AR-XB. Is it worth a new motor?

Thank you all for the responses! Some really good info and education. Here's my take away: keep the table, which other than the motor, is in great shape, check out the capacitors as the motor does turn freely, and if that doesn't work then try and rebuild the motor (I did watch the video) or replace the motor and maybe down the road, replace the arm and headshell as I understand there are some Technics(?) arms that will work with this table. Did I get that right?

This has the similarity of having a boat. A hole in the water you throw money at. Even though I know nothing about this TT,  my advice is to move on and be glad all you threw at it is 200 bucks.

Parted out you should recover your cost with a good frame (revolutionary design), arm, bearings, plinth and platter. BUT the AR-XB is a fine piece of audio history and worth saving or passing on. Acoustic Research was a benchmark company. Here's a news flash, history doesn't start the day a person is born and everything was not made to throw away.... Americans, jeez. I am guessing that the lube in the motor has hardened, and the start capacitor is way past its use by date and you don't need a motor (I like the hum, it's trying). The AR was designed for a conical stylus (the, supposed anti-skate issue), and there are some great MM's, and even a couple MC's available (Denon). Most people set the anti-skate too high, I did until I got a test record. I have a Thorens TD 165 that's happiest with minimum anti-skate. Now that I'm aware I think I'd like to get the O-scope repaired. I was told it was an easy fix, but that was ten years ago.... hmmm maybe chineseium?  I like your Sony backup TT, and will just mention U-Turn Turntables from Massachusetts (born and made in USA by entrepreneurs), I think almost, if not all the TT's in the line are upgraded with the magnesium arm, and the new belt and platter from the Orbit Theory (I'm a proud owner, punches way above its price (hosting my Sure V15 IV but needed VTA spacer), but I know, not made in a foreign country, sorry). If you need a line input for a second TT Orbit has you covered there, too. Good luck, and please don't just toss the AR   :)

The XB retailed for $130 in 1972. 

I just picked one (77XB) up recently with a similar symptom as yours.  On mine, giving the platter a manual spin to help it reach 33.33 rpm works.  I did replace the cap, but it made no difference.  A full motor rebuild is in the future.  For now, I put an M91ED body and got a NSS elliptical N91ED from LP Gear.  It sounds glorious after some lube of the bearings (arm and platter) adjustment of the suspension, and dialing in the cartridge alignment.  Mijostyn is very knowledgeable but I differ on this point…much stylus wear comes from insufficient tracking force to maintain groove wall contact in the misguided pursuit of low VTF. Using the upper end of the recommended range is the simple solution for most non-SOTA arms used by most people.  I realize some A-Goners do have SOTA rigs with superb precision arms, but that was Villchur’s view and I haven’t seen it disproven.  No one is comparing an XA or XB to gigabuck tonearms, it was meant to be affordable high fidelity and in that context is one of the greatest inventions of its time.