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The Rabco/HK Turntable

K

Karma16

Super Member
HI,
A few questions for you Rabco/HK owners. But first a little story about my history with these tables.

Back in the 60's and 70's I followed the progress of linear tracking closely. I remember the Marantz (didn't work), the original Rabco SLT (worked sometimes), on into the era of servo control (capable of working well). Personally, I bought two servo machines (B&O 4004 and Yamaha PX-2) and my current air bearing Eminent Technology ET-2.5 arm.

I had my first stereo repair shop (in the 1970's), which was associated with a local high end store, when the first Rabco/HK turntables hit the market. We were HK dealers so I talked management into carrying the turntable. I had subversive intent because I wanted to buy one using the standard industry discount (40% off) given to employees.

So, we got them in stock and they sold well....at first. But, as fast as they went out the door they came back. Folks could not get them to maintain tangential tracking. It was my job to get these things working reliably.

I was unable to do so. Phone conversations burned the air waves between me and the distributor and the factory. Nothing worked. Remember, I was limited to either replacing bad parts or using the built-in calibration adjustments. These were new and under warranty units and shotgun repairs were not allowed. Nothing worked.

We finally refunded all the customers money and dropped the line. I lost my interest quickly concluding that it was a flawed design. Shortly, the turntables disappeared from the hi fi mags and, as far as I was concerned, the Rabco/HK went to hi fi heaven as another ambitious, good looking failure.

Imagine my surprise when I joined this forum and found that this turntable had a following. This confused me given my history with them. So, here are my questions:

1. ARE YOU GUY'S ACTUALLY GETTING RELIABLE OPERATION FROM THESE TABLES? THIS MEANS IS THE ARM MAINTAINING A TANGENTIAL RELATIONSHIP TO THE PLATTER RELIABLY.

2. HOW MANY MODELS OF THIS TURNTABLES WERE RELEASED?

3. IS IT POSSIBLE THAT THE FIRST UNITS HAD PROBLEMS THAT WERE FIXED WITH SUBSEQUENT MODELS. IF SO, I NEVER SAW THEM.

If the initial problems were fixed, I'm surprised that the unique arm drive did not appear on other turntables.

Thanks, Sparky
 
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I had one new and have another now as a curiosity, they both worked as designed for as long as I cared to use them tho' I an an understanding and somewhat hands-on user. Interesting and somewhat elegant approach but I have more confidence in light-gate triggered servo approaches.
I understand your scepticism, this issue reminds me of the fanboy biker Harley owners of yore that "had no problems whatsoever" with their mounts, leaving out the yearly total overhauls and not considering being disabled by the roadside in the middle of nowhere due to failed and/or vibrationally discarded vital parts as a problem.
 
I have an ST-7 and owned an ST-8. Both work and worked fine with no problems tracking once dialed in. Right now the ST-7 is my main turntable in my system.
 
HI Folks,
Pio, are you saying that you have to tweak your table to keep it running? If so, I don't consider that to be reliable operation.

As far as my scepticism goes, it's far more than that. From the track record I saw, that table flat did not work. At least not for the average consumer.

Also, how many models were there? I don't remember the model number of the original table that I worked on.

Sparky
 
I have multiple ST-5's, ST-7's and an ST-8. Only one was purchased new when they were available. That one, has been my primary table since the late 1970's when I bought it. I did have to go in and replace the tracking roller with an "O" ring at one point, as the original rubber deteriorated. From time to time I have to clean the sliding and turning metal surfaces of the tracking carriage, and of course belts have been replaced thru the years, as with any belt-drive turntable. I have also had to replace a couple of light bulbs, but all that is general stuff. The table (in fact all of them that I have cleaned up and put back in service) are very reliable, and track well. they HAVE to be level though. Gotta make sure they are level. Once calibrated, I never have to re-adjust the tracking control on the back of the carriage. I can't understand why the patrons of the store you worked at, had so many issues.
 
