Cartridge for Dual 504?

[Cloudless]

I have my dad's old 504, and the cartridge needs to be replaced (at least I think it does, he told me he knew it needed replacing and it doesn't sound good on one side especially). It has a ortofon LM20 in it currently. The manual states 5.5-10g, 1/2" spacing. What should I replace this with? I am a complete novice when it comes to turntables, I just enjoy the sound and quality of vinyl. I am not looking to spend a fortune on a cartridge.

Edit: actually, what is the best way to judge the cartridge? Looking at it, or the sound only? I have a microscope available if inspection is warranted...

[Bran Kulez]

If the cartridge is suspect, then the first thing to do would be to test it with an OHM meter (multimeter). One probe to "R" (red) the other to "RG" (green), then do the same for "L" (white) and "LG" (blue). The readings should be relatively close.
If the cartridge is o.k., then the next step would be to inspect the stylus close-up. The cantilever should be centered without kinks and the tip should be free of gunk build-up. Click on the link below to get an idea of what the tip should look like magnified.

viewtopic.php?f=19&t=11431&start=975

[Cloudless]

Thank you for the reply bran! I checked out the resistance, it is identical on both. Under my scope the diamond itself looks to be in good shape. The cantilever is undamaged. The only thing I noticed was the cantilever has plenty of what I would call "residue" on it. Should I try to clean it somehow, and with what? I did use a tip cleaner kit on it so the tip doesn't have any gunk on it.

The cartridge is original from around 1977.

[Bran Kulez]

It sounds like a keeper. I once used an x-acto knife to scrape crud off of a cantilever. Those DUAL headshells can be a little tricky though and may be a source of trouble if the contacts are corroded or if it doesn't seat properly. If you're still having trouble in one channel as you mentioned then you could check for continuity from the headshell leads to the rca jacks using the continuity setting of the multimeter.

[Cloudless]

I think it sounds pretty good now. Maybe I just had a poor connection somewhere? Thanks for the help!