Harmon Kardon ST-7 with Rabco Tonearm

A friend gave me one of these today. It is all brushed aluminum, made in the United states and looks to be from the 70s or 80s. The tone arm is one of those tangential ones where the whole arm moves from outside-in, perpendicular to the record plane. It needs some cleaning and perhaps a new power cord and RCAs. Other than this I do not know too much about it. Was it a good record player in it's time? Is it worth fixing up? Looks pretty solid overall.

Yes it is worth fixing! I personally have a Harmon Kardon ST-8 which I bought around 40 years ago and I love still today as if it was brand new! These are some of the most sought after turntables from that mid-70's era! I'm going to add it into my new home audio/home theater system via a Simaudio Moon phono preamp to my Arcam AV950 surround processor.

"These are some of the most sought after turntables from that mid-70's era!"  Really?  If I remember correctly (and please do correct me if not), that turntable has a built-on version of the Rabco SL tonearm, only instead of the motor-drive pulling a chain that propels the tonearm across the LP while maintaining tangency in the big Rabco, the tonearm on the ST8 is propelled by a rotating rubber tire riding on a metal bar.  The issue with that set-up was noise from the rubber tire transmitting to the rear pivot and hence out to the cartridge.  It's historically very interesting but not the greatest in terms of success in reaching its goal.
I never owned one, but a close friend did.....agreed with schubert (above).   The arm is a dog - don't think it was Rabco.
I have four...(2) ST5's and (2) ST7's.  Incredibly frustrating to set the arm up to ride the roller properly...but...once it all comes together, it's audio Nirvana.  I have had over 15 esoteric turntables, from B&O's tangential tracking tables (not so good) to eclectic pivoting tone-arm turntables on some of the best platters out there, and find the HK / Rabco combination to be one of the best sounding.  I don't hear any rumble (as stated by someone above) and have the equipment to play really loud - if I want. The soundstage and depth of field is quite remarkable.  I highly recommend these if you have patience (yes, a lot of that), dexterity and motivation to really get into a piece of gear that will reward you with a wonderful music experience.
I owned two used HK ST-7 tables with the Rabco arm. Most finicky tonearm to calibrate and consistently maintain. Owned all the relevant repair manuals back in the day. Essential to have them on hand. I junked both tables.
I purchased mine new in 1977 or so. Other than a couple of new belts I’ve never had a problem with it. Tracking has been spot on. 
Stringreen, I do think that the tonearm on the ST7 tonearm was a low cost version of the original Rabco SL tonearm, designed specifically and only for the ST7 and never used again.  Nor was it ever sold separately.  It was therefore either designed by Dr. Rabinow (the originator of the Rabco SL tonearm or was licensed under his patent, at least.  Rabinow lived in the DC area, where I live. Same goes for the much more expensive Goldmund TF tonearm, used on their Reference turntable.