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HK RABCO ST-7 Tracking Roller Questions

cemerson

cemerson

Active Member
I have two ST-7 Turntables and may have made a mistake. I removed the Tracking Rollers from both of them to realize the rollers are not symmetrical. Upon measuring them I discovered one side (axel) is longer than the other. One axel measures approx. 6.6 mm and the other axel 6.8 mm, so there's 0.2 mm difference between the two sides. This has to be for a reason, I think. Does anyone know which side should go in which direction? I'd like to only put this thing back together once.

Also one had the "tire" still on it and the other the tire was melted away. The tire that was there was rock hard, so it broke when I removed it. It was shot anyway, but you could tell it had originally been flat, not rounded like an O-Ring. I had originally tried the #31 O-Rings, and of course they were really tight, a bear to get on. I looked for another O-Ring at the hardware store and found a #48 was snug but not too difficult to get on. I did a couple measurements and found the O-Ring was about 16 mm in diameter but the flat tire had been 17.25 mm. So my second question would be, does this make a difference? Does anyone have a measurement of the diameter of one of the solid metal rollers?

Anyone that can offer up some advice or help, it would be greatly appreciated.

Thanks, Chris
 
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cemerson said:
I have two ST-7 Turntables and may have made a mistake. I removed the Tracking Rollers from both of them to realize the rollers are not symmetrical. Upon measuring them I discovered one side (axel) is longer than the other. One axel measures approx. 6.6 mm and the other axel 6.8 mm, so there's 0.2 mm difference between the two sides. This has to be for a reason, I think. Does anyone know which side should go in which direction? I'd like to only put this thing back together once.
Click to expand...
Previous discussion: https://audiokarma.org/forums/index.php?threads/harman-kardon-st-7-arm-roller.919740/

IMHO, it doesn't really matter, but logic seems to indicate inward, toward the spindle. I've R&R'd mine 3 times and never paid attention to which way it went, never failed to track. So either I got very lucky 3 times in a row, or it doesn't matter.

Also one had the "tire" still on it and the other the tire was melted away. The tire that was there was rock hard, so it broke when I removed it. It was shot anyway, but you could tell it had originally been flat, not rounded like an O-Ring. I had originally tried the #31 O-Rings, and of course they were really tight, a bear to get on. I looked for another O-Ring at the hardware store and found a #48 was snug but not too difficult to get on. I did a couple measurements and found the O-Ring was about 16 mm in diameter but the flat tire had been 17.25 mm. So my second question would be, does this make a difference? Does anyone have a measurement of the diameter of one of the solid metal rollers?
Click to expand...
Also previous, use a Square-ring. Provides the same contact patch as original tire:
http://audiokarma.org/forums/index.php?threads/h-k-rabco-st-7-tracking-roller-revisited.834571/
 
If you have the dimensions of the original roller you can send the part to Terry's Rubber Rollers and he will rebuild them. I wouldn't use an O Ring.
 
I believe the original was an O ring, but my experience is limited to one ST 7 rescued from a dumpster (and a couple of ST 4s from tag sales). I used an O ring from the hardware store, and got the thing working fine, but everytime it was moved, it needed tuning to get everything level. More frustration than I wanted.
 
Iirc, I ran into the same question a while back, and failed to find reliable pix showing the factory installed orientation. I think I determined the shorter end of the roller axle went inward/left as viewed from front, but can't definitely say oem.
I've owned two ST-7, replaced the roller tire on the second.
 
Interesting - mine was definitely a metal wheel with a groove in it that had an O ring. Wonder if mine was a variant, or a modification.
 
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