I bought a vintage Pioneer PL-S40 turntable from e-bay yesterday. (Mint condition) I'm getting back into vinyl. I noticed that my h/k 645 AVR does not have phono inputs. I'm wondering if I could use the CD input (not being used) to connect the turntable? Will their be any issues?

The phono inputs had an additional pre-amp built into them. You'll need to get an external phono pre-amp (Google it). Then you'll connect the output of that pre-amp to any of the line inputs of the receiver (the CD one will be fine).

be sure you don't buy any "preamp", but only a Phono preamp;
phono preamps have built-in equalization.
otherwise, the low freq. will be very weak and you'll get shrieking high frequencies.
this is a standard (RIAA) equalization curve, and the reason is so that low frequencies don't take much space on the grooves.

The inverse RIAA playback equalization also provides greater than 15 dB of high-frequency reduction at its maximum, which greatly reduces the inherent groove noise of vinyl, which would otherwise be intolerable for music playback. It's a built-in form of noise-reduction for all vinyl recordings.

As J.B pointed out, during recording the bass frequencies are greatly reduced to prevent overcutting during disc mastering; at the same time the high frequencies are heavily boosted, so that on playback through an RIAA phono preamp, the circuit reduces the high frequencies and the groove noise along with it.

Regards,
Alan

i did not remember why the high freq. were boosted up.
thanks Alan. :-)

Just an fyi and i realize this post is a little dated but the op should check if the turntable cartridge is a moving magnet MM or moving coil MC. Not all preamps support both as i found on my search of phono preamps.