Repair idle control box

[Barkascsyncro]

I have a problem with my syncro, the idle valve does not function I have bit the bullet and brought a new one to no avail. I have found the fault to be with the black control box on the drivers side behind the rear light, I have carefully taken it apart and found the main transistor to be burnt up. I have unsoldered it to solder a new one in but I can't read what value it needs to be due to it being burnt up, can anyone help?