In the last article I made mention of using a small button as an electrode rather than a large metal plate. This was a pretty common technique for getting quartz crystals to oscillate at higher frequencies with flat (and easier to manufacture) electrodes. We’re going to look at the button version of the ICA 415 holder to illustrate how it works.
ICA holders all look very similar and disassembly starts the same way: two thumbnuts hold the dust cover. Once this is apart you may discover either a flat-plate holder like the BC2, or a button holder like this one. If you have one with a screw in the middle (where the rivet is in the picture) its a variable pressure flat-plate holder. Both of these use contact between the top electrode and the dust cover as one of the electrical conductors. The button holder you see here is the “high frequency” (40-meter) version, but there’s really no reason it wouldn’t work with lower-frequency crystals.
In the button holder, the electrical path is bolted together–there are no hidden wires like in the Bliley holders. The picture below shows how the top electrode contacts only part of the surface of the crystal, while the bottom contacts most, or in this case, all of it. Inside is an X-cut crystal.
When you disassemble this, be very careful as the pressure on the crystal is light and slips easily. Be careful wen loosening the base pin on the circumference, as this will loosen the spring. It’s very easy to accidentally dump everything on the floor. Do all of this over something soft!
After handling a few of these, my general sense is that the brass electrodes in these holder pit and corrode more more than the steel in most of the Bliley holders. Polishing will almost certainly be necessary to restore operation. Fortunately the bottom electrode is easily removed when the center base pin is unscrewed. The cleaning tips in my previous article apply here, so I won’t repeat them.
Since all of the electrical contacts are through bolts and nuts, you want to make sure everything is clean when putting it back together. A bath in a contact-cleaning solution might be appropriate. This also provides opportunity to clean up the ceramic as well, which is probably pretty dirty after 90 years. Another troublesome contact point is where the point of the spring meats the top electrode–dirt can get caught in there–gentle brushing or picking may help.
The crystal in this one measured about 7088 KC. Given that it is marked on the tape at 7081, my guess is that this was finished by the owner and was not etched. Over time this thing has spalled material and raised frequency, and also probably gummed up the works with quartz dust. Cleaning the crystal and reassembling showed good oscillation, and since 7088 is pretty useless for CW today, I chose to etch up to around 7115 where it s useful for CW work in the old-radio events. While I despise chemicals used in etching, grinding an X-cut at 40m is risky business, in my opinion. Etching in a weak solution is more controllable and also results in a more stable crystal in the long term.
You may find that you can affect operation by moving the crystals around and contacting it off center. This is okay, and probably part of being a 1930’s crystal-controlled operator. Adjust it or best performance!
It’s common to find broken crystals in these holders. All it takes is one drop and the sheer inertia of the contents will break a thin 40m X-cut crystal. (This is why FT-243’s for wartime use in this frequency range are thicker BT and not AT cuts, despite worse temperature stability.) In the case yours has been dropped and the crystal shattered, a replacement AT cut can be salvaged from a surplus WWII crystal. A 3/4″ square blank from a DC-34 or DC-35 can be found for 80 meters. A large crystal blank like this is harder to find in WWII surplus for 40 meters, but you might find some HC-1 holders with large crystals in the 6-7 MHz range that you can scrounge. Several HC-1 crystals were re-manufactured for the ham bands after the war, and since they have less-desirable 3/4″ spacing they often can be found cheaper than the FT-243 size holders. I have been able to move crystals from low 6 MHz to 40 meters, but its a lot of work! Time is probably better spent patiently looking around for a 40m HC-1 holder and hoping it has a large blank in it that you can use!
The “button” holder was used in the 40m version of Bliley BC3, a very common holder, as well as the HF2 and HF3 (overtone AT-cut crystals for 14 and 28 MHz). In these, the electrodes are reversed–the top cover serves as the large electrode and the button is attached to a spring inside that presses up against the crystal. These are plagued by the same contact problems as other pre-war crystals and have the same remedies. Servicing these requires a little more dexterity as they can fly apart from the spring-loading of the contacts and even crack crystals if things aren’t reassembled carefully, so perhaps I can address the specifics in a future article.