Harmon Kardon ST-7 with Rabco Tonearm

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. 

You must have got the best one they ever made .

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.