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Reprinted with permission from the Ottawa Vintage Radio Club newsletter Winter 1994 edition.

REPAIRING EARLY CRYSTAL PHONO CARTRIDGES by Lea Barker

Radio-phono combinations from the 1930's and 1940's, after the era of the large magnetic pickups, used Rochelle salt crystals as their active "biomorph" element. Since rochelle salt is very susceptible to heat and moisture, the life of the cartridge was limited. Rather than modify the tone arm to contain a later model cartridge, the cartridge can usually be disassembled and a new barium titanate element installed, thus preserving the authenticity of the unit. The accompanying diagram shows one such repair, on a 1932 Astatic cartridge. Most cartridges were made in two halves held together with rivets, or, in a few cases, small nuts and bolts.

REPAIR PROCEDURE

Firstly, drill out the rivets holding the two halves of the pick-up together. Separating the halves, it will be sometimes found that the cartridge was filled with grease to waterproof the Rochelle salt crystal- this must be removed along with the defective crystal. The needle armature is held to the case by rubber isolation grommets; these will be found to have rotted. The old crystal will be mounted (usually) between two rubber blocks, which are also removed. It is now necessary to procure a cheap replacement cartridge, from which the barium titanate element will be removed. When one finds some of these at a good price (about $2 is good), several should be bought as they are now becoming scarce. This need not be a monaural unit, as connecting the two halves od a stereo unit together converts it to monaural. Carefully remove the element, taking particular care to preserve the tin foil leads of the crystal itself. This element is now affixed to the rear of the old cartridge (using epoxy), making sure that it clears the end of the needle armature. Orientation is important: the biomorph element is designed to produce a piezoelectric charge when bent or twisted across its flat dimension, so ensure that this is the case when affixing the crystal in the holder. Put new isolation grommets on the armature using silicone rubber spread around the shaft. When the epoxy and silicone rubber have cured, the armature may be inserted into the half of the cartridge containing the new element. Next, a compliant connection is needed from the armature to the crystal. This avoids damage to the crystal, particularly when inserting a new needle, and improves the response. If the new crystal has a small rubber end-piece where the needle attached, then this may be used, gluing (again with silicone rubber) a short plastic rod between the armature and the end piece. Otherwise, a short piece of rubber carved from an eraser will do. The foil may be attached to the connector pins using conductive repair paste used for automobile rear window defoggers. If the crystal has wire leads, simply solder them using good heat sinking. Let all the rubber cure then reattach the top of the cartridge. Et voila- a phonograph cartridge indistinguishable from the original, but with better performance and long life!

Addendum, Winter 1997

Returning to the matter of repairing old phono cartridges, I have just discovered a new material for the active element, namely the piezoelectric sound elements that are found in most computers and games. At the Active Surplus store in Toronto, and other such places, one can obtain these in 1.5 inch diameter brass disks with a thin piezoelectric element of about 1-inch diameter affixed to it, and silvered on the top side. The good thing about these is that by using the abrasive cutting wheel on your Dremel tool, you can cut a piece to almost the same shape as the original Rochelle-salt crystal. Having cut such a piece, use a magnifying glass to inspect the cut edges and use very fine emery paper to remove all burrs so the element isn't short-circuited. It can then be anchored at the non-active end between sheets of rubber cut from rubber bands, and the other end is attached to the forked end of the needle armature, usually with a thin sheet of rubber on each side. The brass is one terminal, and the silvered top of the crystal is the other. Thin wires can be used to connect these to the phono cartridge terminals. These elements are not so critical as to orientation (ie, whether they twist or bend) but one does pay a penalty in a slightly lower output. Usually most radio-phonos have plenty of amplification so this is not a problem, except maybe for the cheapest units which feed the pickup directly to the output tube. The versatility of this material makes it a good trade-off, in my view. (Lea Barker)