Macintosh Plus Analog Board Q12 replacement

[macalle69]

Hello

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 ?

 

[techknight]

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

 

[macalle69]

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 

 

[techknight]

Well modern substitutes have a reverse pinout as well. Do keep this in mind. 

 

[Ferrix97]

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

Here it is: https://68kmla.org/forums/index.php?/topic/26604-macintosh-128k-issues/page-2

 

[macalle69]

May the Force be with you Ferrix!

 

[marcelv]

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

 

[techknight]

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. 

 

[Ferrix97]

The best thing to do is to try a bunch of BRX4x, It took me ages to find a working replacement. Best thing is to order an old part manufactured in the late 80s-90s and try that. Newer ones will not work.

 

[marcelv]

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.

 

[Ferrix97]

5.2V and 12.35V are on the limit of acceptable, I'd usually stay a tiny bit lower (like 5.1V), either way that shouldn't cause any damage

 

[marcelv]

Ok , changed R50 to 1K1 (2x 2K2 piggyback) Voltages are 5.1V and 12.25V

Have to find the sweet spot... :mrgreen:

 

[marcelv]

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..

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

 

[macalle69]

Hi Marcel 

thanks a lot for your support ! Will try and update you meanwhile I solved the problem finding another working analog board . 

 

[marcelv]

Maybe the following can help to understand the circuit better, is part of the powersupply schematic

does anyone have the schematics for the international version of the analog board? 

 

[macalle69]

Thanks a Lot Marcel 

Fixed following your suggestions!

 

[techknight]

With absolutely no details of what you meant by "fixed". Please elaborate for the others in this forum that could use this information. (like me

 

[macalle69]

Hi Techk 

telling the truth this fix I have done on an 120v (american analog board) I placed BRX 45 with 1K 1/4w in the central pin , this worked for my analog board 

 

[techknight]

Ah ok, so you added some resistance to the circuit.