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 Post subject: My 'new' Rabco ST-4 turntable
PostPosted: Aug Sat 26, 2017 7:58 pm 
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Hello everyone. For several years now, I've had a thing for linear-tracking turntables, as exhibited by my Bang & Olufsen Beogram 8000, its little brother the Beogram TX2, and my Technics SL-10, among several others. I like the ingenuity of the mechanisms which allow them to track parallel with the groove, and the various ways which manufacturers found to do so (both mechanical and electrical, both good and not-so-good).

Ever since I learned of their existence, I wanted something with a Rabco linear arm. Several years ago, I spotted a Rabco ST-4 sitting in the back of a car at one of the MIT Swapfests; the seller was busy, and I figured there was a reason it was there instead of sitting on his table, so I let it pass. More recently, I spotted a Harman-Kardon ST-7 (using a Rabco arm, as H-K bought the company in the early '70s) at one of the Kutztown radio swapmeets, but it was a bit too expensive for my liking ($250), and by the time I finally built up the courage to buy it, it'd been sold. My wallet thanked the buyer, but my heart still wanted a Rabco.

Fast-forward to this past weekend. I managed to get into the MIT Swapfest early, and started looking around the sellers setting up in the parking garage. Near the top of the seller area, I spotted a seller who I recognized from some local audio get-togethers, and who I'd bought stuff from at last month's swapfest. While looking at his wares, including a pair of Pioneer CT-F900s which I eventually bought from him, I spotted a box in the back of his truck labeled "Rabco". Sure enough, it had a ST-4 sitting inside it! I asked him how much he wanted, and he responded with $100. Sold! :mrgreen:

I find this unit rather fascinating. The tonearm it uses is different from the sort used in other Rabco-based turntables; instead of using a separate motor to advance the arm based on microswitches detecting the arm going out-of-true, the arm on the ST-4 rides along a long metal roller which turns along with the platter. The arm has a small idler wheel at the top which is in contact with the roller, keeping the arm true as it rides the groove. The Rabco arm used in the various linear-tracking turntables made by Harman-Kardon have far more in common with the older Rabco SL-8E tonearm (which cost more than the entire ST-4 turntable by itself!) than the arm used in in the ST-4. :?

Cosmetically, it's seen better days, but is in pretty good shape overall. One side of the funky split dustcover had bent mounting pins, which I've mostly straightened out. I'm guessing the belts could stand to be replaced, particularly the platter drive one, but they seem to be intact for the most part. When I first powered it up, the platter and tonearm roller slowly started turning. When I pressed the CUE button, the metal flap in front of the arm lowered, slowly dropping the arm onto the junk record I'd placed on the platter, and it started to track. There's supposed to be some sort of end-of-record lift, but it doesn't seem to be working correctly at the moment. I'll have to try and figure out if there's some way to adjust it.

Anyway, enough of my blathering, here are some pictures of the ST-4:

Image
The original box!

Image
What's in the box. Note the split dustcover.

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With the dustcover removed.

Image
What's under the pulley cover in the top left corner. The instruction manual shows a shift lever of some sort to move the platter belt between the 33 and 45RPM speeds, but there's no sign of it, nor of where it would've been mounted.

Image
The cartridge it came with, a Shure V-15 IV. It happens to be one of my favorite cartridges, which is good because the turntable didn't come with any sort of overhang gauge.

Restoring it should be interesting. There seem to be some issues with the power switch, as the platter doesn't get much (if any) of a 'nudge' when the switch is moved, and the last time I tried to turn it off, it initially refused to power off. Hopefully, it's not something major, but I'm not sure. Any ideas?
-Adam

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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Sun 27, 2017 3:19 am 
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Thomas Edison would have recognized that tonearm advance mechanism.

-David


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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Sun 27, 2017 4:48 pm 
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I was a Rabco rep during that era. May still have some parts unless they got lost in my last move.

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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Mon 28, 2017 3:23 am 
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dberman51 wrote:
Thomas Edison would have recognized that tonearm advance mechanism.

-David

Yeah, I can definitely see the resemblance, even if the mandrel holds the tonearm instead of a cylinder record. Of course, I'm pretty sure that Edison's patents had expired by them........... :wink:

Bruce Hagen wrote:
I was a Rabco rep during that era. May still have some parts unless they got lost in my last move.

Anything you could find would be great. Parts and service info seems to be fairly hard to find for the ST-4 these days; the only documentation I've been able to find have been some brochures and the owners manual, which doesn't say anything about aligning the tonearm lift sensor. Anyway, what were your impressions of the ST-4 when it was new? Do you have any idea why its arm seems to be radically different from any of the other Rabco arms?
-Adam

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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Mon 28, 2017 3:52 pm 
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No helpful information to give ... BUT, looking at those pictures, I am jealous and turning bright green with envy.

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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Tue 29, 2017 1:04 am 
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Overhang? Well if you can adjust the cart so it is square on, and adjust the overhang so it tracks straight towards the spindle, that should be perfect overhang? That is, if the arm is long enough to position the stylus of the cart so it tracks directly towards the center of the record. IF it does not, say, the arm is shorter or longer so the cart tracks to a point in front or behind the center, then you would have to mount it at an angle to correct it to be square to the record grooves.


