I recently started repairing Marshall Valvestate 8080. First symptoms were vere quiet sound and a hum. So first a checked for bad connections, broken solder joints itc. Because it is the most popular issue with these amps. I noticed that when turning on, the speaker was making a loud knock. So I disconnected it and checked the voltage on the speaker terminals and it read -7.4V. Power supply works fine, there is 15V on each Zener diode. I found a similar post on this forum but it wasn't resolved so I decided to try my luck here. The older post said that the voltage on the speaker says something about which semiconductor is broken. Could somebody help me figure it out?