Sugden Masterclass IA-4

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ampdelectronics

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Joined 2016
#1
Hi Guys,
I have a Sugden Masterclass IA-4. Few years old now.
The left channel outputs noise, heard via speaker as well as seen on the scope. The type of noise kind of changes here and there. Right channel is ok.
Volume attenuation has no effect on noise. I have disconnected the pre-amplifier and just run the left channel amplifier board and the noise still exists.

It appears that the noise is most apparent after the power supply clicks in and the transistors are warming up....?

Im not an expert on Class A amps, and was wondering if someone could point me in the direction of how best to start. Fairly sure the fault lies on the Left channel amplifier board. Possibly caps?

I've sent an email to Sugden - Patrick, but have not received anything further and thought I'd try here.

Many thanks,
jas
 
#2
Here are the waveforms viewed.
 

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#3
A 5MHz output would be oscillation - instability due to a component or soldering failure. The problem for anyone wanting to assist (few repairers would experienced at these newer Sugden products), is finding a schematic to base any suggestions on.
 
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