Texas Instruments TO-5 part Identification

[slim]

Hello All,
I am trying to identify a older TI part I have in my stock.
Please excuse the poor photo.

Thanks
Slim

[Zero999]

My gut instinct tells me it's a JFET.

Have you performed any tests with a multimeter?

[DaJMasta]

+1 to jfet, the J and a three digit number is still a common way for them to be marked.  I've seen a few marked with a U and a number, but U391 seems to be a no-go as well.

[Zero999]

The middle pin is usually the gate and the outer pins drain/source. J-FETs are normally symmetrical: the are drain/source pins reversible. A multimeter can be used to check that it's a J-FET and whether it's P or N-channel.

Use the diode check on the multimeter to test for a 0.6V drop between the middle pin and either of the outer pins. If it's N-channel, the inner pin will show up as an anode and the outer pins cathode and there will be no current flow when an outer pin is made positive, with respect to the middle. A P-channel J-FET will be the reverse of the aforementioned: middle pin cathode and outer pins anode.

Connect the gate (middle pin) to one of the outer pins. The resistance between the outer pins should be under 1k. The resistance may be higher in one direction than the other, which will be because the bias on the gate will change. If the gate is connected to the other pin, the resistance will swap round.

If the above doesn't work, then it could have a different/non-standard pin out, in which case test it again assuming one of the outer pins is the gate. Failing that it's faulty or isn't a J-FET.