Jake the Peg
July 5, 2010 2:51 PM   Subscribe

RS78 L12A. What transistor do I need?

I have a piece of, old but good, audio kit. Said piece is broken. I blame one sole transistor.

Sole transistor is removed and bears the inscription "RS78 L 12 A".

I've checked Radio Spares but their catalogue lists no matching part number.

It's one of your regular black NPNs with a 'D' cross section. I wish to replace like with like.

What transistor do I need?
posted by run"monty to Technology (8 answers total)
Is there any possibility it could be a 12v linear regulator? Similar to the LM78L12?

If it was, one would expect to see capacitors on either side of it, and the middle pin connected to ground.
posted by Mike1024 at 2:58 PM on July 5, 2010

Are you sure it's not a voltage regulator?

posted by Napierzaza at 2:59 PM on July 5, 2010

I'd guess that the "L 12 A" part is a date code, but "RS78" isn't a transistor numbering format I remember seeing before.
posted by hattifattener at 3:01 PM on July 5, 2010

Damn you beat me Mike.

Anyhow, a 78L12 is a 12 volt little regulator and are entirely commodity. You can purchase one from any vendor since it is a standard part that takes low voltage (no overheat worries). Make sure the pinout is the same by looking at the traces, but they likely will. You could even use a large 7812 if you really wanted to cram it in. But it would definitely not have the same pinout and you'd have to swap the pin positions. I have done this though because I didn't have the smaller supply.
posted by Napierzaza at 3:01 PM on July 5, 2010

If it helps, I've just noticed that inside the recessed circle on the round side of the transistor, there's a raised symbol: two parallel lines with an oblique line running through them. Does that help?
posted by run"monty at 3:04 PM on July 5, 2010

Ah, good call. Put "RS78" into this form and it crossreferences to a series of voltage regulators. The "L" in the middle would make sense if it's a lower-power variant, which would be in the TO-92 case you see instead of the larger TO-220.
posted by hattifattener at 3:04 PM on July 5, 2010

I've just had a look at the board and there is, indeed, a capacitor either side with the middle connected to ground.
posted by run"monty at 3:10 PM on July 5, 2010

I think you'll be fine using a standard 78L12
posted by Napierzaza at 5:34 PM on July 5, 2010

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