$keywords = "tech tips,tech tip,FET,Power FET,2SJ50/2SK135,replacement,Robert M. Braught,Robert Braught";
$title = "Power FET 2SJ50/2SK135 Replacement";
$tech_tip = '
I thought I\'d report a recent success with an old problem (at least for
me); namely, finding / replacing \'vintage\' large metal \'can\' style
(TO-3) power FET\'s in power amps (to make them economically \'fixable\'
:-)
<P>
They\'ve become very scarce/expensive (AFAIK no easy out) for old Hafler,
SCS power amps, etc (FWIW Hafler doesn\'t currently have any options for
spares)
<P>
Targets being the 2SJ50/2SK135 devices, 160V, 7A, 100W.
<P>
Going to a TO-247 package, currently available from DigiKey (add -ND to
device number to order.)
IRFP-240 and IRFP-9240 (N and P type, respectively) rated at 200V (20A
for the N, 12A for the P) 150w
at $1.82 ea for the 240 (10 pieces) and $2.70 ea for the 9240 (10
pieces)
<P>
My testing was done on a SCS (Sound Code Systems) Model 2350A amplifier.
I automatically assumed replacing all the devices in the channel
requiring new output FET\'s, both N and P types (in order to \'match
characteristics\'; the safest bet!)
<P>
If you bend the two outside legs *straight* down from the plastic
package, the lead length and spacing will allow you to pop the new part
into an existing TO-3 socket (new rectangular single hole insulator of
course.) The new package single hole should align to the \'far\' TO-3
mounting screw. The remaining middle leg can be bent straight up and
then back, fitted through a single hole solder lug* and one of the
original TO-3 mounting screws through the lug will fix the device in
place and make the third connection. (If the screw is long enough, as
they were in my test unit; otherwise, get a longer screw- duh :-)
<P>
*H.H.Smith part # S450141206 worked *perfect* :-)
<P>
NOW THE REALLY IMPORTANT PART of this substitution: The source and drain
connections will have to be *reversed*. In the amp designs (i.e. the SCS
amp I tested this on) using multiple outputs wired in parallel, these
are the connections that are usually \'bussed\' in line from socket to
socket; the gate connection(s) are usually stubbed out with a small
value/wattage resistor, one for each output. Swap the source and drain
connections either at the socket(s) or on the driver board.
<P>
Voala! No BS worked, double-checked spectral, stability, gain, etc; the
\'original\' channel vs. the \'substitution\' channel as close a match as
one could ever hope for. Customer got a ~$100 channel fix (there are
total 6 outputs in each channel of the particular amp I worked this up
on).
<P>
Hope this helps someone, and of course, as always, *your mileage may
vary* ;-)
';

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