buzzing and pop sound when fender power amp is turned on

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Hello,

I have this Fender 80W power amp (solid state) which I use for electric drums and guitar practice. The amp had some problems - it would start to distort randomly. I re-soldered some of the transistors and it seems to help. However now when I turn on the amp (from "cold start") it makes loud buzzing noise which ends with a loud pop sound after 2-3 seconds. After that everything seems to be ok, amp works fine. I tried disconnecting the preamp and it still happens. I also replaced the filter capacitors in power supply and no change. If the amp is on for 10-15min and I turn it off and on - this doesn't happen or it is not that loud. I am woried that this could damage the speakers.

Any ideas are welcome.
 
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