Sure, it could be a problem with the axxe so try plugging the audio into another input. For checking the voltage from the power supply, you'll need a multi-meter or an oscilloscope. If it is in fact broken, usually due to shorted tantalum blocking caps, you'll need a soldering iron to fix it. These are all good tools to have around if you're wielding an arsenal of 30 year old bad-ass synths, which it sounds like you happen to be
Other likely suspects could also be the CA3086 transistor array, or the 1458 in the VCA, or worse case scenario you'll need a new 4027-1 VCO module that synthrestore.co.uk sells.