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

[Dennis Monticelli]

Ken,

My career was in the semi business.

Regarding Hfe.  That DC parameter is usually measured at a modest current
level. I would not be surprised to learn that the high amperage beta is
closer to what your originals exhibit at similar high amperage.  In any
case 100 to 150 if not out of line at all if measured at 1A.

Regarding replacement transistors that blow up.  Yes, exceeding BVebo can
rapidly degrade or destroy a transistor but does your ICOM circuit really
do that....put a negative voltage spike on the transistors?  More likely
the SOA (safe operating area) of the replacements is no where near that of
the originals.  SOA varies a lot from maker to maker all bets are off with
Chinese sources.  SOA is only stated in the performance curves section of
the datasheet and most abbreviated datasheets don't provide it.  BTW, a
BVebo cannot be as low as 4V because the physics of the transistor's
construction would not allow it.  A true 4V breakdown would represent what
is termed a degenerate junction...not good for proper transistor action and
only good for making zener diodes.

Did you check out the NTE387.  It is listed as equivalent.  Actually it is
heftier.  The only downside (apart from price $37) is the relatively high
capacitance.

Dennis AE6C



On Mon, Jun 12, 2017 at 11:51 AM, Kenneth G. Gordon <
kgordon2006 at frontier.com> 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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