OT: Obsolete Transistor MS1700G

[Howard W. Ashcraft]

I am working on a power supply with a linear regulator circuit containing two MS1700G power transistors that have shorted.  I don't think they are in any way exotic, and I'm sure there is a modern replacement.  But I can't find any specification information anywhere to begin the replacement analysis.  The only internet search results for a MS1700G are from distributors that have $250 minimums and are only providing a "quote."  None of the normal providers (Digikey, Mouser, JAMECO etc. stock this transistor.)  Does anyone have a reference or spec sheet information regarding these transistors?

Thanks

HOWARD W. ASHCRAFT, Jr.
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[Stuart Rohre]

Transistor substitution:   If there is at least one good junction in the
device you want to replace; you measure the diode potential of that junction
to see if you have a silicon or germanium device:   0.3 volts is typical for
Germanium, perhaps a bit less.

0.6 volts is a Silicon junction.
Now you know the case you have on the device, (is it one of the typical
power transistor heat sinkable cases like TO3 (metal) or TO 220 Plastic with
metal tab?)

You can go to the NTE replacement semiconductor catalog and find similar
power transistors, and with knowledge of how much voltage and current
maximum the circuit handles, you pick a device and try it out.   You always
replace a device with a similar device and the gain should be about the
same.  You can get an idea of the gain by comparing the base and emitter/
collector resistors.

Luckily, except for getting too high a gain in critical feedback circuits,
this usually works pretty well for such as a power supply application.

I could not find a direct cross to the MS 1700G in the NTE list; but there
is likely a similar transistor that would do fine; just you have to know if
you are replacing a silicon or germanium type.  Picking a higher rated
voltage and current capable transistor is OK.

Stuart
K5KVH


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