H-K Rabco ST-7 tracking roller revisited

[HalfApt]

re: the H/K Rabco ST-7 tracking roller issue - it's been covered in numerous older threads, but thought it better to start a new one.

I think I've found the ideal replacement: "Square-section O-rings", aka "square-rings"

Briefly, my ST-7 roller tire died the gooey death many moons ago. I knew about square rings back then, but nobody had the right size as normal stock, and having one custom made was too expensive, so I went with the diy alternatives.

Was never satisfied with the standard O-ring (inconsistent between pressings), so made a latex tire using double-thickness of 1/16" wall latex tubing. This worked better than the O-ring. Still had minor issues, but I could live with it for the few times I played vinyl.

A recent urge to find a better fix found a vendor with the right size "square-rings", so I bought them & got one installed. The ST-7 tracks straight and true now!





These are 3/8" ID, 5/8" OD, 1/8" section. A little tough to stretch over the wheel rim, but fit snug once pressed in, and run true.

Buna-N normally comes in 40, 70 and 90 durometer hardness. Would have preferred 40 (softest), but they only had 70 & 90 in the square-ring, and the 70 is working fine so far.

Vendor: The O-Ring Store

 

[squirrelnest]

Thanks for the information.....need to check my H/K Rabco ST-7....

 

[Montycat]

Good info. I will have to put together an order. I hope I can find more than just 52 cents worth of stuff to buy there.

 

[986s]

If you can find one, the st-5 or st-8 all metal roller works very well with no issues.

 

[rread13]

re: the H/K Rabco ST-7 tracking roller issue - it's been covered in numerous older threads, but thought it better to start a new one.

I think I've found the ideal replacement: "Square-section O-rings", aka "square-rings"

Briefly, my ST-7 roller tire died the gooey death many moons ago. I knew about square rings back then, but nobody had the right size as normal stock, and having one custom made was too expensive, so I went with the diy alternatives.

Was never satisfied with the standard O-ring (inconsistent between pressings), so made a latex tire using double-thickness of 1/16" wall latex tubing. This worked better than the O-ring. Still had minor issues, but I could live with it for the few times I played vinyl.

A recent urge to find a better fix found a vendor with the right size "square-rings", so I bought them & got one installed. The ST-7 tracks straight and true now!

View attachment 1261606

View attachment 1261607

These are 3/8" ID, 5/8" OD, 1/8" section. A little tough to stretch over the wheel rim, but fit snug once pressed in, and run true.

Buna-N normally comes in 40, 70 and 90 durometer hardness. Would have preferred 40 (softest), but they only had 70 & 90 in the square-ring, and the 70 is working fine so far.

Vendor: The O-Ring Store

Thanks for the information! I pulled the trigger and placed my order for a dozen. There cheap, but the shipping cost $6. Figure what the hell, I'll give it a try and see how it performs on at least one of my ST-7's.

 

[rread13]

I received my package of flat tire O-rings. I just need some time this next weekend to get into the change over on one of my ST-7's. I'm predicting I'll have to make some adjustments for the arms set down and pickup height on the lower pin.

 

[HalfApt]

.... I'm predicting I'll have to make some adjustments for the arms set down and pickup height on the lower pin.

Rick:

If the new tire OD is different than what you had in before (O-ring?), then yes, the cuing might need adjusted. Mine did just a little, simply because my funky double latex retread was smaller in diameter.

Needed tracking adjustment as well, about 1/2 turn, but it's still spot-on since I first posted.

 

[rread13]

The weekend finally arrived! Did my weekend lawn mowing and laundry, then finally had enough done I could justify getting to my turntable to start my mini tune up. I had a perfectly working round tire O-ring roller but you know me chasing the tweek and upgrade path to perfection! The easy way for me is the complete teardown and disassembly and remove the "hub/tire". The removal went smoothly as did the installation of the flat tie O-ring on the "hub". Installed the hub with the new flat tire O-ring, then put the hub in my cordless drill and turned it over a few light passess on emory 400 grit paper (sanding off the seam) and off to testing the table with the side and top panels off. The adjustments and tweaking are done. Mine did not require hardly any changes.

