Mesa MK3 Graphic EQ Switch

[Chubbs]

Hey guys, picked up a mesa mark 3 DRG recently and noticed the graphic eq is always in despite the switch being in auto, out, or in. The previous tech did replace the caps and a eq slider, but everything looks to be done very well and correctly. From looking at the schematic it looks like there may be diodes, transistors, ldr’s, and or vactrols in play, perhaps a failure somewhere causing this problem. Not sure if this is common or if they’re any way to test these components but figured I’d ask, I’m a bit out of my element coming from Marshall’s.

 

[Chubbs]

I'm stumped, my transistor voltages look good for the most part and my voltages out of the switch to the LDR also look good. 2/3.5v when the switch is out and 4/5.7v when the switch is in. Both are 0v out on the resistor side yet it doesn't measure as an open circuit. The diodes around the switching look good as well. The only questionable thing I see is base voltages for Q1 and Q3, and am not sure what exactly controls the EQ being in or out. The LDR? I have some voltages/ part of the schematic below if anyone notices anything. S stands for mesas stock schematic and the following voltages are mine where applicable.