What a project manager would do?

See for yourself!

How to conduct effective sprint retrospective meetings?

1. Do retrospective after every sprint. No exceptions.
2. Involve all stakeholders. Developers, QA, Product Owners and whoever is involved in the sprint.
3. Define action items at the end of the retrospective and follow through on action items. In every retrospective, review the actions taken on the action items from the previous retrospective. This is very important. When a scrum master follow through and review the action items from the earlier retrospectives, team will start to believe in the effectiveness of the retrospective. If no action is taken, retrospective is just another boring, useless meeting!
4. Make sure every participant shares his/her feedback. Define a ground rule to state that no one interrupts when someone shares the feedback. "Talking token" would be useful in this situation. You can use a fluffy toy or a coffee mug or even a marker as talking token. When someone holds the talking token, he/she can talk and everyone else should listen. Once he/she is done, the token is passed to next participant. This approach will yield two benefits. (a) This reduces the interruption and makes the token holder comfortable to share the feedback without worrying about getting criticized in the middle of the conversation (b) Passing token among all participants helps the shy people to talk as well. 
5. Make the retrospectives fun. Try to use starfish approach detailed here. Nothing beats the wall and stickies. I usually record the retrospective output in Confluence pages as well to measure the retrospective effectiveness and also for historical purposes.
6. When you end the retrospective, ask the team if they want to thank someone for the help he/she provided during the sprint. This will help the retrospective to end in a positive note.