This is the first in a series of articles on pre-WWII quartz crystal restoration. This article walks through the restoration of an early 30’s crystal holder that is fairly common on the surplus market, pointing out some of the characteristics and challenges that are unique to 1930’s crystal technology.
Lately there seems to be a bit of a renaissance in using crystal controlled transmitters. I think vintage-radio operating events like the Linc Cundall, Classic Exchange, and Novice Rig Roundup have have much to do with this. By and large when people acquire “vintage crystals”, they are referring to WWII surplus–either original or re-manufactured after the war by outfits like Texas Crystals, Petersen, JAN, Hundley, and a host of other smaller resellers/re-manufacturers. Sometimes one comes across an older “doorknob” type crystal like the Bliley BC2 pictured above. We’ll explore what these are, how they work, and the issues involved in to restoring them to operation.
He’s dead, Jim.
The war taught us a lot about how to mass produce things and control quality; before the war technology was cruder and less dependable. In an age of coal dust and transmitter designs that over-stressed a lot of quartz, the vast majority of early crystals will not oscillate in as-found condition. This very often true even of pre-war crystals that are new old stock! So what gives?
Let’s dig into our BC2 to see what the issues are and how we can resurrect an awesome piece of radio history….
BC2’s have a finely threaded screw cover. Inside is a 1-in square “sandwich” that is lightly pressed together with a piece of spring brass. In some ways this looks like a large version of the spring-pressure FT-243, but we’ll see there are some important differences.
The first thing to know is that Bliley originally sold the BC2 as just a holder–the actual quartz crystal was sold separately. It was not unusual in the first part of the 1930’s for the owner to grind his own quartz, and many outfits would sell you the rough blank for you to finish. For more money, you could buy crystal blanks pre-ground to a percentage of specified frequency. (This is one reason why pre-war crystals are often several KC off of their markings.) Also, because these were entirely user serviceable, crystal blanks were often swapped out from time to time of its life. There is no guarantee that the crystal in the holder is anywhere near the marked frequency, if there even is one. This adds a little mystery and excitement to the whole enterprise.
Once we have our holder taken apart we can begin restoration to operation. This involves cleaning the electrodes, identifying and cleaning electrical contacts, cleaning the quartz itself, reassembly, and finally testing and adjustment. Yes, in the 1930’s periodic service of your crystal was part of station maintenance!
This particular holder had no crystal in it, though it had evidence of use: a piece of old masking tape across the front that I tried to clean off, and signs of brush discharge or arcing inside, which probably destroyed the last occupant!
One thing to notice is that the plates are flat, and that the side that contacts the crystal is more polished. This is typical of early 1930’s crystal electrodes. This made them very sensitive to pressure, and in fact several variations on this idea were used to make an adjustable-frequency crystal. To much pressure will impede the motion of the crystal–something to keep in mind if yours refuse to oscillate!
For comparison, here is an air-gap plate from WWII:
The air gap holder made contact with the crystal at the corners, where you can see the electrode is slightly raised. This allowed for a more more reliable operation but was harder to manufacture. This kind of pressure-air-gap electrode is typical of all WWII AT- (<5 MHz)and BT-cut (> 5 MHz) crystals, until plating the electrode directly to the quartz became the norm after the war. It can also sometimes be found in later pre-war crystals like the Bliley B5 (BT cut) and the occasional LD2 (AT cut).
I cleaned my BC2’s electrodes by grinding them against some 800 grit on a flat surface, not unlike how you might grind a crystal blank. Ithen pollished off with 1200 grit. This ensures that the surface stays flat while it removes any pitting and oxidization on the surface. The bottom plat is harder to clean since it is not easily removed from the holder. I did what I could here. If you really go to town, make sure you indicate which surface contacts the crystal so you can put things back together in the right order.
After this is done, you should wipe the electrodes clean to remove all dirt and grease. I’ve found that pure isopropyl alcohol is good for this. The top plate should probably be immersed in a bath for a while and wiped clean with a lint free towel…or a clean paper towel as long as you are careful to make sure no fibers are left on the surfaces. The bottom electrode, assuming you haven’t removed it from the holder, is best wiped with a Q-tip or similar item. You might want to blow the thing out just to be sure you didn’t leave anything in the crevices. You want to hold the electrode by the etches so that you don’t get skin oil on the surface. A good way to spot dirt is to look at the reflection of a bright light off the surface.
Finding and cleaning contact points
On thing to keep in mind about all pre-war holders is that they largely rely on spring pressure and screws rather than solder or welds for electrical connections. Since these were designed to be user-serviced this makes sense, but over the years these have become unreliable due to oxidization and dirt. This is probably the #1 problem why these things don’t oscillate, even when found as NOS.
Above is a typical example of this problem. It shows a wire lead that connects the top electrode to the nearest pin underneath. There is also a paper or fiber insulator that keeps it from shorting to the bottom plate on the way up. Thus the connection to the top of the crystal consists of the point contact between the top plate and the spring brass, as well as the screw pressure of the spring brass to the wire you see. This particular wire has some corrosion on it that will need to be cleaned off. Once this is done the we should check the continuity of the wire to the base pin. If this is bad one should try reflowing solder into the pin before attempting further disassembly. I have not had a problem with this connection, but given that old tubes sometimes go open this way, it’s worth mentioning.
