Transistor Replacement

[teenix]

Q1 PNP passes the battery charge current under heavy display current demands
Q2 PNP passes the display current (larger tabbed transistor)
Q3 PNP display current monitor and switch for Q4
Q4 NPN switch for Q1

Q3 and Q4 probably similar specs.

Note: If you changed NPN Q4 with a PNP 2N3906 it won't work properly, although your text may have a typo.

cheers

Tony

(04-20-2022 03:32 PM)Dreato Wrote:  I had the dim display problem recently, too. I replaced all 3 transistors (Q 2, 3 and 4) with generics: 2N3906 (I had a bunch of these) for Q 3 and 4, and MPSU56 for Q2. This last is an audio transistor I had on hand, but it worked fine.

The original Q 3 and 4 tested bad, but Q4 was fine, so I kept it in the parts box.

This leads me to a question: Generics have worked for me in many HP applications over the years. In this case, Q 3 and 4 have different parts #s. Do you think they actually have significantly different specs?

What does everyone think??