Marshall AVT 20 Loud Humming

[andomando]

I KNOW THIS HEADLINE WAS POSTED IN 2012...buy,,i have a similar problem with a very loud hum as soon as i turn on my amplifier..Marshall avt20x.
I have changed the filter capacitors,,but alas,,no change,This hum does not vary when i try volume or other controls and is so loud i have to turn it off right away in case something else goes wrong..
I dont know anything about "cycles" in sound measurement,,but it sure is a very loud hum.

any help would be great..
regards
Andomando.

 

[Tarzan]

Without a speaker connected, do you measure a dc voltage on the output leads going to the speaker?
In that case, the power amp IC (TDA2050) is fried. Replace the chip.

 

[Enzo]

DC on a speaker can ruin it. So do check for DC. Visually, if you turn it on and the speaker cone moves one direction and stays there, then you have DC on the speaker.


If the amp makes DC, you won't get any guitar sound coming through. If your filter caps were bad, it would hum loud, but you'd still hear some audio.

 

[andomando]

marshall avt20 loud humming=2

Thanks to Tarzan from Belgium and Enzo from USA for your help about my problem.
I just want to ask ,where to connect my multi-meter leads to test the tda2050 chip at the speaker outlet.?do i connect to x & pos connectors or x & ground.
Im new to electronix ,

 

[Enzo]

You can look at the schematic for pin numbers, but the easiest place to take that reading is at the speaker wires. They are much easier to get to than the IC legs.

 

[andomando]

Thanks for your help Enzo,i will give that a try.
regards
Andomando

 

[andomando]

Thanks for your help ,i tried the multi-meter over the speaker wires test and it showed up with a reading,so i think i,ll change the tda2050 chip.Will let you know how it goes.

regards
Andomando

 

[Tarzan]

Before you fire up the amp with the new TDA chip, place a light bulb (40 or 60 Watts) in series with one of the ac wires coming from the wall ac socket.
This will eliminate a to high current if there is another major problem.
There is a lot of writing on the subject of the light bulb protection; google for it to understand the working.
If all is well, remove the light bulb and connect the amp direct to the ac wall socket.