You should also check the continuity between the bottom electrode and the base pin. It’s best to test on the side of the electrode if you an so that you don’t scratch it with your test leads!
Finally, it’s probably a good idea to clean the spring as its point much make contact with the top electrode and its washer end with the wire.
Cleaning the crystal
The crystal should first be inspected for damage. A large number of crystals from this era–and almost all home-ground ones–will have identifiable surface defects, such as light scratches or small edge-chips. This isn’t necessarily a problem, and I would encourage you to try to salvage any pre-war crystal, particularly the old X- and Y-cut crystals all crystals have defects. A crack is going to destroy the crystal, although if the two pieces can be separated you may find that they will oscillate independently, despite their irregular shape. If the crystal is not usable (or completely missing, as in my case) a replacement can be found in WWII surplus. This won’t be an old X- or Y-cut, but will probably be a better oscillator. AT- and BT-cuts became available around 1935, and hams who could afford the premium may have done something like this anyway.
Before I clean I like to measure the thickness of the crystal. Since pre-war crystals like these are almost always in a ham band, an average a few caliper measurements can usually reveal both the frequency and the cut to within 100 KC or so, limited by the accuracy of my caliper. A crystal with edges that look “tumbled” and are not quite square is almost certainly an early home-ground X or Y cut.
Cleaning the crystal is done the same way as with the electrodes. I immerse the thing in a bath of clean alcohol, and wipe with a clean paper towel, always holding the blank by the edges. I like to wipe from the center out, rotating the crystal each time. Then I inspect in the light that all is well. On a frosty surface, oil will show up as as dark or clear patch that doesn’t evaporate when looking through the crystal. Dust particles show up when looking at a reflection from a light off the surface. Either indicates the need for cleaning again.
At this point we can begin reassembly. Start by reassembling the “sandwich”. The crystal goes on the bottom electrode in the holder and top electrode above it. Be sure that the polished side of the top electrode is in contact with the crystal.
Be sure that you use a tester that is proper design for the frequency of interest. A BC2 or other holder with flat plates like this is usually a 1.75-2.0 MHz or 3.5-4.0 MHz crystal. Many modern testers are downright awful at this frequency and made worse by the fact that we’re using poorer quality crystals to start with. If in doubt, I would use a real radio or at least a common tuned-plate oscillator like one would have used in 1935.
If you have a test oscillator with some wire leads, you can check for oscillation now without having to screw together the pressure contact. Connect one side to the base pin and the other side in contact with the top electrode. You may find that oscillation is dependent on the amount of pressure involved. If you get oscillation at all you are in good shape. If it is stubborn, there are some tricks in order of simple to drastic:
- Check for a short or open again. There might be an intermittent contact in the base pin.
- Rotate the top electrode
- Flip the crystal blank over, then go back to #2
- Clean everything again. Go to #1
- Try a different top electrode. A small brass button about the size of a dime can be made from brass strip. Square or round is fine so long as it is flat (you can grind it on glass to be sure). You you might want to put a dimple in it to seat the spring contact later. This trick was standard for 40M crystals. I have found 40m X-cut crystals in flat-plate holders like these, and they have generally been poor oscillators.
- Substitute 1″ air gap electrodes from surplus WWII holders. They can sometimes be found in HC-5 holders and very occasionally in the first FT-171B holders, all in the 2-4 MHz ranges. Large crystals like this are in the minority, however, as quartz became a scarce commodity during the war.
Please, PLEASE don’t gut a perfectly good holder and solder a modern crystal in there. Every time you do this a little fairy falls down dead and clapping your hands will not bring it back.My apologies to J.M. Barrie
If you are convinced its the crystal, you can try grinding or etching to restore operation. That’s beyond the scope of this discussion, but the larger crystals from DC34/35 and FT-171B holders can work well (about 3/4″x 3/4″). Some of the 2-3 MHz HC-5 (three-pin) holders from the beginning of the war have 1″ square AT cuts in them, but you will most likely need to grind them up several hundred KC (that’s where I got the one shown here). If you are crazy enough to try grinding an X-cut crystal, you should consult the handbooks of the early 30’s. I’ve done this, but it is decidedly more challenging and labor-intensive than dealing with WWII stuff. I suspect a smaller FT-243 size crystal will work (about 1/2″x 1/2″), but you might need a small spacer to keep the electrodes from rocking if the crystal slides . All that said, you may find that the original crystal will work fine in another holder, so I usually hang on to them.
After all this, it’s time to screw on the spring contact. You’ll want to retest, and adjust the pressure so that it is reliable and keys well (if you key the oscillator–many people chose to key a buffer stage in the 1930’s, which helped poorer oscillators). Once secured and oscillating in your transmitter, you can screw on the dust cover. These were not designed mechanically for a tank radio, so treat them carefully. You now know how to adjust them if they get bumped out of alignment.
Good luck! If this helps, I’d love to hear about your experience.