Loud buzz from deck, seems to be coming from RCA cable

First of all remove the wires to the cartridge. With the amp on, touch each of the wires to the cartridge with your finger. one at a time. On one you should hear a loud buzz on the left and on another a loud buzz on the right.
If you get no loud buzz on the left. There is something wrong with either the connections between the deck, or the pre-amp of the amplifier. This will not effect any other function on the amp such as tape or cd.
If you do get a loud buzz on the left, check the cartridge connection or replace the cartridge.

You have two options. First a faulty cartridge. Or a faulty pre-amp in the main amp.
You can find out if the cartridge (and amp) are playing up. By removing the connectors on the cartridge.
Then with the volume turned up to about half, with your finger, touch each one of the wires one at a time. It will not harm you at all. But you should hear a loud buzz on one wire on the right and a loud buzz on the left on another wire.
The two other wires will either produce a crackle, or a tiny buzz, or nothing at all.
If you do hear a loud buzz on two wires. The cartridge is faulty and needs replacing.
If you hear a slight buzz on them or weak sound. There is either a bad connection between the deck and amp. Or the pre-amp inside the deck which is only used by the record deck so will not effect any other function, is faulty.
You can remove the record deck connections to the amp and again with the amp on half full, touch the inputs with your finger. If you are not hearing a loud hum or buzz, the pre-amp is faulty. If you do the connectors are faulty.

You could have either a cartridge problem or a pre-amp problem.

To check. Remove all the wires to the cartridge. With the deck connected to the amp and powered up, set the volume at normal for what you would listen to it. Now with your finger touch each wire one at a time (it will not hurt you). You should hear a loud buzz on two of the wires, one on each channel.
If you do not hear a buzz or its weak, then either the connections between the deck and amp are faulty, or there is something wrong with the pre-amp for the magnetic cartridge inside your amp. If you have only recently connected the deck to your amp, it might be that your amp has no built in pre-amp, and you will need to purchase one. The internal pre-amp for the deck failure will not affect any other part of the amp.
If you do hear a loud buzz, check either the cartridge connections or replace the cartridge.

First thing: Make sure ALL and I do mean ALL of your interconnected equipment is POWERED from the SAME receptacle. Powering different parts from different receptacles in a building will surely get ground bounce buzz and ALSO subject the equipment to hazards of circuit damage.
The audio connecting cables should be BALANCED lines where possible... either XLR or TRS cables. Problems like you are having are common when CONSUMER grade equipment is mixed with PROFESSIONAL equipment. Anyway, get all the stuff powered from the same receptacle and get back to me if you still have the problem. Note you may have to run three wire extension cords if devices are separated a bit... the pro's have to do that to avoid noise. All devices should have three wire (grounding) cords and/or grounding wires connecting chassis...

Yep! Get a decent audio interface that attaches to the PC with a USB connection and uses BALANCED lines to your mixer. I use a UA-25EX Roland/cakewalk as an example. (about $200 and there are cheaper units such as M-Audio Fast Track MKII ) Note these interfaces BOTH record and playback with two channels for stereo.
Essentially the audio board that is in PC's and Laptops are too near the switching noise of the processor and some of that is just bound to get into the audio lines as they don't even have balanced audio lines from the computers. Make sure you use either TRS or XLR balanced cables from whatever interface you get. Also connect ALL interconnected equipment to the same power source or receptacle. You can try to reduce the buzz with the mixer EQ's with what you have, but that sacrifices audio quality.

First put your finger on the audio imputs for the deck in your amp. They should buzz with the amp switched on. If you can't get them to buzz, then the magnetic cartridge pre-amp in your amp has failed. If they do buzz. Take the wires to the cartridge off (make a note of where each one fits). Then with the amp powered up and deck plugged into it. Touch each wire one at a time with your finger. You should hear a buzz on two wires (one on the left one on the right) and nothing (much anyway) on the other two. If you don't hear any buzz there's something wrong with the connection cables to the amp. If you do hear the buzz, then the cartridge is dead.

try grounding the shield of the rca cables and see if the buzzing goes away. if so you have a bad ground to the rcas either in the deck or the amp. grounding the rca cable shield should take care of the problem.

Buzz is caused by a bad ground or interference in your shielding check out your cabling or plugs that is where the problem lies.

Hiya, first try a normal cassette, see if that plays OK. If so the deck is probably fine, if not then the deck is faulty (probably not serious though and just a loose wire on the head).
If the connection between the tape and MP3 is a cable, the fault is quite likely there (if bluetooth then not). I strongly suspect the cable from the previous issues also.

Best of luck

Usually what causes the buzzing sound is the shielding on the RCA cables. If all the connections are tight, plugged into the correct receptacle, and you still get the buzzing, then this is probably the problem. If the cables are poorly insulated, the other electronics will cause interference and the buzzing sound will result. Do you have a number of electronic devices stacked together, or in close proximity? A more expensive RCA cable is probably the answer to your problem - one with preferably gold connectors and thicker, better insulated wiring. Hope this helps...