transistors going weak??

[2879]

Was wondering if transistors can get weak after time? Do they either work or die or do they slowly loose gain/amplification with use? Lets say you have a 2x4 box that does 400avg, well we know that is over driving the transistor. And you use this box a lot. How long should you expect it to keep doing "400 bird" assuming all the caps, resistance values are good?

 

[Shockwave]

Large bi-polar RF transistors like the 2SC2879 can do all of the things you mention. It is true that the gain of this transistor can be destroyed by excessive abuse. The transistor will continue to make output but with a lower amplification factor. The transistor has many fine gold wires inside that connect to the individual junctions in the transistor. I've seen cases where you can burn open some of these junctions but not all. There may be other scenarios that create this same loss of gain however, it's not something that you can reliably estimate since this damage can be done quickly or slowly depending on the operating conditions.

 

[-=PEAKABOO=-]

Was wondering if transistors can get weak after time? Do they either work or die or do they slowly loose gain/amplification with use? Lets say you have a 2x4 box that does 400avg, well we know that is over driving the transistor. And you use this box a lot. How long should you expect it to keep doing "400 bird" assuming all the caps, resistance values are good?

If you are showing a noticeable amount of power loss like say 20% then more than likely it is something besides the "pills"

 

[Captain Kilowatt]

I must confess. In more than 33 years of being in electronics I have never seen a transistor that was not either GOOD or BAD.That goes for bipolar or MOSFET's. I have never seen one that was "damaged but still working" other than one that was thermally defective.

 

[Shockwave]

I must confess. In more than 33 years of being in electronics I have never seen a transistor that was not either GOOD or BAD.That goes for bipolar or MOSFET's. I have never seen one that was "damaged but still working" other than one that was thermally defective.

If you run the transistor "normally" the reduction in gain will never happen. When you continuously over drive and over volt the transistor, over a period of time the transistors can lose gain. Most people never notice this loss in gain until they burn up one transistor. Put a new one in with transistors that have years of abuse on them and the new one will draw more current then the old ones. This is due to more gain on the part that hasn't been abused. The new transistor is now the weakest link in the chain and most likely to blow up again.

I've seen one technician attempt to replace a 2SC2879 in a 16 transistor amplifier with a new one. Just as expected the output balancing resistor across the combiner of the bank with the new transistor got hot. An RF probe on that transistors collector also showed more RF voltage. He took a resistor and proceeded to hold it from the base to the collector of the transistor for about 5 seconds and then would check the balance of the circuit. This over biasing eventually scrubbed enough gain off the new part to balance it with the old transistors.

While the vast majority of transistors either work or they don't, there are other possibilities. As you mention there are thermal intermittents. Transistors can also have leaky junctions that pass current in the reverse direction. Additionally when subjected to harsh operating conditions, you can reduce the gain of a transistor without completely destroying it.

 

[Captain Kilowatt]

That's probably why I have never seen what you describe in over 33 years. I worked in commercial broadcasting where gear was never run at max potential much less beyond that and I have never run my own gear beyond what it is speced for.

Well there was that time many, many years ago when I ran a pair of 6KD6's in a home brew amp with the plates the same color as the filaments but we won't talk about that little supernova incident.