Same is also true of the six summing amps in the 688 section of a 6000.
Old electrolytics right at the summing node are a common source of imbalance... If you do one 26 card, be sure to do the other, (or the other three, in the case of a 4000... or the other 5 summing cards in the case of a 6000!) because the caps will all be the same age, and of one is old enough to fail... they're ALL old enough to fail.
It's a virtual earth node, so the impedance to ground is -in theory- zero ohms precisely. Caps with high ESR (equivalent series resistance) will throw the gain off, because it's as if the values of the equivalent resistance feeding the summing node has changed, and therefore the ratio of input resistance to feedback resistance (the precision resistors on the 26 card) is therefore thrown out of balance.
I'm not usually a habitual "recap everything, it's been almost a week since the last recap" kind of guy... but THOSE caps are fairly critical. Use low-ESR replacements of good quality. -At this point in more opinion-based forums, someone usually chimes in with their favourite types. -I look at what's available, what's the right size & shape, and what has exemplary specs on paper. -It's always worked flawlessly.