Phototone said:
I have multiple ST-5's, ST-7's and an ST-8. Only one was purchased new when they were available. That one, has been my primary table since the late 1970's when I bought it. I did have to go in and replace the tracking roller with an "O" ring at one point, as the original rubber deteriorated. From time to time I have to clean the sliding and turning metal surfaces of the tracking carriage, and of course belts have been replaced thru the years, as with any belt-drive turntable. I have also had to replace a couple of light bulbs, but all that is general stuff. The table (in fact all of them that I have cleaned up and put back in service) are very reliable, and track well. they HAVE to be level though. Gotta make sure they are level. Once calibrated, I never have to re-adjust the tracking control on the back of the carriage. I can't understand why the patrons of the store you worked at, had so many issues.
Click to expand...

HI Phototone,
Well, I don't understand either. Perhaps the problems were due to the fact they were brand new to the market and the problems were sorted out later. But there is no doubt there were serious problems at least initially.

I'm happy that you and others seem to be getting good service. This confirms my original ideas about the turntable. Frankly, I'm surprised.

Would someone please tell me all the model numbers?

Sparky
 
Mine were/are both ST-7, I regularly cleaned the roller shaft and upper glide rod and checked the tracking to ensure the gimbal block stayed square in the carriage when tracking. I don't remember any particular issues with it when it was my main TT. I later got a deal on a Thorens TD-125 MkII and then an SME-3009 and the ST-7 went bye mainly for that reason. The Thorens subsequently got sold off as unshippable when I was transferred to Germany, I didn't need a broken Thorens there.

I know of the ST-6 (AC motor, manual controls, otherwise sim to 7 with the roller tracking and Rolamite bearing arm), ST-7 (Pabst hall-effect brushless DC motor, light gate triggered end-of-play cue-up, touch conduction controls) ST-8, last improved iteration of the roller Rabcos (same as 7, in black with bubble level and adjustable leveling feet, cue muting, permanent hard tracking tire, and belt driven rotating upper glide rod to reduce friction further. Added machinery and hard tire raised the background noise a bit, still very low but not as low as the 7. I don't know about the others offhand, think there was an ST4, and 5 at least, the 5 being the first of the later style roller Rabco series. The ST-4 was something quite different.

BTW, there was someone that posted somewhere a while back about being a tech at an audio shop that maintained the Marantz SLT-12s and claimed that he kept them working, as unbelievable as that sounds.
http://www.thevintageknob.org/marantz-SLT-12.html
 
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Hi Sparky, I have an ST-5 (no speed adjustment beyond 33/45). I bought mine locally recently, supposedly "working fine". Yeah, right. I fiddled with it for quite a while, but actually got it working; working well in fact. However, I have a love/hate relationship with this TT. I love the linear tracking mechanism, even if it would bring a smile to Rube Goldberg's face. When properly set up, there's virtually zero tracking error, because the tracking roller is always active, not just when a servo activates. Also, it's mechanical--no unobtanium IC's to fail. However, H/K's implementation of the system sucks eggs. A sheet aluminum chassis? Are you kidding? Didn't they know anything about turntables and chassis vibration? Also, the motor vibrates very badly, inducing vibration that you can feel into the entire unit, and it is audible when playing a record with quiet program material.

But hey, their feet are adjustable.
 
Dynamat can be your friend, and get rid of the leftside gantry dress cover or find a way to damp it.
 
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Yes, Dynamat can be your friend. However, the motor vibrates something terrible. Fixing this would be the first order of business. THEN, damping the chassis would be icing on the cake.
 
Karma16 said:
1. ARE YOU GUY'S ACTUALLY GETTING RELIABLE OPERATION FROM THESE TABLES? THIS MEANS IS THE ARM MAINTAINING A TANGENTIAL RELATIONSHIP TO THE PLATTER RELIABLY.

2. HOW MANY MODELS OF THIS TURNTABLES WERE RELEASED?

3. IS IT POSSIBLE THAT THE FIRST UNITS HAD PROBLEMS THAT WERE FIXED WITH SUBSEQUENT MODELS. IF SO, I NEVER SAW THEM.

If the initial problems were fixed, I'm surprised that the unique arm drive did not appear on other turntables.
Click to expand...

I also worked for a shop that sold these, and they has very few problems other than that the tracking adjustment was exceedingly fussy and had to be spot-on or the arm would drift off tangency after a few inches of travel.

