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IIci Motherboard Transistor D7/D8
Posted by: jmacz on 2023-08-21 22:33:27
My search skills are just not working great today so asking for some help.

I've got two transistors on a IIci motherboard that are pretty corroded. Not sure if they work or not but given their appearance, I wanted to start working on procuring proper replacements. These are the transistors I'm referring to:

IMG_5071.JPG

It's D7 and D8. What's interesting is that one of their legs seems to be disconnected. Looking at the Bomarc schematic, I see them referred to as 5D diodes?

IMG_5104.JPG

So these transistors seem to be used as diodes? Using a magnifier, the transistors have "5D Z underbar" markings on them (both D7 and D8). The markings on D7/D8 are barely visible so the below picture is from D3/D4 in the soft power circuit which seem to be used the same way (with 2 pin used) and have the same markings on them:

IMG_5103.JPG

I think they are in a SOT-23 package. But are these transistors serving as diodes, and if so, is the third pin (I think it's the emitter?) left disconnected? Does anyone have the proper part number for this (Mouser)? Thanks.
Posted by: Bolle on 2023-08-21 22:49:27
It says what they are right there in the schematics. 5D = MMBD914

Bildschirmfoto 2023-08-22 um 07.47.30.png

They are indeed diodes. It's not uncommon to have diodes in SOT23 package with one leg unused.
Posted by: jmacz on 2023-08-21 23:02:39
Man, I'm too tired. Thanks for pointing that out @Bolle.
Posted by: David Cook on 2023-08-21 23:12:42
I think they are in a SOT-23 package. But are these transistors serving as diodes, and if so, is the third pin (I think it's the emitter?

@Bolle beat me to the reply.

Perhaps the SOT-23 package was just more convenient for the pick-and-place machine setup.

"5D" is the marking. "Z" is the month.
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