Hello - I have purchased a 2nd hand Carlsbro Cobra 90 Amp head. It powers up, and the connected speaker has a "hum" - so seems there is power and audio feed "at the back end". When I plug a guitar into one of the inputs - I get the guitar sound coming through the speaker - but only for a few moments. Then sound gradually gets quieter and then fades out completely.

I have tried cleaning everything inside with electrical contact cleaner. I cannot see any bad solders or scorch marks on the boards - no visibly damaged components.

If I power the amp off for a few minutes, and then back on, I get a few seconds of sound , but then it fades off again, and nothing.

Please help!

 

Check for any swollen or distorted capacitors, burnt resistors and poor soldered connections.
Good luck.........