hifi engine

Why does my Roberts 770X SLOW DOWN in "PLAY" Mode and Not REWIND after Approx. 15 Minutes ON?

I just received a used Roberts 770X Reel to Reel Recorder.

When I test the machine, PLAY and REWIND both function very well until machine has been powered "ON" for approx. 15 Minutes.

Around the 15 minute mark, there is a "beep, beep, beep beep beep" noise in the speaker and the PLAY Slows Down and the REWIND pretty much stops -- however the reels move freely so I can "hand rewind".

With this "beep, beep, beep beep beep" noise and the 15 minutes of "heating up time"...I'm wondering if this is a simple matter of lubrication or bad fuse, etc.

Thanks for your help! I really look forward to recording with my new, used Roberts 770X.