At 03:21 AM 1/23/2003, Todd Last wrote:
>I wonder if there is a beefier power transistor that you could replace the
>BD438 with that would not be so prone to burning up?
>
>http://www.unofficialbmw.com/repair_faqs/idle.html

The BD912/NTE332 is such a device, recommended by someone.  However this is
already a four-amp device carrying half an amp -- reading the above link
indicates that the failure suggested lack of heat-sinking (none of these
power transistors can carry anywhere near their ratings without external
heatsinking -- this one expects to be bolted to some chunk of aluminum or
such; that's what the hole is for), and/or lack of mechanical support for
the solder joints.  The two effects may well be synergistic -- I'd be
inclined to carefully add an aluminum/copper/brass strip for heatsink,
bearing in mind that it will be electrically live and must be protected
from contact with other components -- and goop some electrical-grade (no
acetic smell when curing) RTV compound around the pins after making sure
they are properly soldered.  The strip would not have to be huge to make a
considerable difference -- doubling the existing surface area should help a
lot.

It might simplify things to bring out an external socket for transistor and
heatsink, or just a set of flying leads.  If the latter, make sure leads
are wrapped at least a turn-and-a-half around the transistor leads before
soldering them -- solder cannot make a good mechanical joint, only provide
electrical contact to an existing joint.  Also it would need to be
protected from environment -- small metal box siliconed to existing unit
might work, box itself acting as heat sink.  If necessary to electrically
isolate it, insulating washers are available (mica washer with ZnO
heat-sink paste, or special heat-conductive heat-sink washer -- both
available for various package types but may be hard to get in onesies...

Note that the BMW fella also had a cracked trace (the copper foil "wire" on
a PCB) at the transistor (prolly hard to see without a microscope).  This
might have been a result of local board heating -- apparently the BMW unit
had the device mounted horizontally on the PCB instead of standing clear as
it does in the VW one.

david

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"