[Elecraft] OT: Obsolete Transistor MS1700G

[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