Harman Kardon Rabco ST-6 tracking roller replacement help

It seems that replace this tracking roller (O-ring) is a very common service for ST-6 and ST-7 Rabco tonearms.

So after a year of postponing, today I got into this project. Sure enough the old rubber roller is disintegrated, totally gone. Other then that, TT seems to operate fine. Both belt are fine.

1. How to get to this roller? I removed a couple of pieces from the top, and I see it (the place where I need to install the new one). However, the whole Rabco tonearm is sitting around the tracking shaft and I don't see how to get there. What is the correct approach to get there? From top? From bottom? Need help.

2. How to remove the platter (to check the condition of the drive belt)? Normally I pull them up, but this one is sitting tight and I don't want to force it.

There is a 'C' circlip under the mat center piece that has to come off first to get the platter off.
The tire may be replaceable on the tracking roller without total disassembly, check past threads on the ST-7. Do make sure the roller goes back in the same L/R orientation it came out, IIRC it's not perfectly symmetrical side-to-side.

Found the C-clamp. Thank you.

Good to know that no total dis-assembly needed to replace the tire. Still stuck and need a bit more specifics.

I was able to replace the rubber wheel on my st-7 by loosening up the wheel holder and gently it pops out. You get to it from the top on the back. Two screws and lift the cover over the arm with the level. Then two more screws and take off the back plate and your there. Here's a pic after I finished...

Thank you very much. That is exactly where I am now. 2 screws and 2 screws to remove a back aluminum plate.

And what now? I see another 2 hex screws on tonearm (facing rear) with a tracking adjustment lever. I am reluctant and I need a confirmation that this is the right way.

Actually when you open it up all you need to do is loosen a nut that holds the wheel holder together and the roller should pop out, with a little persuasion.

Thank you very much. I was able to take the roller out. Indeed, "persistence" is the key, but it's understatement. Real pita. It required rather special tools to loosen the nut in very tight space.
But then metal roller just doesn't have a room to get out, so I have to bent this gray plastic clamp and it's dangerous, could be broken. Need to unscrew some more tonearm screws to make a little bit of room.

Finally I have this metal roller out.
I have a feeling that it would be next to impossible to install it back. Any tricks?

Just take your time you can't go out and buy new parts. If you got it out, it will go back in. Clean all the old rubber gue off, put on the new o ring, and carefully reinstall. A small skinny pair of needle nose pliers helps. If you wedge some small pieces of whatever so the roller can't drop to far helps a great deal, you'll understand. Did I say take your time?

It might have been easier to leave the roller wheel inside the yoke but loose, and work the new tire into then onto it. Then get the wheel back into the yoke in the same original orientation. I'm fortunate to have an assortment of surgical instruments.

Pio1980 said:

I'm fortunate to have an assortment of surgical instruments.

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It's a micro-surgery, indeed. I have to make a tire (O-ring), then I report back.

Ok, it's been two weeks until I got a time to work on HK ST-6 and I'm glad to report that tracking roller has been replaced, everything adjusted, and TT works perfect now.

I would like to point a couple of notes, however.

The replacing a tracking roller is not that easy. In fact, it's quite hard. In scale of 1 to 10 it's about 8.5, when 10 is impossible, and 9 is nearly impossible. In other words, it's quite possible that job will not be completed (or even worse if something break).

I used the exact approach as I was advised in this thread above. Therefore, I worked in extremely tight space. Removing the roller holder was quite easy, but the hardest trick was to install it back (this time with a rubber roller, that eat even more of tiny space). I used surgical tools and spend 1 hour trying to stick it back and gave up. My wife tried it another hour without any progress. Next morning I woke up and my wife told me that she already installed it, I have no idea how, and she cannot explain too, the only thing I got is she used the same surgical tool.

Another consideration was not to break something. Tonearm is very delicate, has old plastic parts that I have to pry, multiple little springs, etc. One wrong move and Rabco is gone.

Another difficulty I had with tracking mechanism. Its handle has two smallest Alien screws I've ever seen. My smallest Alien wrench is maybe 3/32", but it's way too large. Then I got twice smaller 0.050 wrench, but it's still twice large then needed. I ended up fabrication my own smallest Alien wrench to overcome these 2 screws.

To wrap it up. HK Rabco works great.
However, this project is difficult, require a lot of patience and many different tools.
And final result could be different if someone trying to do it for the 1st time. Everyone should consider this before attempting this project.

I'm trying to do the same repair on my ST-5, but I can't even get the platter off. There is a "curved washer" under the mat (not sure if it's the same thing as the "c-clip" on the ST7 or not...) and I can't get the thing off. I feel like this might be one of those things where once I figure it out I'll be slapping my forehead and wondering why I didn't think of that!

Nevermind... got it! It WAS a c-clip ONTOP of the curved washer. I thought it was all one piece until I looked closer. I'm an idiot, haha.

