If you only replaced the pitch belt and not the drive belt then get a new drive belt. A ten year old drive belt will be slack.
After *drive* belt replacement, if you do have a problem with the pitch control, here was my experience:
I have a Dual CS 506. It's a belt drive with pitch control. (My pitch problem may apply to your Dual 505...)
The pitch is a dial type control at front left of the deck. My problem was that I was able to twist the dial in one direction only - then not move it back in the opposite direction due to slipping toothed pitch belt.
First turn off/ unplug the turntable and move it to a work area with good light.
To fix my pitch, I had to assist or ease the pitch mechanism by pressing down on top of the mechanism's brass/metal bracket with one finger while at the same time turning the pitch dial back using my other hand. (The pitch and the belt pulley share the same bracket mechanism).
The bracket for drive belt capstan and pitch belt and is front left, underneath the platter. I removed my platter but there are platter access holes under the platter mat. You could use an access hole (with the mat off) to reach in and press the bracket without removing the platter, I believe.