Hey Renee, I was just reading on sci.electronics.design about transistors degrading over time when they get alot of reverse Vbe. Actually they were talking about exceeding the rating or the transistor but how is that rating determined ? They obviously test it destructively and don't take 20 years to do it. So I think it possible that it could degrade even if the rating is not exceeded. Looking at the circuit, it might have quite a negative spike in the base drive.
I tried to measure the rise time but I am not 100 % sure of the scope. (one of the reasons I want the 422 running). I am not sure of the geometry, as in \_\ vs. /_/. I also made no attempt to measure the fall time when the transistor turns on. The Miller effect causes the rapidly falling collector voltage to get to the base and fight the base current. It also increases whatever negative spike.
Hmmm, just thought of something, I could use the 422 to cop the waveform from the 422. Why not ? It's not like trying to tickle yourself. It works well enough (once I get that filter in that poofed on me) to take waveforms in itself. The only time I guess you couldn't is trying to view a waveform in the same vertical channel you're using. But conceivably in channel one you could view channel 2 and vice versa. I want in the power supply, there should be no problem. I have alot more confidence in the Tek than my elcheapo daily driver scope. Eventually I am going to end up with a 7000 series, I can smell it. I think I can talk my buddy out of the 7603 if I get his 7834 up to snuff. But he has been too busy to do anything so, oh well.
Turn off time is most important with an inductive load, but turn on time is also important. I already measured the saturation voltage and it is low enough, at least by my (hopefully accurate) math. Turn on time is not as critical of course with the inductive load, but it does matter.
The design may be flawed. (the hell you say !) It is possible that at higher input voltage when the regulation is kicked in more, that the forward base current drops. I dunno, but it doesn't seem to be turnoff time, it doesn't seem to be the saturation voltage, when else could this dissipation be happening ? Leakage when off ? Well I see maybe what, 50 volts there ? Even at 10 mA that's only 50 mW.
I might just wire in a pair of 2SD427s I THINK I got and I do have a heatsink. you might want to try that with the 2N3055s. All you need is a piece of metal, a drill and for 50 measly volts you can use wax paper for an insulator, or use 2 pieces of metal and don't worry about insulation for now. I happen to have a bunch of heat sinks around but you don't want a 1,200 cm^2 thing for the test because the 422 doesn't have that. In fact the 422 doesn't even have any vent holes. It was designed to be rugged. A Tek rep told me years ago he saw one that had gone down a hill in a car rolling and it looked much better than the car. I also read that someone had grabbed one and slammed it hard on a table repeatedly and there was no damage. If ruggedness was their goal I guess they wouldn't want a spill or a few raindrops destroying their masterpiece.
And it is a masterpiece. Madman Muntz would be proud. Other than the power supply it only has I think 49 transistors in the whole thing, 3 nuvistors and a couple HV regulator tube. I think that on top of good engineering, Tek has had better CRTs. I think they make their own. Check out :
when you have spare half hour. And I was 7 years old at that time.
Muntz was a TV maker who used so few tubes that other engineers said there is no way in hell it could work. He is said to have walked around the prototype department and just remove parts from the units until they quit working. I think wiki has a page on him.
Jeff Urban