You need to replace all the belts with new ones. Belts lost its elasticity and it might have deteriorated. It is slipping on the pulley.
Take your time and do it carefully. Download the service manual that is available here. Take pictures when you work on the M70. This will really help when you want to put it all back together. Step by step pictures always help a lot.
https://boomboxery.com/forum/index.php/files/file/63-jvc-rc-m70-sm/
You have to lubricate the button movement mechanical parts with white Lithium grease. That should solve the issues with deck buttons locking up. Grease only lightly and don't get it on any rubber parts or idler tires.
Main belt FBS 9.6 (Flat belt/small width) inner circumference 9.6 inches. Medium width slips off the pulley so buy the small width.
Counter belt SBM 7.0 (square cut .035 X0.035 medium) inner circumference 7 inches.
You can order these belts from studio electronics
http://www.studiosoundelectronics.com/
Here is some excellent instructions from our member Superduper ( Expert Electronics Guru) which I copied from the following thread.. https://boomboxery.com/forum/index.php/topic/6685-changing-belts-on-jvc-rc-m70/
Posted 21 August 2011 - 10:35 PM
No. One belt is replaced from the backside. The other belt is replaced from the front side. Many folks think that rebelting a deck is easy, falling for those ebay descriptions..... "needs belts, no big deal, EZ job." For experienced boombox hobbyists, yes it is often not too difficult. For the novice, it can be anything but easy and screwing something else up in the process is not uncommon. Anytime you feel uncomfortable and over your head, I suggest you stop and take a break or farm this out for more experienced individual to rebelt for you.
On the M70, the best way to rebelt the deck is to:
1) Remove the back cover.
2) Remove the top panel.
3) Remove the chassis from the front cabinet 1/2.
4) Disconnect and remove the deck assembly (includes music search led board and appurtenant mini pcb's) from the chassis to rebelt on the work bench.
5) Replace the small belt from the front side which will be exposed once the chassis is removed from the cabinet.
6) From the backside, remove the flywheel backing plate (counting the number of turns on the adjustable backlash screw)
7) If lubing the capstan shaft......... then:
8) Loosen and/or remove the auto stop solenoid
9) Remove the flywheel, clean and lube the shaft near the base of the flywheel with light silicone grease.
Reinstall in reverse order.
Notice that NONE of this process involves tampering with the main circuit board.
Not ever step is enumerated above and I presume you will use common sense to remove and untether the proper connections to facilitate remove of components.