While in the process I pulled out my digital micrometer and took some measurements of both the the O-rings:

Round O-ring O.D. 15.55 mm while mounted on the hub
Flat O-ring O.D. 15.95 mm while mounted on the hub.

I am having an issue with the end of play arm pickup with the bare chassis I know the light reflection off the cabinet wall is potentially some area of light boost. But placing a drink coaster as a temporary reflection surface and then using a business card to cover the black cone light sensor the circuit is not detecting the closed circuit. My sensitivity adjustment variable resistor is the faulty part. I have used some cleaner and lube for electronics and then my CRC Mass Air Flow sensor cleaner and kinda hoping with some time it will help some oxidization clean up and magically start working again!? If not I'm gonna have to order a replacement and put some additional parts on order as a preservation and maintenance step on the main circuit board.

More to come as this gets sorted out!

The fat tire is working very nicely. Kinda like a slick on there instead of a bicycle tire! It's like the ST-8's metal roller but in rubber. I think it's quieter than my ST-8, I don't detect any subsonic rumble with the drive set up this way. I'd say it's a draw, the upgrade is easy to do, but you won't hear any difference IMO. But it's still a worthwhile upgrade.

This by the way was done to my Factory Modified/Upgraded ST-7 which did not have the alloy wheel ( I've read some did), this one also is an international voltage model. Makes me wonder how rare this one is? Still no end of record pick up. Gonna have to order some electrical parts and do some soldering!

 

[HalfApt]

... put the hub in my cordless drill and turned it over a few light passess on emory 400 grit paper (sanding off the seam)

I thought about removing the tire seam too, but it's so small that even slight weight pushed it flat to the tire, so I didn't bother.

I saw a German web page where the guy used an O-ring, then spun the hub in a lathe holding an emery board to the O-ring to create a flat surface. The square-rings are so much easier!

I am having an issue with the end of play arm pickup with the bare chassis I know the light reflection off the cabinet wall is potentially some area of light boost. But placing a drink coaster as a temporary reflection surface and then using a business card to cover the black cone light sensor the circuit is not detecting the closed circuit. My sensitivity adjustment variable resistor is the faulty part.

I presume you already know, but if not, the motor has to be running for the lift circuit to trip.

How did you determine the adjusting pot was faulty? Maybe try jumping it with a fixed resistor. The pot is 22k ohms, so a 5/10/15k might restore it temporarily.

 

[rread13]

No electrical testing has been done yet. I had very erratic and intermittent results with the pot, moving from near centered to opposite ends had no effect of increasing sensitivity or decreasing. Most times I got no electrical triggering. When I have some more time this next weekend, I'll dig back into my project and test this some more.

 

[kmp14]

So several months later, are you all still happy with the flat roller? Any thoughts on more or less mechanical noise than with a regular "bike tire" style o-ring?

Here is why I ask: I am hearing a fair amount of mechanical noise finding its' way into my speakers, possibly due to the all-metal roller on my ST-8. I have an extra roller from an ST-7 that was parted out, and I am wondering if I should replace the all-metal roller with a regular "bike-tire" o-ring or flat wheel o-ring to eliminate some of that noise?

 

[HalfApt]

So several months later, are you all still happy with the flat roller?

Absolutely! It tracks straight & true at all times.

Any thoughts on more or less mechanical noise than with a regular "bike tire" style o-ring?

I've never have had any mechanical noise with this table. The issue I had with O-ring was that it couldn't quite keep up with some large groove pitch direct-disc albums, the carriage would lag a bit. The "Square-ring" tire cured that issue.

