I have a repair shop in the Netherlands, for all kinds of Guitar and Bass related amplifiers, so also Blackstar amps!!.
I thought, maybe it's usefull to share this one with You.
After a long search on the internet, I could not find anything usefull to determine what the cause is, for the popping sound when switching from clean to overdrive and back.
Also there's a popping sound when operating the voice switch in both channels. The sound is at that moment very bad, lack of power etc..
But there's NO popping sound when the voice switch, of the clean channel, is "off" and the voice switch of the overdrive channel is "on".... Strange is it not? When there is no popping sound, during switching, the sound is good!
The sound from the SEND jack is at all times good, switching also has no popping sounds. The RETURN sound is good, only at the above described switching position, in the "popping position" the sound is bad..
Ok, long story short: here the solution.
The problem is caused by a bad mosfet (FET-TR2, IRF830) in the phase inverter. In the Soloist the phase inverter (PI) is made out of 2 mosfets, almost the same way as a tube PI.
So, somehow Blackstar is switching a (switching) FET in the feedback part of the PI, Why? i'm not sure of. But there is a switching going on at FET19.
Now, when the mosfet is bad at that moment, the bias value of the corresponding (EL34) tube is way off.. And is causing the bad sounding endstage, and also the popping sound, Yes! because the bias Voltage is going up and down at that moment...
Allright replaced the IRF830 (and also the other-one) for a more heavier one. The original has a max voltage of 500Vdc, the new one can handle more than 600Vdc.. (N-Channel, btw)
Cheers! Gerrit Peet
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