[ARC5] Some questions...transistors... (I'm well OT too Ken)

Mon Jun 12 16:13:10 EDT 2017

  Hello Ken,
  It's hard to say without either circuit, BUT I have the (ICOM) manual
  showing (what I think is a circuit card number) U-192-8443 and I
  believe this is the DC-DC converter.    Have "A" circuit diagram, but
  I don't know if it is "THE" circuit diagram.   The document I have
  appears to be A-302 (under the words, Printed in Japan).  Circuit is
  over many sheets and I must print  (and tape) it to read with any
  degree of ease.  Do you want to try further discussion off-list?

  I'm in a similar situation to you.  I'm repairing things for others.
  I have a faulty battery charger.  It was a present from a  (now
  deceased) father to a son, and the son sees sentimental value.  

  The secondary of the center-tapped power transformer puts out (only)
  12.3 V (AC, no-load) and I can't see any sign of damage to the
  transformer.  So obviously the PSU cannot produce the 16 or 17 volts
  needed to charge a 12V battery.   Hmmm.  I'm starting to think that
  some weird circumstance physically removed turns on the primary and
  then re-joined the circuit so there isn't a shorted turn.   How else
  would an (apparently undamaged transformer) out out low voltage?   

Next I'm going to put a 2A load on the secondary and see if that yields
additional info.  

The circuit is weird - completely.  The transformer has two 110V-110V
primaries (standard), BUT!! 
For normal operation these are wired in series (bec here we run 240V).  
BUT a "start" (crank) function switches the two primary circuits in
parallel.  (i.e. the full 240V appears across each 110V winding).  The
manual says "crank" power (80A) is available for 5 seconds only!!  I
can't figure this.

 Returning to you "Q" about transistors.    As I recall (from a long
 time ago) Vebo is different from Vebr.  The (or "a") resistance across
 the junction changes the break-down voltage.  That's what I recall a
 guy called Leo Gunther telling me years ago.  Leo taught physics at
 Hobart Uni. and published a magazine called "EEB".  Electronics
 Experimenter's Bulletin.  He would say things like:   "Vebo is
 different from Vebr.  Don't forget that."    He was a wonderfully
 precise fellow, and insisted that all students pay close attention to
 matters like Vebo (and so on).    Leo built amazing 3-D solid state
 circuits - but once built the "inner-mystery" would be unavailable to a
 soldering iron.  (Get it right first time!!)  But I only knew him when
 I was 15 (or 16) and after that lost contact with him.    He knew lots
 and I knew little.  (Nothing changes).

   73 de Les Smith
   vk2bcu at operamail.com

More about Leo Gunther.
A FUNERAL NOTICE.

FOSTER
A Service to Celebrate the Life of ELAINE MARY FOSTER (AKA Ralph Leo
Gunther Silent Key) will be held at the Bethel Funerals' Chapel, 2d
Cochrane Street, Mitcham on FRIDAY
(December 12, 2014) at 10am.
Private Cremation

On Tue, Jun 13, 2017, at 04:51, 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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