Analog playback issues

The other week my mom got out some of her records she had not played for several years. When she put one on it sounded garbled(like when you plug a cd player into the phono pre-amp on a receiver) the turntable is a technics sl dd24 from the mid or late 70's, and she assumed that it was just time to replace it with some budget table from Sony. When she set the new table up the same result occurred and she wound up returning the item. I'm fairly certain she had both the old and new turntable set up correctly. What could be the issue? Component info: Onkyo TX-8511 receiver, Technics sl dd24, Cartridge Technics P338
Can you get more specific about the nature of the "garbled" sound? Distorted? Highs or lows exaggerated (which?) No volume or too much for a volume knob setting that is OK with the turner or tape player?

Did the replacement turntable have a new cartridge or did you use the old one? That could be an explanation if the cantilever or stylus were damaged.

Was this a moving magnet, moving coil or crystal cartridge? Each of these needs a different input configuration.

In what condition were your records? Vinyl can be easily damaged from rough handling, being played with a defective or damaged stylus, dirt in the grooves, or even mold growth.

This isn't the end of the list of possible explanations, so more detail would be helpful is diagnosing the problem.
Sounds like she used the same " bad " cartridge for both tables..That would make sense>>>>>>>
To be more specific in regards to the sound: you could barely hear the music, but then there would be leaps into the high register accompanied by outbursts of static(again, not sure if you have ever mistakenly plugged a cd player into the phono pre-amp of a receiver, but this was the exact effect). Like I said the cartridge is a technics sl-dd24 that is 30 or so years old which I presume to be MM. The sony table she tried briefly came with its own cartridge, but had the same result, so I rule out the possibility of it being a cartridge issue specifically. Could it be that the phono pre-amp on the receiver went bad? I thought initially that this TT has not been used in a long time, but apparently it worked fine a few months before the sound distortion started showing up. The records looked clean to me, but I am hardly an expert--I hope this expounds on the issue enough
if two entirely different TTs end up having identical malfunctions, it's pretty likely the receiver is the issue.
Does the receiver have phono inputs? If not, that's the problem.
Sounds like a phono preamp problem.

One thing you might try is getting some spray contact cleaner. Turn off the receiver and unplug it. Remove the top case of the receiver and spray the cleaner on the selector switch (the switch that changes between tuner, phone, tape, aux input.) Rotate the switch knob back and forth for a couple minutes. It could be that crud or corrosion was affecting the phono input. Since moving magnet cartridges run at a much lower voltage than tuner/tape/aux lines it would be more susceptible to this issue.

If that doesn't fix the problem (and your aux input is still good) get an outboard phono preamp. These start as low as $25 or $30 at Radio Shack and the quality really isn't too bad. Obviously you can spend a lot more if you wish.