Welcome, Spider, you've come to the right place. There are many folks here that can help you.
As far as replacing needles, check out the FAQ in the Electronic Gramophones and Phonograph section. It's near the top of the page. Get the number(s) off your cartridge to find a suitable replacement. There could be several numbers on that cartridge, including a Magnavox number (560345) and an Electro-Voice (EV275), or a Magnavox number (560355) which has an Astatic number (165D). I don't know what model you have, so this is just a ball park figure to go from. Numbers can be cross-referenced. These are just samples of what I have and have used them all.

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I forgot to mention, there is a single clip-in needle that seems to work in the smaller cartridges.

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All of my Astro-Sonics have extension speakers connections. Terminals are on the back near the bottom of the cabinet, and there is a switch in the record storage section for Internal/External/Both. I'm only speaking for the consoles that I have. There are also Aux/Tape-record/play jacks in the same area and optional external antennas. If you want to know your model, remove the back and you may see a sticker pasted at the bottom with something like Model RP637, Finish Pecan, 120 cycles, 50watts, Fort Wayne, Indiana. There may also be a wiring diagram for the speakers, turntable, receiver, amp.
Not knowing what your setup is, there could also be small date stickers on the receiver/amp sections to give you date of manufacture...that does not apply to the cabinet since it can be used for several years.

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As far as turntable issues, I think some adjustments and thorough cleaning/lubing are in order. I think these turntables are pretty hardy, and there may be some accumulated gunk in the wrong places. Check with MattTech and other qualified/knowledgeable folks about those issues.

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Oh Crap, I went off the deep end...you have the older series turntable, and they don't use those smaller cartridges. The smaller cartridges use .25 volts, while the larger ones use about 1volt. Gave you too much info. At least, you'll know how to look and cross-reference model numbers, date stickers, matching cartridges with needles, etc. --BB
Welcome Again, Spiderman!
Bobby Basham
Tucson, Arizona
Welcome again, Spider! --BB
Bobby Basham
Tucson, Arizona