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Macintosh Plus Analog Board Q12 replacement

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Thanks Macalle69 for giving me a good start. My analog board is international 240V.


I did find that R53 and R54 were dead. About Q10 I am not sure if it is fine, as my dmm shows that b->e and e->b are shorted when tested in diode mode. C to emitter or base shows readings when minus lead is on c and OL when positive is on c.


I will continue tomorrow to check if there is more broken components.


Will you think that both fixes mentioned earlier in this thread would work for Q12 after i've founded and replaced all burned parts.


Many thanks!

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Have you also checked the diodes and the feedback circuit close to the optocoupler?


You should check the main switching transistor off-circuit, as it's connected to primary of the transformer (which is a very low "DC resistance") but that rarely fails since it's beefier on 240V boards.


As for Q12, I've had luck in the past with old BRX49 and BRX46. The older the better, the ones I've got don't require any resistor mod, they just have a different pinout.

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Hi Ferrix, thanks for tips, I will start to check those tomorrow.


I believe it will take time from me to find the circuits from the board so I would really appreciate if you could point me where they are actually located. 

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