Need help identifying ST 99240 21789 Has 3 leads, was in a Power-One SPL53-4000 ASSy 75114 101-Y quad output power supply +5 -5 +12 -12 volt outputs.

[GClif.1]

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[Peter BENSCH]

Welcome, @GClif.1​, to the community!

The power supply dates from the last millennium and should therefore be checked very carefully because, for example, the electrolytic capacitors may already have dried out and have therefore very probably lost a massive amount of capacity.

Is the device numbered U1, U2, U3 or Q3, Q4. Q7?

Would it be possible to upload a photo of the device where the marking is as legible as possible?

Regards

/Peter

[GClif.1]

Hi Peter,Thanks for the reply.

The component in question was Q5 of the power supply.

Here are a couple pictures of the component and where it was located on the board.

Would you happen to have a schematic for the power supply?

Best regards.

Greg

[Peter BENSCH]

Hi Greg,

thanks for the photos, but please delete the double-posted images below as this only confuses other users.

Unfortunately, there is no more information about the 21789, which was discontinued in 1997. I could only find out that it is a bipolar transistor (BJT) in package TO220, almost certainly also an npn with 500...700V collector voltage. Probably the BULD38D or a comparable type could be used instead, which can be found out by re-engineering the circuit. Only the manufacturer Power-One, now called Bel Power Solutions, has a circuit, but rarely does anyone give out their documentation. However, a photo of the bottom side of the power supply can also be found on Ebay, from which it is quite easy to reconstruct the circuit for such single-layer boards.

If the problem is solved, please mark this thread as answered by selecting Select as best, as also explained here. This will help other users find that answer faster.

Good luck!

/Peter