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Have a Marshall AVT 150H, main power light comes on, fans are spinning, tube is glowing but no led's on the front and there is a buzzing that builds louder and louder than gets quiet and repeats.
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Check your power supply voltages. -
the tranformer? or the speaker outputs?Comment
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Well, the power supply does include the transformer.
But what I was aiming at was the overall 'health' of the power supply.
The output ic's run off of PSU +VE & PSU -VE.
There are no voltage markings on this schematic, but the VE capacitors are rated at 63 Vdc.
So you should measure +60 & -60 Vdc. (roughly).
At that time you can switch your meter to read Vac which will measure the ripple voltage of the capacitors.
There are +15 & -15 volt power rails.
These should also be checked.
There is a +5 volt regulator. Check that.
The amp will not function properly unless the power supply voltages are present & fit.
That is why I suggested that you check the power supply first.
These IC output amps generally do not put dc on the speaker if the IC is good.
When the Ic's are bad they take out fuses.
But, yes, you can check for unwanted Vdc on the speaker jack.
Until you resolve the issues, refrain from attaching a speaker.Comment
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