Ashdown bass amp buzzing loud

[Throstur]

I´m trying to repair a bass combo, it´s a Ashdown, model EVOII 500. It´s a solid state amp with one preamp tube and 4x10 speakers. It is supposed to be 500W.

It suddenly started to give out extremely loud buzzing noise. It does not go away by turning down the volume or by pulling the jack plug out - only by switching it off.

Has anyone got any suggestions what might be causing this? The solid state amps I have repaired up to now have either been dead or had some crackling noises, usually bad solder points really. This one just screams at me! Loud!

[Shrapnel]

Look at the filter caps and check the power out section for blown silicon. Also check voltages. You may want to do all this without speakers. Also check the rectifier.

[Throstur]

Thanks Shrapnel. Is it OK to work on SS amps in general with the speaker/speakers unplugged?

[Shrapnel]

SS could care less if it sees infinate impedance, or no load at all. Tube amps are the ones that can get very fussy about that.
Hence with SS - 750W 2-Ohm, 500W 4-ohm, 300w 8-Ohm.... 16-ohm, infinate impedance (open circuit), doesn't care as long as the load isn't too low (i.e. it may have a hissyfit @ 1-Ohm, and go into full meltdown with a short if circuit protection isn't present. Tube would be same power rating for all impedances it had taps for.

The reason I say with the speaker disconnected is the buzz can be annoying at the least... if one half of that power section is shorted, it *could* be more voltage/power than the speaker(s) can handle. Better safe than sorry, won't harm a SS amp at all.

[bnwitt]

after you disconnect the speaker check for DC voltage present on the output jack

[Throstur]

It was a blown filter cap
Thanks
T