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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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  • 2 years later...

Resurrecting this old thread in the hope of some advice / guidance from the pros...  I have a Mac Plus which decided to go "pop" recently when I powered it up.  Inspecting the 'board shows a blown fuse and a disintegrated Q12...




...further inspection, also seems to indicate that the insulation on the flyback transformer is gone... 



This happened a couple of months back... and I vaguely recall finds bits of 'something' - which may have been the fragmented insulation...  A visual inspection of the 'board doesn't show anything else amiss... it's actually pretty clean.



From reviewing this thread... it sounds like I should also check Q11 - or should that be Q10?  The post refers to Q11, but Q10 was what's circled. :-/ It also sounds like I should check R53 / R54.  Anything else?


Also... could the insulation-less flyback have caused the original blowout?  I plan on replacing it anyway (along with the RIFA), but curious if that could have been the cause... 


P.S. although clean... the 'board has obviously been repaired in the past... 



Ooo... that C18 doesn't look too healthy... adding that to the list...  

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  • 3 months later...
  • 68kMLA Supporter

I'm feeling like this is the 'Plus Q10 / Q12' thread, so I'm going to add some additional insights here for reference. No guarantees it will make sense.


From some digging around, it would seem the determining factor in finding a working replacement SCR for this component is a very low turn off time (tq), in order for Q10 to avoid conflicting with Q11 during their self-oscillation cycle. The E0102 / E0122 have a tq MAX of 10μs, where the suggested newer 30V/0.8a replacements (2N5060, NTE5600, even the BRX44) have a tq of 10μs TYPICAL, or slower, if tq is even listed on the datasheet...


The 2N5060's and BRX's sometimes work because of production inconsistencies, and some will happen to be fast enough but it's a gamble. A resistor on the gate may also work with some as it may desensitize the circuit... but it's also a gamble without an oscilloscope for the same reason.


Unfortunately 'fast' SCR's are no longer being made as the kind of circuit we're dealing with here has been obsolete for a while. One option that still seems to be kicking around at the moment is the BR103 - there seem to be a few available (mostly on the east side of the Atlantic (https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=br103+thyristor&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=br103+scr), I got mine from Germany), and they have a tq max of 6μs and I can confirm they work as a direct replacement (CGA is reverse from E01012/E0122/2N5060 with an offset gate pin, but the board is ready for it).


What I'm interested in is what we do when reliable replacements are no longer available. I'm intrigued by the idea of *making* fast SCR's with a pair of fast high-voltage bipolar transistors and some passives in SCR configuration, and something that seems to end up there in this Hackaday post: https://hackaday.io/project/168100-fat-macintosh-repair/details


This is beyond my capabilities to design (I'm no EE that's for damn sure), but the idea of breadboarding a fast SCR in this way with modern components seems like a path forward.


E01012YA datasheet

2N5060 datasheet

BR103 datasheet attached



Edited by desertrout
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I do like that solution. Its neat, he used a specific SCR and a "clamping" circuit to prevent the SCR from false-triggering during a pulse cycle. aka, the dv/dt scenario. 


It works, and I like it. :-)


Not sure if I would have figured that one out. Even though I am an EE, I am a different type of EE and these old analog/discrete self-oscillating circuits mystify me sometimes. One thing is for certain though, those particular self-oscillating switching designs are HIHGLY sensitive on the components you use. Some early Class-D Audio amplifier designs are too, MTX being one of them. change the outputs to a different type, the circuit no longer works. And the outputs are also NLA... 

Edited by techknight
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