From what I can gather (you haven't made it easy..) you have a mains transformer, the secondary is tapped to give 17V-0V-17V, and you've slapped a 1000uF cap across the two 17V nodes. So you have 34V RMS across the cap. We have no idea how you measured this, what multimeter setting or anything else involved. But if we find the peak... 34 / 0.707 = ~48V peak. Pretty close to 50V. How did you test this transformer?
Even if it was rectified, you still have 34V RMS, which when rectified can easy get to 48V I assumed you rounded up.
I didn't see a rectifier, so that poor cap gets 34V, then -34V across it - did it complain via the standard procedure of exploding and making nasty smells?