API 550 not passing signal

Hi there - I have a pair of orig API 550 EQs (not the A version, the 550 which has a few less frequencies), anyhow one of them has decided to quit on me.

I have swapped in known working 2520s but she no go still... wondering where to start to fix this sucker.  The supply voltages are correct and are getting through the circuit to the op amps.  I'm wondering if its the output transformer (hoping not)...

Any ideas where to start?  Should I just swap in one of my Classic API output transformers to see if it works? 

if you can send tone into the unit and use a multimeter to check along the signal path of the circuit you'll find where the audio is getting "lost"

at the very least you can check the primary side of the transformer this way... see if audio is even getting to it before you swap it with another transformer.

good luck!