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Bradley Bell edited this page Jul 30, 2023 · 5 revisions

Q2 and Q3

The original Disk II cards from Apple use the MPS-U51, an obsolete and very hard to find, high-wattage audio transistor, for what seems to be a kind of "power-saving" circuit. Some period clones use different transistors, and some seem to do away with this circuit altogether. I for one don't understand why Apple really thought this feature was necessary, or chose such a high-wattage transistor for it, because they only power two ROMs and one 74LS chip, if anyone has any idea on this, please let me know.

To eliminate this feature

  • Omit Q1, Q2, Q3, R1, and R2
  • Connect pads 1 and 3 of Q2 together
  • Connect pads 1 and 3 of Q3 together

You can also replace Q2 and Q3 with an easier-to-find PNP transistor. I've tried it with simple 3906s and it worked fine. Make sure you connect them with E, B, C on the correct pins. YMMV.

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