Rebuilding Crystal Phonograph Cartridges
Sean R. Barton
Oftentimes the crystal cartridge in many 1930s and 1940s phonographs will be found to be "dead." The active element in these cartridges is a Rochelle Salt crystal, which does not hold up well to heat or humidity and over the years becomes inactive. Perhaps this is one reason these phonographs do not command very high prices. There are a few places that will rebuild the cartridges; however, that service is usually somewhat expensive. In searching for information on rebuilding such a cartridge for a Spartan radio/phono I came across a webpage that gives a couple of ideas for doing such. One of the ideas was based around using a cheap and readily available piezo element to replace the original crystal. In experimenting I came up with a method for rebuilding a cartridge which I will detail here.
In order to rebuild a cartridge you will need the following items: an electric drill, soldering iron, tin snips, silicone bathroom sealant, toothpicks, spring type clothespin, piezo element (Radio Shack 273-073 or equivalent), sheet rubber (a piece of an old inner tube would work), #2 machine screws and nuts or rivets of comparable size.
The first step is to drill out the rivets that hold the two halves of the cartridge together. Pry the two halves apart to gain access to the inside. There you will find something similar to what is shown in Figure 1. The Rochelle Salt crystal is the white rectangular part in the middle of the cartridge. This needs to be removed. Usually the crystal is falling apart anyway and removal is not difficult. The fork and armature will also need to be removed in order to replace the rubber mounts between it and the case.
If the piezo element recommended in the parts list is purchased, the element will need to be removed from the casing. This can be done by prying off the back cover and carefully breaking the plastic case of the speaker to relieve the element that is super-glued into the plastic case. To prepare the replacement element, cut a 3/16" x 5/16" piece of piezo with the tin snips. The size for your cartridge may vary but I have found these dimensions to work quite well. Make sure there are no burrs on the edges of the element that would short out the two contacts. With a low-wattage soldering iron, solder a new lead to the brass side and one to the silvered side. The wires that were originally soldered to the piezo will work for this. Try to make the connections as close to the mounting end as possible. Cut some small rubber mounts for the piezo element and glue them to the inside of each half of the case as shown in Figure 2. These mounts will support the new element inside the cartridge. You will also need a piece of rubber to go inside the fork as an insulator to keep the fork from shorting the two sides of the element together. Slip the element and rubber insulator inside the fork and position the element so it will be sandwiched between the rubber mounts when the case is put back together. Before closing up the case, use a toothpick to put a dab of silicone sealant in the troughs on both sides of the case where the armature will be resting. Use enough to fill up the troughs as the excess will be squeezed out when the case is put back together.
With everything positioned, put the two halves of the case back together and hold them with a clothespin. Make sure the armature is centered in the trough and allow the sealant to cure. After the sealant has cured, you can pull the case apart for a final inspection and solder the leads from the element to the connector on the back. The rubber mounts originally used to secure the crystal can be left to hold the leads in place. If everything looks right you can put the case back together and secure it with screws or rivets.
If done carefully and correctly you will have a rebuilt cartridge that for only a few dollars will perform as well and last longer than the original element. For more information, this is the page that gave me the original idea: http://home.earthlink.net/~mrkevo4/radiocity/pages/cartrepair.html
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This site created and maintained by Sean Barton.
Last updated 9/13/2004.