Vinyl Old timers: HK's and Rabco's

Sean, One of the mods I did on the early flat tone arm Rabco's was to reduce flex in the giant head shell (arm).

Originally this design was way too flexible, having been formed from wafer thin aluminum sheet with the edges bent over to increase strength. Certainly it was lightweight, but provided less than a perfect platform to secure the cartridge.

Add to this, the counterbalance was large and heavy in order to accommodate (heavy) cartridges of that era. I had a machine shop mill the counterbalance where it's weight balanced against my reference cartridge when fitted very close to the bearing point. (Reducing the moment of inertia).

I substituted other materials for the flat aluminum sheet design with balsa wood, Oak, steel and Plexiglas. Each offering trade off performance over the stock material.

Today I would consider Carbon fiber or other modern lightweight, but stiff materials to substitute for the aluminum.

I never heard the tubular version of the Rabco, but would assume it would be superior in rigidity to the flat design.

Sean,

You should follow this recent thread at AA for info on Rabcos, esp. Dave Shreve: http://www.audioasylum.com/audio/vinyl/messages/129076.html

Shreve is well-known for his modified Rabco SL-8E and his pioneering work on optimizing VTA/SRA.

Rabco marketed the expensive (active servo) SL-8E and cheaper turntable/arm ST-4 combo under their own name, and contributed the rotating tube/tire-rider arm design to the H/K turntables. In the H/K ST7 and ST8, the tubular arm is terminated at the rear with a tire that rides on the rear rotating tube. The arm moves laterally, "passively", to maintain even tire contact with the rotating tube. When the tire wears down or when the tube rotation drive becomes uneven, tangential tracking is compromised as angular errors become larger before the rear of the arm moves in reaction to stylus drag motion at the front.

THANK YOU for all of your help. I appreciate the quick responses and tips / tricks / links for further info.

As to Steve's description of how the tubular arm is driven, i'm sure that you did a bang-up job of describing it. However, i'm having a hard time understanding exactly what you mean. I'm sure that it would all fall into place once i could see a diagram or actual unit that was "gutted".

I will check the thread on AA and see if it answers my questions. Just in case though, i was curious as to whether or not the tubular Rabco arm could "easily" be installed onto another TT ??? Judging by Steve's description, it doesn't really sound like it. Sean
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Sean,

There is a better description of the mechanism in Jason Gold's review of the ST-7 at Audio Review (of all places!):

http://www.audioreview.com/TurnTables/Harman,Kardon,RABCO,ST-7/PRD_121324_1597crx.aspx

Because the rotating tracking arm is coupled to the rotation at the platter spindle, I don't think the HK versions of the Rabco arm can be fitted readily to other turntables.

I thought the HK was a pretty good-sounding turntable, but I don't know how it would compare to today's "meat and potatoes", like Regas.

1. It would be very difficult, but not impossible, for an adept person to remove an arm from an ST-5,7 or 8 to install on another turntable since, on these, the main turntable motor drives the rotating armtube via an additional pulley on the main shaft of the platter.

2. The rubber drive wheel that engages the rotating arm tube can deteriorate or wear but it is replaceable by an "O"-ring used for plumbing. A ring with 3/8" i.d. and 19/32" o.d. is available and a perfect fit. I get them from Home Depot at 5 for 79cents.