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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Wed 30, 2017 2:12 am 
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charlief64 wrote:
No helpful information to give ... BUT, looking at those pictures, I am jealous and turning bright green with envy.

Yeah, I know the feeling. :mrgreen: I've been very lucky when it comes to finding turntables; of course, some might say too lucky, given that I've amassed at least a couple dozen thus far. This Rabco was one of my holy grails; I wouldn't mind finding another linear tracker capable of using a 1/2"-mount cartridge (most of the later ones used plug-in cartridges), or a broadcast-quality turntable like a Technics SP-10, Garrard 301/401 (almost acquired a 301 once, but I didn't feel myself capable of restoring it at the time), or a Thorens TD-124.

wazz wrote:
Overhang? Well if you can adjust the cart so it is square on, and adjust the overhang so it tracks straight towards the spindle, that should be perfect overhang? That is, if the arm is long enough to position the stylus of the cart so it tracks directly towards the center of the record. IF it does not, say, the arm is shorter or longer so the cart tracks to a point in front or behind the center, then you would have to mount it at an angle to correct it to be square to the record grooves.

Yeah, I think I misunderstood the instructions in the owner's manual with regards to setting the overhang. What I thought was a drawing of an overhang gauge was actually an illustration of how to place the tonearm on its roller. To set overhang, you move the tonearm over the spindle (which ends in a point), and adjust the cartridge mounting so that the stylus is centered over the spindle. Gotta love the (apparent) simplicity of the early linear trackers.
-Adam

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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Wed 30, 2017 5:20 pm 
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Adam, fascinating project... one I'd like to do!
Just had a parallel-tracking idea... To drive the arm across the record at the right rate without any side-thrust, pivot the arm on the moving bar, then servo the pivot point motion along the bar so that the angle between bar and arm (optical angle sensor, no friction) remains at 90 degrees. Any small pivot moving error is taken up by the arm swinging just a tiny bit, far less than a standard arm at a fixed pivot point.
Did Rabco do this? Likely not (EDIT: yes it did, see below) as the arm seems driven by a roller and lead screw (EDIT: No) off the T/T itself (is it?) Did any other T/T manufacturer use my "angle servo" idea?
Cheers,
Roger
EDIT. It seems they all used a servo, albeit mechanical or micro-switch. But an optical sensor would be better as no friction or backlash.

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Last edited by engineer on Aug Thu 31, 2017 5:09 am, edited 1 time in total.

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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Wed 30, 2017 10:18 pm 
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I think that's exactly how the Rabco works. It would have to. You couldn't just drive a lead screw at a fixed speed to index the arm because record pitch is highly variable.

-David


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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Wed 30, 2017 10:21 pm 
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engineer wrote:
Adam, fascinating project... one I'd like to do!
Just had a parallel-tracking idea... To drive the arm across the record at the right rate without any side-thrust, pivot the arm on the moving bar, then servo the pivot point motion along the bar so that the angle between bar and arm (optical angle sensor, no friction) remains at 90 degrees. Any small pivot moving error is taken up by the arm swinging just a tiny bit, far less than a standard arm at a fixed pivot point.
Did Rabco do this? Likely not as the arm seems driven by a roller and lead screw off the T/T itself (is it?) Did any other T/T manufacturer use my "angle servo" idea?
Cheers,
Roger

Not quite sure that I quite get what you're going for there, or if any of the linear trackers used something like what you mention. The original Rabco arm (SL-8/E) used a microswitch on either side of the tonearm to detect when the arm goes out-of-true, which then triggers a battery-operated servomotor to move the arm via a chain-drive mechanism. The Rabco arm used on the later Harman Kardon turntables was similar, but simplified to some extent (no more chain drive or battery, for one thing) and rather finicky.

The ST-4's arm uses no leadscrew; best I can tell, the roller is just there to keep the arm true, and facilitate its movement as the groove pulls it across the record. Bang & Olufsen's linear arm replaced the microswitches with optical sensors, and used either a precision leadscrew (4000/8000 series) or a cable (Beogram 5005 and onwards) to advance the arm when triggered by the sensors. Probably the coolest was the Pioneer PL-L1000, which used magnetic levitation to 'float' the tonearm as it tracked the groove. Quite fascinating what they came up with in those days, really.
-Adam

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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Wed 30, 2017 11:04 pm 
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So you're saying that the roller is rotated by the turntable only to make it easier for the stylus to drag the entire tonearm across the record. I can't see how that wouldn't result in intolerable side loading.

-David


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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Thu 31, 2017 2:31 am 
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dberman51 wrote:
So you're saying that the roller is rotated by the turntable only to make it easier for the stylus to drag the entire tonearm across the record. I can't see how that wouldn't result in intolerable side loading.

