Marshall blues;(

[ski4]

Hi
i have an old marshall master reverb 30.
it's on old single channel solid state amp, and i still like how she sounds.
lately out of the blue when ever i turn the pre amp up past 1/2 way it cuts out or the volume will greatly reduce.  it i have the pre amp maxed  it almost stutters!

i've taken it apart to look for some loose connections or a fried component , but every thing looks ok?

how should i start to trouble shoot this?
btw im a complete newb at trying any sort of amp repairs

thanks in advance

[ski4]

i have found a schematic if that will help at all?
it's strange cuz i can leave the preamp at just under 1/2 and max the main volume , things will sound great.
just can't get the amp to overdrive anylonger???


http://www.drtube.com/schematics/marshall/5203.gif