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Macintosh Plus Analog Board Q12 replacement

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after burning i'm looking for a replacement of SCR E0122 YA 8 located in Q12 international  , i founded in the forum a discussion in which suggested replacement with equivalent SCR 2N5060 , i try with it but does not work.

Any suggestion , or somebody has  original spare parts ?



Edited by macalle69
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There was another thread on this somewhere and turns out that this part is pretty picky, it wont go into regulation with modern substitutes. 

Hi Tech! 

Yes you're right, unfortunately I've tried with a modern 2N5060, apparently with the same features but it does not work properly 

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  • 11 months later...

Am also looking for a replacement for the E0122YA

     _ _
    | x |
    |_ _|
    | | |
    | | |
    1 2 3
E0122YA    (K G A)

tested the foillowing replacements:

BT169D     (K G A)    flupping set

P0102DA    (K G A)    flupping set


BRX49    (A G K)reverse!    mac starts but is not stable


According to the other post, it may be possible to tweak the power supply (alter some resistors) to accept the replacement.


Think it's R50 but can someone let me know and the value to change it to?



Altered the schematic from Larry Pina's book to the international version of the analog board.


R45    390
R46    4,7
R47    1K
R48    39
R49    1,5
R50    2k2
R51    220
R53    1,2
R54    15
R55    33k

C42    470uF
C45    470uF

Q10    2SC3153 BCE
Q11    2N3906    EBC
Q12    E0122YA    KGA


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I dont understand the early circuit topology enough to attempt to modify the design. I tried giving some input on this awhile back with Ferrix but I dont think it went anywhere honestly. You have to think, Switching power supplies were still in their infancy when this circuit was designed. 


Only thing the SCR can do is crowbar. once it latches, it hangs up until zero-cross. or the voltage completely depletes. Not to mention the schematic is drawn in such a weird uncommon way for todays standards to understand. It almost has to be re-drawn to understand the circuit better. 

Edited by techknight
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Thanks for your replies.


Put a 10K potmeter in place of R50, the mac is stable at 1K, so I replaced R50 with a 1K resistor.


Problem is I have very little regulation on RPx to adjust the 5V, voltages are 5.2V and 12.35V


Mac is working ok but don't know if this is the best solution.


Edited by marcelv
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tried some other resistors,

1K5 is too much (unstable),

1K , voltages too high.

1K1 and 1K2 work


this is using a BRX49, don't have other BRX4x to test.


You have to regulate the 5V to get as close as possible to the desired 5V value, too high or too low => crowbar.. :unsure:



Still looking for a beter solution...suggestions...


Edited by marcelv
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Yes Techknight I do not know if this trick works on international board , as I previously swapped the original  scr E0122 YA  from this 120V American Analog Board and installed on my international analog board with faulty scr bourned.

Then I have tried directly on the USA analog board with missing SCR , I have tested several SCR before but unsuccessfully.

Hope this could be of great assistance for everybody (like You):-)

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So I figured it out. macalle69 you in fact did unlock the secret. 


I have been comparing datasheets from old SCRs to modern SCRs, the threashold voltage and current for triggering is the same. Except for one thing. the load impedence is an order of magnitude more on the old SCRs than on the new SCRs. So when you do the math, to match impedence, that means the old SCR requires 2mA of threshold at 10Ohms, whereas the newer replacements are 200uA at 10ohms. old SCRs are 200uA at 100Ohms. an order of magnitude off. So what this actually means is the circuit is latching the SCR too early as the current across the 2N3906 transistor is ramping up.


So by increasing the gate impedance, you require more voltage to trigger the gate. The more resistance you add, the more voltage/current its going to take to trigger the SCR. Precisely what we want. 


Thats my view at this point. Yall will have to experiment more to see if my theory is correct or way off base here. 


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  • 3 months later...

Hi! I just bought macintosh+ and it also has blown fuse and Q12 EO122.


Did I get it right that there's 2 ways yo fix it?

1. Change it to brx45 and add 1k 1/4w resistor into central pin like macalle69 did or

2. Change it to brx49 and replace R50 with 1k1-1k2 resistor


Many thanks for your help!


I have't checked if there's something else broken yet, just wanted to get this clear to me.

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Hello Ochim

don't understand if you have international 240v analog board or US 110 V analog board, I'm pretty sure there is something else shorted , you better check and take measurements before burning one more time Q12 , if fuse blow there is a short .See red marked zone and check with multimeter Q11 at first , and then resistors near 


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