On 02/16/2017 02:57 PM, Jon Elson wrote:
I was repairing a piece of old equipment, and needed a replacement RF
transistor. Checking at Digi-Key, I found very thin pickings, so i checked
a few other distributors, and found the same situation. They seemed to have
a modest choice of SMT transistors, but REALLY thin variety in T0-92 and
similar plastic packages. Not a good sign, as we have a lot of 30 - 40 year
old nuclear instrumentation here.

Another issue is the circuit is a differential NPN pair set up as a one-
shot. So, it was running about 23 mA through one transistor at idle, and
all of these on-at-idle transistors have failed. (The other transistors in
the pairs seem fine.) The part actually in the unit is an FMT1190, which
certainly seems like it should have been able to handle that current long-
term. After replacing it with the best thing I could find, the transistor
only has about 2V C-E, so the power dissipation is less than 50 mW,
shouldn't have burned them out. I'm wondering if somehow the startup
condition exceeded the base ratings.

Anyway, I'll replace the transistors and burn it in for a while and see if
it gives any more trouble.

Jon


A whole lot of TO92s went away in the last couple of years. I bought
several thousand as a lifetime prototyping supply.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net