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Old 17th Apr 2013, 11:54 pm   #109
GP49000
Hexode
 
Join Date: Mar 2012
Location: Sonoma County, California, USA.
Posts: 405
Default Re: Garrard record player deck identification.

An improved tonearm bearing for the Unimechs:

Mentioned in prior installments of this series with regard to the introduction of the Unimech series record changers and single players has been their inability to operate at tracking weights demanded by the better hi-fi cartridges of the day. This was due to friction in the tonearm's horizontal movement. Garrard knew this; a minimum tracking weight of 2.5 grams was specified in the service manual for the entire Unimech series when it first was introduced.

This was acceptable for the original intended market for the Unimechs, which were mostly to be sold with ceramic cartridges tracking at 4-5 grams. But expanding the line into hi-fi models proved irresistible, especially since the Unimechs cost less to manufacture than the Autoslim chassis while hopefully being sold at the same retail prices, or even a bit higher due to their contemporary styling.

In fact, the USA first saw the Unimech in a hi-fi version equipped with magnetic cartridges from Pickering and Shure, with broad-radius elliptical styli that could be operated at up to four grams without damaging records. The model number "42" placed it just slightly above the longstanding entry level Autoslim model 40B. Tracking weight was preset at the factory to 3 to 3.5 grams; without a built-in tracking weight gauge, they had no means by which users would know that these models could not track as low as the two grams recommended for the 40B.

But as the Unimech line expanded upward, something had to be done. The Model 70 had a built-in calibration for its user-adjustable tracking weight adjustment, and it was placed in a considerably higher price echelon; indeed, promotional literature claimed it was "in performance, features and styling, every bit the equal of last year's SL72B." Never mind that this was flatly untrue; if it couldn't track below two grams (the SL72B was even better than that), the customer would know it.

The Unimech vertical tonearm shaft rotates in a machined bore in the pickup base assembly casting. A flange in the shaft indexes to one in the bore, to prevent the tonearm from being lifted out of the unit. On the bottom, thrust of the vertical shaft is taken by a ball-bearing, preloaded by a compression spring that sits in a plastic tnrust bearing, built like a "socket" that seats into the Unimech subchassis plate. It is all easy to assemble, with no finicky adjustments necessary, or possible. However, with the spring preload, the flanges in the shaft and in the pickup base assembly rub on each other. This is the cause of most of the friction in the horizontal movement of the Unimech tonearm.

Garrard's answer to this was an adjustable thrust bearing. The plain plastic socket was replaced by an open-ended, threaded plastic fitting. The ball bearing and the compression spring were eliminated; a screw with a polished ball contour on its end, its screw thread fitting into the threads on the plastic socket, was substituted. The screw was to be adjusted by the assembler to provide the barest amount of free axial play in the tonearm shaft, so that the flanges in the shaft and in the pickup base assembly would not rub when in use. Thus, a low-friction bearing assembly resulted, which Garrard specified at 1.5 grams.

The new design is low-friction only if properly set up. I acquired a Garrard 775 because its prior owner got fed up with its not being able to handle cartridges tracking under 3.5 grams; with a Stanton 500 rated at 2-5 grams, slight record eccentricity would cause the stylus cantilever to flex in its suspension, because the arm wasn't following the eccentricity. I traded an SP25 Mk III to him; it tracked the Stanton 500 at its lowest-specified tracking weight, and he was happy. I verified the excessive horizontal bearing friction on the 775; it was so stiff that the antiskate, at its maximum setting for five grams tracking weight, was not able to move the arm past the friction (this puts the approximate lateral friction at about a half gram, as seen at the stylus)! Digging into the 775, I found that the screw in the tonearm thrust bearing had been adjusted too tight...and then was secured with lock paint. Removing the lock paint and readjusting the screw fixed it.

Because of the declining sales volume of record changers by the late 1970s, I have not seen enough Unimech samples to know precisely when this change was incorporated, and in exactly which models. It is easy enough to tell by looking at the underside of the record player, which type thrust bearing is fitted. The older, spring-loaded thrust is made of white plastic, with a closed end. The newer, adjustable, lower-friction design is moulded of black plastic and has a hex-headed screw threaded into it. Specific samples I have seen of the 6-200 and 6-300 models, the 440 and 62, and the 630S and 620S models have the nonadjustable, spring-loaded type; and of the 770 and 775, the adjustable, low-friction type.


Photos:

Exploded diagram of the Unimech pickup base and subchassis.

Schematic of the 6-300 pickup arm bearing, original spring-loaded type.

Schematic of the 775 pickup arm bearing, adjustable type

Photo of 6-200 underside, note WHITE, closed-end thrust bearing.

Photo of 775 underside, note BLACK thrust bearing with adjustable screw.
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