st-70 with no stereo seperation

Hi, I recently came across an ST-70 that has no stereo operation. When I set the preamp balance control to the center, things sound okay but no seperation. When I move the balance to either right or left the sound volume goes down slightly but still hear equal sound volume from each channel, although a little distorted at higher volumes. I also notice that when I turn the volume to off, I can still hear output only very low. I have three other ST-70s and I have never experianced this kind of thing before. I have tested with two differant working preamps ( PAT-4 and Marantz 7T-set to low output). I have a PAS-3 also but am afraid to try since something is wrong with this amp. I tried to run one channel at a time with an RCA shorting plug in the the unused input with the same results. I disconnected the mono/stereo switch per Bob Latino's diagram and still no effect. I was told this amp was home built so I went through the factory wiring directions and checked each wire. I found a few cold solder joints and repaired them but found no wiring errors. I was told this amp was sitting in a basement, maybe damp seeing some rust on the cover. I wonder if the power transformer is damp or shorting? Any ideas would be appreciated.

You can email me a photo of the top and bottom of your amp and I will post it here if you like.

Sal

salb203 at optonline dot net

Thanks, I'll try to take a picture of the top and bottom as best as I can.