Thyristor issue with Macintosh Plus ED

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Thyristor issue with Macintosh Plus ED

The past months I have been trying to repair a broken analog board. I checked a lot of measurements on both the broken ED and a working Plus.

I concluded that the Q11 thyristor I had replaced did not meet the required specifications (the original one was blown up along with some resistors).

I then ordered three different types, each with a different specification, but they didn't work.

 

I ordered the S4X8ES (200uA), P0102DA 1AA3 (100 uA) and a S4X8ES2 (50uA).

I ​​can't find the original E0121 anywhere. If I put the thyristor from the working Plus in the broken ED, it works fine so it's definitely the thyristor.

I also replaced all capacitors, some burned resistors and more.

 

Does anyone know a good replacement type?

Thank you for the help!

 

 

 

 
 
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Last seen: 32 min 39 sec ago
Joined: Feb 27 2021 - 18:59
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must be fast

See the discussion at Replacing old SCR's

The speed of the thyristor is critical because it controls the duty cycle of the switcher and thereby the output +12 V.

It looks like E0122 and E0102YA were also used for Q10.

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Last seen: 32 min 39 sec ago
Joined: Feb 27 2021 - 18:59
Posts: 716
Q11?

Also, I'm sure it was just a typo, but Q11 is not a thyristor, it is a NPN transistor, type 2SC2810 (can substitute 2SC3039).

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Last seen: 2 days 18 hours ago
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Managed to fix it

Sorry it was indeed Q12. I managed to fix it with a BRX49 thyristor.

It got pretty hot so I attached a heatsink to it.

I also had to increase C40 to 660pf and add a diode in the gate circuit of Q12.

 

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