Friday, 2 March 2012

Grooving on a Tangent - Harman Kardon Rabco ST-7 Turntable

After a week of my car being in the shop, and work getting in the way, I was finally able to get down to Point Roberts to pick up my most recent turntable acquisition.

This one is the most exotic yet, and will hopefully hold top spot for quite some time to come. Considering it cost me more than any table I have purchased recently I hope it does. Of course it seems for now to need nothing in the way of upgrades or maintenance as was the case for many of the other pieces of audio gear I've recently acquired. The fellow I bought it from on AudioKarma did a bunch of work on it already so there is little for me to do, except perhaps polishing some blemishes out of the lid which I am not only getting pretty good but it costs me mostly just time.

Harman Kardon ST7t

The turntable in question is a Harman Kardon Rabco ST-7 and has, as the owners manual says, a "straight line tracking system." Otherwise known as linear or tangential tracking this method of tonearm design essentially means the tonearm does not travel through an arc, but always stays at a tangent to the groove. In theory at least this should sound better with less potential for distortion.

tracking error

The principal reason this should be better is that the stylus is always positioned in the groove in the same way that the record cutting lathe created the groove on the master discs. Doing this ensures the stylus reproduces the sound accurately. When a tonearm describes an arc as the vast majority do there is simply no way to have the stylus always at the correct angle to the groove. The best you can achieve is a compromise, though that can be a very good compromise and for most is more than adequate for good sound reproduction.

I've always been intrigued by linear tracking turntables but have never seen one I would either want to own or could afford. They only represent a very small percentage of the turntables made and in the shops I go to usually the only ones I ever see are the lowest end versions and usually in very bad shape.

Rabco Tonearm

I honestly am not sure how much better this type of turntable can sound for me. It eliminates what is know as "inner groove distortion" which is concern for conventional tonearms. In the inner grooves the difference between the angle of the stylus/tonearm to the groove is the greatest so there may be more audible effects or differences from the way the source material was added to the disc. I am not really that critical a listener, rarely sit in the sweet spot and really listen, and don't have the ultimate is system quality so...

In other words the potential differences are really quite subtle to most mortals. However even if I can't hear as much of a difference as some folks, the engineering behind this design also fascinates me.

HK Rabco drive

There are various ways that the tonearm follows the groove. In the case of the Rabco ST-7 this is done by a secondary belt driving the tracking shaft. Riding on that shaft is a rubber tracking roller and based on the easily adjustable tracking angle the speed at which the cartridge moves across the record can be changed. Because there are varying lengths of music on record sides, there are not always the same number of spirals so the rate at which the stylus reaches the centre of the record will also change. Being able to adjust this angle can compensate for this variance.

From a maintenance standpoint this arm and drive system should need little work. The tracking roller is know to wear out or disintegrate but relatively easily found and modified O rings can be used in its place. The belts are also supposedly quite normal in size.

Rabco tonearm with Audio Technica

Installation and alignment of the cartridge can get tricky. Harman Kardon did sell these with a gauge that would come in very handy, as would another tonearm wand for easy swap of cartridges, so I will be keeping my eye open for those.

This one is well set up right now, so hopefully these issues will be sometime down the road. For the time being I will simply try to enjoy the sound and the Bauhaus like design. 


  1. Hello there, just found your site when I was searching for a manual (found it) for the ST-7. I bought it some 20+ years ago at a flewmarket, but failed to understand how it worked (or SHOULD have)and so it rested in my parent´s cellar since last spring they asked about whether to eliminate it...I tried it once more and this time I realised the former "missing link" in the arm´s moving; i.e. the deteriorated silicone(?)-ring whose relics I had assumed to be a rotten ´gummibaerchen´ (sort of wine gums here in Germany). As you write I took an O-ring instead, did some cleaning, balanced the arm - and donated it to my daughter as an easter gift due to her interest in analog HiFi. She likes it !

  2. I have one that looks mint but it has been on the shelf for 10 years. Bought it used but never even got around to hooking it up...think I might give it a try. Heavy and very cool looking for sure. Thanks for the article...

  3. Bought my Harman Kardon Rabco ST-7 new in early 1980 and it still works today, only replaced the belts after stored for several years. Still plays my half speed master recording like new!

  4. I bought an H.K. ST-7 new in 1976 and it was the love of my life until it needed service in the mid 80's. No one savvy enough in this town to fix it for me. With tears in my eyes I gave it away just a few weeks ago along with a Revox table that I could not get serviced. I loved both turntables and wish I could have kept them running. Nothing makes sense like the linear tracking designs.

  5. I too got bit by the "audio bug" and have made a return to vinyl. I owned an Onkyo PL-25F linear tracking table that was in storage for years in a closet and needed a new stylus. It began here looking for stylus and cartridges on Amazon and Ebay. I did a search on Ebay for linear tracking tables and saw a Harmon Kardon ST-7 and was intrigued to say the least. I began researching the table and it siblings and learned all I could about these models. Over the past year I bought (3) ST-7's and (1) ST-8 and brought each back to their former glory. I sold off one of them and a Sony PS-X5 that snuck in my home. I really enjoy the ST-7 and ST-8 tables and have been following them on AudioKarma's website.

    Here are a few links to post that I am sure will be of interest to other fan's out there!

    Anyone who owns one and has basic skills can work on them. Once dialed in they really are worth the time and effort, what a joy to own and listen too!

    Rick Read