loud hum from amp

[andomando]

I am trying to repair a CARLSBORO COBRA 90 BASS AMP with a very loud hum.seems someone plugged an extra speaker into it ,and did,not check the ohm,s.it kept blowing fuses all the time you switched it on.
I HAVE REPLACED THE FILTER CAPS,TRANSISTORS SOME DIODES AND POWER RESISTORS..ALAS NO SOUND FROM GUITAR ,JUST A MASSIVE LOUD HUM.
Any help would be great.

regards
Andomando.

 

[JonSnell Electronic]

Have you replaced the output transistors and checked the emitter resistors? You have a circuit?

 

[andomando]

Yes changed both...not sure if rectifier could be faulty.

 

[JonSnell Electronic]

If the bridge was faulty, it would blow the fuse straight away. No DC at all. They only go short circuit. Have you changed the driver transistors and checked where the DC off set starts from?

 

[JMFahey]

1) disconnect the speaker NOW
2) measure to check whether you have DC at the speaker out terminals. How much?
3) build anbd use a bulb lamp limiter.

Report results.

 

[JonSnell Electronic]

Check your circuit. Only use either RCA 40636 or 2N3055 output transistors!

 

[Mooly]

Moved to I&A forum.

 

[andomando]

Thank you all for your help.