-David

That's how I understand it. The arm has a small idler wheel in it which rides on top of the big roller. Apparently, the later Harman-Kardon versions employed that as well as some vestige of the servo mechanism from the original Rabco SL-8E arm. Some more info on these arms can be found here:

http://mellowgroovy.blogspot.com/2014/05/rabco-st-4.html
https://forum.audiogon.com/discussions/vinyl-old-timers-hk-s-and-rabco-s
http://www.audiokarma.org/forums/index.php?threads/haman-kardon-linear-turntables.4937/
Popular Science, November 1971
Electronics World, June 1971 (page 14)

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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Aug Thu 31, 2017 2:57 am 
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Thanks for posting the links -- they provided the answer. How it works is very clever: the little idler wheel (called a "steering wheel") and the rotating roller together form a mechanical servo. As the arm moves toward the record center it changes the arm angle which changes the angle of the steering wheel relative to the rotating roller. This creates side thrust between the roller and steering wheel which drives the arm along the roller until it is again straight. The stylus does not drive the arm, the roller does. Very simple, should be very reliable, but depends on a very good surface on the steering wheel. Any dents or gouges and it won't track smoothly.

-David


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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Sep Fri 01, 2017 3:03 pm 
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dberman51 wrote:
I think that's exactly how the Rabco works. It would have to. You couldn't just drive a lead screw at a fixed speed to index the arm because record pitch is highly variable. -David
Correct. You would servo-control the lead screw speed from the angle sensor, the latter optical in my suggestion.
Cheers,
Roger

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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Sep Wed 20, 2017 6:23 pm 
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My first post. Timely discussion.

I've had my Rabco ST-4 since the early 70's and now have refurbished it for service again.

My only problem is replacing the drive belt. My old belt, that still works, is 36.5" but is spliced and 35+ y/o.
I purchased a FM 34.0 that was supposed to fit, but TOO tight.
I purchased another FRX 35.1 and it too is tight.
According to my CAD layout the belt length should be 37.6".

Attachment:
Rabco ST-4 Drive Belt.JPG
Rabco ST-4 Drive Belt.JPG [ 28.81 KiB | Viewed 1122 times ]


By tight, I mean the motor spindle will not turn the belt. The spindle seems to bind under the tension and will not move. The drive spindle is odd in that it is two piece. The brass piece is presumably attached to the motor shaft and the outer (silver) spindle is free wheeling. See pic.
Is this some sort of torque limiting coupling?

Can someone check the drive spindle and verify it is two piece?
Anyone have a contact for the proper drive belt? The two services I contacted don't seem to have the answer.

Getting a new 36.5" belt has been difficult.

Is the ST-4 that sensitive to belt tension or is something amiss in my ancient TT?


Attachments:
Rabco ST-4 drive spindle.jpg
Rabco ST-4 drive spindle.jpg [ 127.64 KiB | Viewed 1122 times ]
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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Sep Thu 21, 2017 1:25 am 
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Mine has the pulley seen in that photo (appropriate, since it's one of the ones I took). I have seen a photo of a ST-4 with a different style of pulley (as seen in a small picture here) which has the larger section on the bottom. I'm not sure exactly how similar the pulley is otherwise. Does your ST-4 have the doohickey which is supposed to shift the belt between pulley sections in order to change speeds?

As far as the belt goes, I've seen pages which recommend either the FM 34.0 or FBL 35.1; have you tried measuring the belt path length using a piece of string? I've heard that modern belts sold by whoever took over PRBline are stiffer than the ones available back in the day. I've seen a picture of an ST-4 on this blog which appears to use an entirely different style of belt. I don't know off-hand what sort it is (gum rubber?), or where they got it. Perhaps you could try sending the blog owner a message asking about it. Good luck!
-Adam

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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Sep Thu 21, 2017 2:42 am 
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Adam thanks for the reply.

I don't have the speed doohickey. Mine is just like yours in the picture.

Does the silver spindle stop turning when lightly touched with your finger?
I have video but don't know how to attach it.

If so, then it s is some sort of torque limiting mechanism which tells me the system is sensitive to belt tension and that an exacting belt dimension is critical.

If not, then I have a De-coupled spindle/motor shaft.

The string method measures 37 3/4. 3%-5% (recommend stretch) is 36.62 - 35.86 which tells me my 36.5 length should work if I can find one. I probably will need to talk to these guys, http://www.elexatelier.com/.


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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Sep Thu 21, 2017 3:19 am 
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If the belt tension exceeds the light finger touch, the spindle stops turning. It's a very light touch.

The motor must be very very low torque.

Belt length/tension seems critical.

YouTube;

https://youtu.be/Fj3cDdsng9I


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 Post subject: Re: My 'new' Rabco ST-4 turntable
PostPosted: Nov Tue 14, 2017 6:47 pm 
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Adam:

Sure took me long enough but I do find that I have lots of pulleys and a few of the two belts. The units were shipped without the pulley on the motor shaft because UPS was known to throw cartons and when the box stopped the pulley didn't bending the shaft. We stocked the pulleys because the dealers would misplace them and panic when they sold a unit but could not deliver. Funny, but sad.

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