Mark IVa Circuit Questions

For those who might know, what function is associated with the 750pf capacitors at the top of the tone stacks? I know that it will shift the EQ "notch" over and effectively allow more mids through, but which function uses this cap: presence shift, pull fat, etc? There's one on both tone stacks, so I assume that it is the pull fat feature of Rhythm 2 and Lead. The schematic I dug up doesn't show the switching matrix or power supply. Does anyone have a complete schematic for this beast? I'd also like to see a IVb schematic too. I'm curious exactly what was changed between the two. My IVa is knocking on 18 years old, so it's going to be time for a cap job pretty soon. I didn't see any bulging caps last time I was inside, but I know its coming.

They are for the "Pull Fat" and are only on for R2 and R3 when the switch is engaged. They bring the treble down and midrange up.
In parallel with the 250pf they give you a 1000pf treble cap. LDR's 1 and 3 switch in and out for lead and R2 and do not turn on for R1.

My IVa is knocking on 18 years old, so it's going to be time for a cap job pretty soon. I didn't see any bulging caps last time I was inside, but I know its coming.

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Just curious, how do you know you need to change caps? How does it sound? I have a Mark IVa also from '92 and it has become "muddy" lately...I don't think tubes are ones to blame, 'cause they are pretty new :?

When the big filter caps start going, the amp starts to motorboat and power can fade in and out, hum is a key sign too, as AC is leaking through. I think a safe guideline is about every 10 years for the big caps, but there are still vintage Fender and Marshall amps using original caps, (but I would change them before the fail). The smaller caps, used for the tone stack and coupling, PI and other areas of the amp you start getting hiss and loss of power, etc. Take it to a pro and they can determine what caps need to be changed.

Jack