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!
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.
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).
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.