Unbalanced inserts and balanced gear - some channels work, some don't?

[petereugenefoley]

So I have a balanced outboard EQ I just bought and it sounds great... until I try and put it in the unbalanced insert point on 5 (and only 5) of the channels on the console. The channels pass audio, but I get this weird pulsing/phasing, almost like there is a tremolo effect.  I replaced one of the channels with a spare and it fixed it, but I'm wondering what could be causing this. I find that if I use the - of the input and the + of the output for the send and return of the insert point, it goes away, but that puts it out of phase and we don't want that.

The EQs work great when you patch em in after the channels on the board; it's only on the insert point of a select 5 channels that it gets funky.

What the heck is going on?

 

[pucho812]

what console? You may have an opamp out or need a recap on those channels.

 

[petereugenefoley]

what console? You may have an opamp out or need a recap on those channels.

That would not surprise me in the least; it's an early 80s Tascam M16 console.