However, the tech who worked on them before me said that he was told the factory had great difficulty with the early production of the very first model out (the ST-7) because of difficulties in molding the rubber tracking roller to be perfectly circular and still be of the exact resilency they wanted. These samples could not be reliably adjusted to keep the tracking perfect. He was told that the fix the factory finally came up with was to freeze the roller material in liquid nitrogen and then precision cut the now rock-hard material on a lathe or similar machine. When they thawed, they were perfectly circular.

I never worked on any of those, and the 'table I owned was made after this period, so other than the touchy setup, it always worked perfectly.

Keeping this history in mind, I am still somewhat amazed that many others here at AK who have refurbished these machines have had such good success with essentially o-rings from a hardware store, but-- hey, if it happens it must be possible! :D
 
HI Classic,
Thanks very much for your input. You have confirmed the problems I was having. Now I can feel secure knowing that I was not imagining the problems. At the time I was working on them apparently the factory had not actually found the solution. They were just cranking them out and trying to fix them as necessary which wasn't working.

The arm had a unique pivot that actually had a name that I don't remember. In the early advertisements, HK was bragging about it. It caught my attention. Edmonds Scientific actually had a working plastic model of the patented version (not the Rabco version) which I bought so I could learn. I don't remember the name of the bearing (pivot) which was named after the inventor. Anyone remember the name?

Everyone should understand that the Rabco/HK tangential tracker was ahead of its time. At that time there was only one linear tracker that actually worked-the original Rabco SLT which was far from perfect. Servo controled arms were still several years in the future. Air bearing arms were a decade in the future. The Marantz arm was a brave first attempt but being totally passive with mechanical bearings, it had problems. I'm surprised anyone could get it working. I'll bet it didn't work very long before it needed a new setup. Then there is the Souther which is the only passive design (except air bearings) that actually works at a high level of performance. Air bearings are really in a different class and represent the best solution to date. The whole tangential tracking journey has been a fascinating trip with great engineers working the problem.

BTW, I'm still seeking the complete model number list.

Sparky
 
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Sparky;"The arm had a unique pivot that actually had a name that I don't remember. In the early advertisements, HK was bragging about it. It caught my attention. Edmonds Scientific actually had a working plastic model of the patented version (not the Rabco version) which I bought so I could learn. I don't remember the name of the bearing (pivot) which was named after the inventor. Anyone remember the name?"

Rolamite bearing, invented by Sandia engineer Donald F. Wilkes.
http://en.wikipedia.org/wiki/Rolamite

Some more info on LT arms with emphasis on the Rabco SL-8E;
http://www.soundfountain.com/amb/ttrabco.html
 
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HI,
Thanks PIO. You are a great help.

And, your article provided the model numbers I was looking for. They are ST4, 5, 6, 7, and 8.

Sparky
 
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jhoyt said:
Yes, Dynamat can be your friend. However, the motor vibrates something terrible. Fixing this would be the first order of business. THEN, damping the chassis would be icing on the cake.
Click to expand...

I have several ST-5's, and my motors do not vibrate. Perhaps you have a defective motor?
 
Ant1-Hero said:
Does anyone know the size o ring roller I need for the ST-7?
Click to expand...


I took my cleaned tracking roller (it is removable) to an industrial "O" ring, and rubber hose vendor locally and gave him the roller and said find me a fat "O" ring that fits. He did and I bought several, but I never knew the size.
ha ha. Since they are rubber, they stretch, you know...you need to fit one that is slightly smaller so it will be stretched and tight. You may, or may not want to scuff up the surface that touches the metal rotating tracking rod underneath for more traction.
 
Karma16 said:
HI,
Thanks PIO. You are a great help.

And, your article provided the model numbers I was looking for. They are ST4, 5, 6, 7, and 8.

Sparky
Click to expand...

But note, the ST-7 was the first of the Harmon Kardon models, the lower number models were less expensive models that came after the ST-7, and the ST-8 was a later "more delux" model. I'm not sure, but I think the ST4 was actually a Rabco turntable, not Harmon/Kardon Rabco. It used similar principles though, it was implemented differently...totally mechanical tracking. I have one, and in fact it does not require any use adjustments other than belt replacement from time to time and cleaning.
 
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