Here is why I ask: I am hearing a fair amount of mechanical noise finding its' way into my speakers, possibly due to the all-metal roller on my ST-8. I have an extra roller from an ST-7 that was parted out, and I am wondering if I should replace the all-metal roller with a regular "bike-tire" o-ring or flat wheel o-ring to eliminate some of that noise?

I've never seen or heard one with the metal roller, so can't comment on noise, but I believe the square cut ring is the better choice. Provides the same rectangular contact patch as the original.

I'd never even heard of the metal roller until I joined AK last year, and don't know what to make of it. Doesn't make sense to me why they'd go that route.

My trusty ST-4 (1971) still has the original flat rubber tire and tracks perfectly.

 

[986s]

There is so much going on inside that metal box, I don't think the metal roller creates more noise. I use my ST-5 daily, with a metal wheel, and it tracks fine with no adjustments ever needed.

 

[Bobkat07]

Good info. I will have to put together an order. I hope I can find more than just 52 cents worth of stuff to buy there.

I

 

[Bobkat07]

Thx halfapt for the upgrade on the track wheel will order the flat square O-Rings from o ring store for my st7 haven’t used it in 20 years still works after replacing fuse and cleaned power switch with electronic cleaner platter belt was stuck but freed up easily and all lights and strobes work. Like the flat surface on the wheel better than o ring should track better as you say. Any points that need lube?? My track wheel was a soft reddish mass of gel amazing to me that is was rubber st one time. Appreciate any other tips. Thx and looking forward to enjoying my vinyl

 

[HalfApt]

The ST-7 has very little lubrication need. I've marked up the exploded view showing the only factory lube points. Refer to the color legend in lower left corner.

Most important is to keep the red "D" marked items clean and dry.

I've never added any lube to mine yet. In fact I removed the "grease" called for in the purple areas, particularly on the guide bar, as it woud get gummed up with dust & hair and cause the guide roller to stick. Works perfectly without grease there.

 

[Bobkat07]

I am grateful for the info. I did not encounter any grease on that guide I will not put any on it either. I ordered a pack of tracking square rubber o rings from McMaster Carr there is 25 in the pack which is more than I need in 3 lifecycles so if anyone wants some I will mail them out to you as I have benefited from the AK rabco fans advice so I will reciprocate. Hopefully I will get the wheel back in easier than pulling it out and not disturb the assembly. Thx bob

 

[Phototone]

I am grateful for the info. I did not encounter any grease on that guide I will not put any on it either. I ordered a pack of tracking square rubber o rings from McMaster Carr there is 25 in the pack which is more than I need in 3 lifecycles so if anyone wants some I will mail them out to you as I have benefited from the AK rabco fans advice so I will reciprocate. Hopefully I will get the wheel back in easier than pulling it out and not disturb the assembly. Thx bob

I could use a few.

 

[kmp14]

I have been using some dry teflon lube wherever I see moving parts that might benefit from it. Thanks HalfApt for posting that image from the service manual! I am going to back thru all the moving parts on my ST-8 and see if I can track down/eliminate the noise I am hearing.

 

[HalfApt]

I have been using some dry teflon lube wherever I see moving parts that might benefit from it.

Me too, on both the upper slide rod & lower guide rod.

Thanks HalfApt for posting that image from the service manual! I am going to back thru all the moving parts on my ST-8 and see if I can track down/eliminate the noise I am hearing.

Here's the exploded view for ST-8.

I've never seen an ST-8 in the flesh, but the discussions got my curiosity up, so I've been looking at the manual & photos to try and find all the changes.

One notable change that I've not seen mentioned before is to the carriage guide mechanism.

The ST-7 used a pulley-type plastic wheel, mounted in front of the carriage and riding on the guide rod.

The ST-8 instead uses a shaped guide piece mounted inside the carriage, with a U-shape channel that fits to the tracking shaft. On the side of that is attached a "Guide damper".

See part #s 140 & 329 on the diagram.

The word "damper" stands out to me. What is it supposed to damp? Vibrations? I can only guess.

If you look into it, let us know what those 2 parts are made of?