Hi folks, I have to repair one channel of the power amplifier inside the Spirit powerstation and I want to share with you something that drive me nut for too much time.
It's came in with intermittent problem at one channel, low output and in distortion. I restrict the problem to TR204, a MPSA92 pnp transistor. When the proble occur simply probing on his leg solve the problem. So I did think to a dry solder joint but redoing properly the solder didn't solve the problem. So I did think to maybe some sort of corrosion that the transistor suffer. I did replace it with a modern Diotec semiconductor MPSA92 and until it don't burn the power amplifier did work. Now I have some experience and some basis of electronics but this things take to to a rubbit hole that never end. At the beginng I did think that TR204 overheat it's due to some other problem with the load of this stage, I did think that because the Vce it' quite stable, it may vary some tenth of volt in a almost 60V region, and because while heating up the ddp across FR202 increase. I did test almost every single component downstream with no luck untill I decided to swap TR201 with TR204 and problem solved. The trasistor is still burning hot but the ddp acress FR202 is stable and equal to the other channel and that means that also the Ic is stable. So with some electronic basics theory is TR201 and TR204 two current source? So does everything downstram TR204 (under some boundary) not inflencing Ic in TR204?
but the main question is why the old motorola work great and the new Diotec doesn't? (In did try more than one transistor)
This is the link of the SM.
https://elektrotanya.com/soundcraft_spirit_powerstation_mix.pdf/download.html
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