Re: 5440 PS transistor selection


I don't know the details of that circuit but unless somebody at Tek had been smoking something well out of the ordinary they certainly wouldn't fine tune the output voltage by selecting transistor parameters.

No sane designer would ever select for max hfe. In an ideal world you don't want any base current so a perfect transistor would have infinite hfe. Real life isn't so kind so you need a minimum hfe so it doesn't upset the circuit driving the base. With a darlington I can just about conceive a designer might need to select for min hfe but it seems unlikely.

My view is just replace with something that's similar and try it. If it works within tolerance and doesn't get horribly hot then you've fixed it. You say there's a +/-10% tolerance on the output voltage. That's big enough to drive a Sherman tank through. In any plausible design the voltage will be set by a reference (such as a zener or another stable voltage rail) and some resistors. If you can't hit +/-10% without trimming then that's like not being able to hit a barn door while standing inside the barn!

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