Would like to know if the jensen jta-450 can be connected directly to my computer, if so, how?

Check the belt is fitted correctly. If it is not it will affect the speed. If the speed is slow on both 45 and 33 rpm, then it could be the motor. Sometimes there are speed presetts at the none shaft end of the motor. If it doesn't look like the motor has any built in electronics in it, then look to the turntable power supply look for faults in that.

Could be the belt for the pulleys is stretched a little, or there is something making it slip, or the potentiometers were set incorrectly from the factory . It is normal to have one for each speed. It is unknown if you can access these from the bottom of the unit. Slippage is less likely to be caused by something oily, since it would alternate the speed faster and slower and yours seems to have worked then reached a constant slower speed. Do your 45's and 78's play ok? (yeah I know, does anyone have a lot of 78's on hand)
Your model does not have an outside pitch control to fine tune the speed faster or slower. Another Jensen model JTA-230 does. Entry level turntables such as yours are not speed accurate reliable. After reading reviews on your turntable, it's possible that the belt is the problem. If the slow speed problem had happened from the start, it would be safe to say that the speed wasn't calibrated correctly and you could have returned it and perhaps had better luck with another one but because it worked fine at the start and then now plays slower, it's either the belt or the potentiometer for the 33rpm speed has somehow changed.

If a cable speaker is shorted to ground or (in your case) the short was on the casing the radio mutes the output to save the amp. So
remove all cables and put them back to the radio & connect the speakers one by one to see if it works or mutes again

Jody, the arm the needle is on, usually adjustable with a dial on the end that can lighten, or put more weight on needle to help it track better on records. Other then that, the belts normally are changed to address slow speed issues. Check the attached links,instruction and guides, Good luck
"I hope this helped you out, if so let me know by pressing the helpful button. Check out some of my other posts if you need more tips and info."
Jensen JTA 220 Instruction Manual Page 2

http://audio.manualsonline.com/manuals/mfg/jensen/jta220.html

How To Calibrate Your Turntable For the Best Possible Sound

How to Adjust Tracking Force

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The record that you are trying to play maybe a 45rpm recording. Some 45's were released in album size recordings.

Tech Help call 1800 686 079 Mon -Fri 9am to 5pm I have a JTA 380 Manual ? similar spec's.

There is a speed selector directly under the needle arm.

Try moving the arm up to the end of the vinyl, if it does go till the end of the vinyl then try adjusting your counter weights, a penny will do, if it doesn't go till the end of your vinyl then there's a mechanical problem check any obstruction at the botton of your arm.