities.

Astron:  x 2 Beta should be within design range, Beta having always been 
specified over a wide range in the first place.

Peace,

Scott

On 6/12/17 11:51 AM, Kenneth G. Gordon wrote:
> 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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