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Transistor confusion on 502 Board

Posted: Tue Apr 30, 2019 11:35 pm
by Jeff
I am confused.

In the schematics for the 502 board, the Transistors for Q1 thru Q6 in the RS-232 section are written in pencil.

Q1 2N5226 NPN
Q2 2N5526 NPN
Q3 2N5526 NPN
Q4 2N2255 PNP
Q5 2N2255 PNP
Q6 2N2255 PNP

Note... Q1 is different than Q2 and Q3... a 5226 not a 5526... note the transposed second digit.

The SAMS schematics DO NOT specify a part number, AND.... they show all 6 transistors as PNP!

So... my question is... does anyone know what are the real part numbers for these transistors?


Re: Transistor confusion on 502 Board

Posted: Wed May 01, 2019 1:50 pm
by dave
Just looking at the circuit, the SAMS is wrong. It is showing PNP transistors for Q4-Q6, but they should be PNP. It is also has the +5V and -9V backwards for those transistors.

The 502 schematic is correct, but penciled-in transistor names are all over the place (by the guy who sold me my C2-4P, most likely).

I believe the NPN transistors are specified as 2N5225, and PNP are 2N5226, which are real parts. I think they are not routinely stocked at most distributors.

However, there's nothing special about this application that would require those parts. Any jelly bean NPN such as a 2n3904, 2n2222, or BC547 will work for the NPN, and any jelly bean PNP such as 2n3906 or BC557 will work for the PNP. The 2N5225/6 have slightly higher max beta values, but there is plenty of base current available, so the difference ends up not being a factor. Looking through other OSI schematics, it looks like they use the 2n5225/2n5226 as jelly bean transistors throughout their boards. They probably just bought them in bulk and had no need use anything else for basic switching.

Most hobbyists back then would have used whatever was they happened to have, usually 2n3904 (or 2n2222) / 2n3906, since those were stocked in 10-packs at Radio Shack. In fact, it's not improbable that OSI would have bought whatever bulk transistors it could get a good deal on, and use those in production.

Be aware that this circuit expects -9V to be present. You can either supply -9V at the backplane, or disconnect the -9V from the backplane and ground it, and it will probably work.



Re: Transistor confusion on 502 Board

Posted: Thu May 02, 2019 10:39 pm
by Jeff
A very comprehensive answer!

Thanks, Dave. :) :)