Topic: Replacement for FST290 transistor needed

[Phil]

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:


While the base pin at the BC557 looks like this:


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

[radiomarty]

Hi Phil - replaced most items in the control box but never in the pod - hopefully Plod or Chas may be able to help on this one - good luck

[Karl]

Gotta love the Pod - I reckon even NASA would struggle with them 

Must be time for someone with a bigger brain than me to design a simpler replacement that fits within the original "shell".

[PLOD11]

Just made a phone call, hope to have an answer by this evening, just had a look at one and i am sure there will be a alternative ...

[Phil]

Ahhh stupid!

The BC557 works - I only foolishly reversed it in polarity!
Guess it was just too late yesterday...

However, this is how it works:
When looking on top of the original FST290 transistor, the flat side points to the left.
If you replace one of these with a BC557 the flat side must point to the right.

Have a look at this:


The BC557C is a very common transistor. You will get it in every electronic store. - Just in case someone has the same problem.

-Phil

[radiomarty]

Well done that man - good info to have on here cheers

[PLOD11]

Well done, that will certainly save time in the future.
Maybe a listing of all the components on both the POD and the Control board with alterntives for those that are no longer availble might be something to think about for the future (during the winter months  )

Compile all the data into one place for reference ........

Maybe even part Nos from malins etc and source details if not stocked by maplins.

Just a thought.

John

[Karl]

Plod - I've set up a new section for that ...but it's all a foreign language to me so you techy types will have to update it yourselves 

http://c5alive.co.uk/forum/index.php?board=24.0

You've been added as a moderator for this new section,plus radiomarty and retroandy should be able to help as the global moderators. If you want anyone else to help out let me know 

[radiomarty]

Top idea Plod ! - if you need any help just ask - ps nice gloves