Hi Folks,
one of the transistors in my instrument pod which are used for led multiplexing (TR1 and TR2) just burnt down.
And now I'm having some trouble finding a replacement.
This transistors are PNP types. sinclairc5.com says:
These transistors should have a saturation voltage of less than 0.5 volts at 100 mA collector current and 3.75 mA base current. The FST290 is a specially selected transistor to this specification.
I thought a BC557C could be applicable. But it is not - the LEDs are glowing just a bit.
So I analyzed the signal at the base pin of the remaining 2nd FST290 which looks like this:
![](http://www.cypax.net/projects/c5/images/DSC02785.JPG)
While the base pin at the BC557 looks like this:
![](http://www.cypax.net/projects/c5/images/DSC02786.JPG)
As you can see, the base signal of the BC557 won't get down enough (and so the LEDs won't light up).
But this is also a little bit astonishing to me since I just don't understand why.
However, has anybody here ever replaced one of the FST290 transistors? What type did you use for replacement?
Thanks for advice!
-Phil