Carlsbro Bass Cobra 90W Combo Issue...

[theosd]

This was my first ever bass amp and it never really worked right. I've just fished this out of my mum's garage and plugged it in at rehearsal last night. Worked fine at half volume for about 5 minutes but then would begin to distort when I played heavily, even at lower volumes. The distortion would phase in and out every so often, each time it would seem to rectify itself if I let it rest (overheating??) but if I played prolonged heavy lines the volume would drop also as if the amp is failing, but would still be ok after a rest. Most strange.

Any ideas? I'm not electrically minded and haven't had a look under the cover. I'm reluctant to get rid of the amp because when it does work the parametric eq is great and can get some really cool vintage tones from it. It's pretty loud too for a mere 90w.

[thepurpleblob]

At the risk of being facetious - it's broken. Take it to somewhere that repairs amps. It could be almost anything. The actual speaker, followed by the output transistors are a good place to start but the fault could be anywhere in the circuit. Without the proper test equipment you have no chance.

[theosd]

Hehe fair enough!

[thepurpleblob]

Repairs are actually like that. I used to run a repair business fixing car audio equipment for the motor trade. We tried to avoid the public - "can you just fix this - it's got a wire off / loose connection / etc.". We'd seen all the common faults in everything and a reply like "nope, it's the pair of BC578/579 surface-mount transistors buried underneath the board" elicited a range of blank expressions

Now I've got a workshop behind my house I keep thinking about setting up doing audio repairs again. You become an avid collector of circuit diagrams and index cards with notes on !!