As this is a somewhat unusual package, I suspect you're going to find
it difficult to find a direct drop-in replacement.
Depending on the circuit application, you might be able to substitute
a more modern sort of part. It looks as if the 2SD829 is a power
Darlington, intended for use as a saturated switch. It might be
possible to adapt the circuit to use something like an IGBT, and use a
different package which can be mounted on the existing heat-sink.
Take a look at the International Rectifier IGBTs (they're page 992 and
993 in the last-year's Digi-Key catalog I have here).
There appear to be some tradeoffs in the selection. The 250-volt
parts in TO-247 packages have Vcd(sat) values similar to those of the
2SD829, and much higher pulse- and continuous-current and
power-dissipation ratings, but don't have an internal reverse diode.
The parts that do have an internal reverse diode, and current and
power ratings which match or exceed those of the 2SD829, all seem to
have a slightly higher Vce(sat)... but as these are at a higher
current level, this may not be an issue.
As one possible example: the IRG4PC40 is a 600-volt part, in a
TO-247AC package, with Vce(sat) maximum of 1.85 volts, 49-amp
continuous current rating at 25 degrees C, pulse current IC(max) of
200 amps, 160-watt maximum power dissipation, and it has an internal
reverse diode. You *might* need an external gate-pulldown resistor,
or might not, depending in the circuit driving it (and might need a
series resistor of a few ohms on the gate to avoid snivets, if the
existing bipolar-drive base resistor won't serve). $5.74 in onesies
in last year's catalog (the newer ROHS equivalent part IRG4PC40PBF
is a couple of bucks more).
