I have a JVC RXC50 dual cassette CD changer boombox. The dual cassette decks will play tapes okay but niether will work to rewind a tape or to fast forward. Can the decks be fixed so they will work co

Check the belts, inside the unit you should see a rubber belt which is what pulls the tape fast forward or rewind. Some belts with time get loose and slip, they can also break and fall off which will not let you fast forward or rewind.
Open your cassette player and look where the plastic pulleys are located and check for the belts, they might not be broken at all but very loose and slip on the plastic pulley.

The ancient tape is junk. Buy a new one. You might also try slamming the old tape against a wall or desk to loosen up the tape layers. Then FF and RW all the way.

I think this should be a simple fix! The deck will have switches at the top of the cassette compartment. These detect the tape and also select it for type - metal or chrome etc. These switches have become bent or damaged or have dirt on them.

I may not have worked on one of these before but all tape decks have some common drive parts. Yours is missing the proper torque to drive the reel table in take-up for play and your fast wind observation also points to either drive transfer problems or the motor itself. I suspect the fastest answer is to have a tape deck technician work on it. Dual cassette decks are not known for being the long lasting products like a singles cassette deck is.

It sounds like the belt is slipping or broken which monitors the speed of the tape movement. When you play or rewind the tape, it thinks the tape has reached the end of play and shuts off. You can try a different tape to make sure the tape does not have too much tension on it. If the second tape has the same result you will need to remove the top cover of the deck to inspect the tape drive mechanism. Make sure the belts and pulleys are clean and replace any belts which have glazing or cracks. If the belt is broken you will need to replace it with a compatible belt of the same length and width.

There is an idler wheel behind the platen ( black plate behind the cassette)..replace the idler wheel.

The guy in texas is Marrs communications

Belts have become too slack to allow mechanism drive. Have replaced

{11-30-13)
TO FIX THE CASSETTE DECK BELT PROBLEMS:

- Disassemble the player.

(you will need only one, long, phillips head screwdriver $5.00)

[It is a tedious job. (first,study how it is put together) There are 6 screws on the back - three down deep holes, three at the bottom, and two dark colored ones on the side. Once the front covering is off, then you start the process to get at the Cassette deck. You will have to unscrew a ground wire; pull the "tape" connections and one wire bundle snap fitting; and then unscrew the two screws on the back, near where the wires for the speaker connections are placed... then remove the Cassette Deck.]

{I sat in my recliner with the whole thing in my lap as i worked with a foot stool near by to put things as they came apart... and i used a metal salad bowl to through the screws into as i went along.}

- Study the drive mechanism.

- See where the drive pulleys are.

- Use small elastic bands to replace the broken belts.
(small = 1/2 inch diameter ... the size of your little finger, but will stretch out to two inches or so.)

https://www.google.com/search?q=small+elastic+bands&espv=210&es_sm=122&tbm=isch&tbo=u&source=univ&sa=X&ei=6rGaUrTlCszloASJw4KQBw&ved=0CGYQsAQ

I happened to have these on hand:



- I did this on my JVC PC-XC70BK and it worked.

- Both the player-side and record-side decks had broken belts.

[The fast forward and rewind wouldn't work on either side.]

(Per each deck: one drives play, one belt drives both Rewind & Forward)

- Now both decks work.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

TO FIX THE CD PLAYER ISSUE:

- My guess is the laser pick up is dirty (because they all face upward and oily film builds up on them).

- First try a disk cleaner with a brush on the underside of it.

- If that doesn't work, then (after taking off the front plastic cover plate {see above}), you can see where the upward pointing laser lens is. If you can, use Rubbing Alcohol on the end of a Q-Tip to probe into the area and thoroughly clean its surface. Then wipe it clean with a dry Q-Tip.

- If you can't get at the laser lens, then Disassemble the Unite.
[There is another ground wire that must be unscrewed from the deck assembly. And there is the issue of the tape connections leading to the top part of the unit ... they will flop around and fall out of their place once you have pulled them from the connectors ... with a bit of observation you can get them back correctly.]

- Unscrew two screws at the back ... then pull out the tray assembly.
.... then go as far in the disassembly as you need to so you can swab the laser lens with the Rubbing Alcohol.

- Re-assemble. I had one screw left over.

... ROTFL but i know where it goes and i don't care to get it there... so to hell with it, right?

Everything works fine now. Not bad for a 17 year old piece of equipment that has a Ten CD storage tray... that you can't get anymore... right?

Sounds like the tape tension sensor is stuck. The sensor is always telling the mechanism that the tape has reached the end.