It took me two evenings to get it all figured out, but I finally got the ST5 I bought working!!

Used the old O-Ring trick to replace the worn-out tracking roller. Worked like a charm.

Trickiest part was getting the lift-pin screw adjusted correctly. It seemed like no matter how short I made it it was still too big to let the tracking roller sit properly when cued in the down position. I ended up taking the lift-pin screw out and replacing it with one of the two screws from the tracking shaft pulley-wheel since it was the same diameter but could be adjusted shorter since it has no head on the end. This obviously isn't ideal since now my tracking-shaft pulley wheel is only held on with one screw instead of two. However, it seems pretty sturdy with just the one so I think it's fine.

Re-assembled and tested and everything worked fine. I had heard these HK/Rabco tables were notoriously difficult to get the tracking of the tone-arm right, but frankly it seemed pretty simple. Took me a couple tries to keep the tone arm from racing ahead or lagging behind the carriage, but it's tracking great now!

And best of all, it sounds FANTASTIC! Was totally worth all the work.

986s said:

I was able to replace the rubber wheel on my st-7 by loosening up the wheel holder and gently it pops out. You get to it from the top on the back. Two screws and lift the cover over the arm with the level. Then two more screws and take off the back plate and your there. Here's a pic after I finished...

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That looks like an original equipment tire instead of a replacement o-ring substitute. Where'd you find one?

So I realize this is an old thread but I have been working on my ST-6 today so will bring this to the top. I think I may have a handle on it now but would also like to incorporate a few more things to make this more of a one-stop shopping experience.

First the O-ring size reportedly is 3/8" ID and 19/32" OD. I don't have one yet but after working on this I am not sure I need one. I will pick up some anyway as I have an ST-7, this ST-6 which is on the market, and soon will be tuning up a friend's ST-7

I think my tracking is now ok, though the pic below is before I quite had it dialed. My original roller looks to be in good shape. However before I worked on it today the arm was leading and did not appear to start to move right away.


Rabco ST-6 by Buhduh, on Flickr

I made some adjustments to the dash pot and it seems to cue a bit better now, and with less drift to the arm as it comes down. I also checked the threaded lift pin gap and found it to be very small with possibly some evidence of zero gap, i.e. friction which would cause tracking issues. I did not make an adjustment to the gap per se but found that there was a possible solution.


Rabco detail by Buhduh, on Flickr

The lift plate had some crap under it! The circle on the left is where one of the nuts is that the top of the table screws to. There is one on the other side too. They sit loosely in slots and could easily drop out, and someone sometime added some putty or something to hold them there. One hardened piece dropped off when I pulled the screw out. The screw is now held there with black electrical tape. Anyway, I found a couple of extra little hard pieces of that crap under the lift plate! You can just barely see one in the other circle.

And below you can see both chunks and where I found them. They could have cause the lift plate to be pushed up and touching the lift pin I reckon. I found that I had a gap just large enough to slide a piece of business card under, slightly over .01 inches. The manual says nominal should be .02 or less so I think I am ok.


Rabco crap by Buhduh, on Flickr

So after taking them out and playing with the tracking adjustment lever I have virtually eliminated the tracking error. :thmbsp: I also added a tiny touch of oil to the pivots that the lift plate moves on in case it has some stickiness, and worked that in a bit. I still have to get the shut off tripping adjusted a bit.

I might still replace the roller soon though or may have to on the ST-7. When you guys are getting at that are you taking this nut off (or just backing it out) to allow you to pry apart the whitish plastic part lower down by the arrow to release the tracking roller holder?


Rabco roller nut by Buhduh, on Flickr

Just loosen up the nut and you should be able to wiggle out the roller. If you take the nut off things start to fall apart and it's a lot harder to reassemble.

986s said:

Just loosen up the nut and you should be able to wiggle out the roller. If you take the nut off things start to fall apart and it's a lot harder to reassemble.

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Did you remove the roller from the assembly or just slide the pin out of the plastic enough to insert the new O-Ring?

On the crap that held the nut in place. I believe that is factory original crap. My St-7 has dried loose putty. I replaced it with some epoxy putty. which should be good for the next 30 years.

Hello, I'm very new to this forum, and I've been working on an ST-6 for a little while now, after changing belts and all the little adjustments I'm now gonna try to replace the wheel/o-ring since it is completely gone. I have the o-ring I need and I've followed everyone's instructions on how to get the roller off by loosening the one nut and prying the plastic, but the roller is now sitting on the carriage rod and I can't see how to get it out to change it. Please help!!! Thanks in advance for any tips!

Barrington said:

but the roller is now sitting on the carriage rod and I can't see how to get it out to change it. Please help!!! Thanks in advance for any tips!

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Can you post a pic of what you are seeing?