Fender Studio Bass EQ repair

[gjesse]

I'm a newbie and looking for advice on fixing my Fender Studio Bass's 5 band eq -see schematic...
http://johnkvintageguitars.homestead.com/studiobass.gif
The footswitchable eq includes 5 potentiometers.   The response is flat when the pot is centered and  cuts or boosts when turned right or left, respectively.
The problem with mine is that the 1850 Hz pot will cut and boost just fine, but the other 4 pots will only cut, no change when boosted. Presence, high, mid and low work fine too.  The only problem with the entire amp is the boost function on the 80-1000 Hz eq pots.
I'm assuming it'snot the pots, as they work fine when turned right.  So it's either the caps or the inductors on the 4 problematic channels, or perhaps both, but I'm hoping to minimize the cost and time for this repair, so I'm hoping to get people's advice who understand these circuits much better than me...

[Mike Burgundy]

It's quite rare for multiple bits of circuit to experience the same error caused by equally multiple (more or less identical) failures. Could be though, especially if the amp has sustained some kind of heavy physical blow taking out the coils, but that surely would leave more damage too, plus the cut wouldn't work either. This almost cetainly means there's a common flaw in there. My first reaction was to look at the 4u caps on the common EQ lines (one driven by anode, one by cathode of the tube) *but* the fact that the 1850Hz works suggests otherwise. There may be a break in the common line between the 1850 and 1000Hz - is this pcb-based? I suspect the connection to V3's grid is on the correct side of the break, somewhere between that point and the 1000Hz (and the rest) there's an open circuit. Treb/mid/bass are in a different bit of circuit altogether.
That said, for a newbie I'd strongly recommend taking great, great care tinkering on tube circuits. Never ever do it with power on. Preferably get help, from someone with experience in tube circuits.
Seriously - there's more than enough juice in there to kill you very, very dead, even when the power is off. There are bleeder resistors according to this circuit, but I wouldn't blindly trust that.

[gjesse]

Mike,
  Thanks for the thoughtful reply.  I'll look for any breaks in the common line.  It's all point to point handwired, so it should be fairly obvious...