I have a optimus ssm-1750 radio shack mixer that when hooking my phone to it to play music i can only get it to play through one speaker

Try a good local electronics supply shop as Vince says or eBay. If you can remove it is will have a generic part number from the maker. With that you can Google the number.

https://www.ebay.com/sch/i.html?_nkw=Mixer%20Replacement%20Fader&norover=1&mkevt=1&mkrid=711-156598-679417-4

Or you can get the whole unit for $50

https://reverb.com/item/11188486-radio-shack-ssm-1850-dj-mixer

https://www.google.com/search?q=RadioShack+SSM-1850+replacement+fader

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I agree fully with expert .

Connect the dvd audio outs, normally red and white phono jacks, to the inputs of your mixer. You may need phono to 1/4" phone jack adapters to do this, but they are available at Radio Shack, but hurry as they are going out of business.

Sure you can, but that mixer amp is a mono amp instead of stereo. Head for radio shack and get an 1/8" stereo male phone jack to 1/4" female mono adaptor. Plug the adaper into the headphone output of your device. Then run a shielded cable with 1/4" mono phone plugs on both ends (like an electric guitar would use) from the device to one of the channels on the Peavey. Bring the volume up on both the channel and master on the amp up maybe 1/3 the way. Play a track on your device and adjust the devices out put as high as it will go with out distorting the channel input. Then adjust the amp's channel and main output as you would normally to get the final volume you need.

What you've done should have worked on the video input. The phono will not sound like you'd want. Have you made sure the mixer is working correctly by trying it plugged into a different amp using the same outputs and cable?

You have NOT told us what speakers you are using or anything to help us help you. That mixer will produce about 250 watts a side into 8 ohm speakers. That should fill a 30 by 50 foot area. You will need at least 15 inch speakers. Proper mixer setup is important. Look at videos on the subject on YouTube for instructions. Many people use too much high frequency EQ setting. Set the EQ flat to start. Use minimum of reverb to start. Use good quality mics with XLR to XLR cables... none of this Radio Shack or Karaoke quality. Minimum mic cost iis around $90 for decent ones... or about $70 when bought in lots of three or more. For your speakers make sure they are elevated on speaker stands. Use the PFL to solo each channel and set the TRIM level for each as the first step.

Which inputs are you using? If you are using the 2T RTN, you will need to engage the 2T RTN button under CONTROL ROOM SOURCE and engage ASSIGN TO MAIN.

you need a cable with two A3F  XLR s going to a 1/8" stereo phone plug, male. plug the XLRs into the Main out L&R on the mixer. plug the 1/8" into the LINE Input on the laptop. You should be able to get the cable from any music store,or Radio Shack. below is a link to the manual,in case you don't have one. Good Luck!

http://www.behringer.com/assets/1622FX_P0557_M_EN.pdf 

Couplers and adapters available at any Radio Shack

use 1/4 inch speaker cables. hook one speaker up to "main 1 R" out and the other to "main 1 L" out. if you have additional main speakers, hook them up the the "main 2" outputs.

look for the "bridge amp mode" switch and move it to the "main L- main R" position.