jaymen

Hello to all.
I wanted to report that after some exhaustive troubleshooting, and testing I found out something we all need to be aware of, and that is frequency drift of old crystals. A particular Controlaire was off frequency by about 2 KHz, and replacing the crystal did not correct the problem, or so I wrongly assumed. After trying several different crystals of the same fequency (yellow) I tried an orange and found that the frequency was within 200 Hz, almost perfect! This then led me to testing all of my crystals, and guess what? 95% of the older ones were off by 1.5 to 2.5 KHz, which is unacceptable and problematic. Thus, I had to toss my sizable and coveted collection of 27 MHz band Tx and Rx crystals; only the new ones (under 15 years old) were any good. The thing is, I had checked them all in the past, and they were good then, but that was years ago. I also had to toss some nice NOS wire lead Xtals I just got off Fleabay as well.

I can say with certainty that if you have a 1950s to 1980s radio, there is a very good chance the crystals have drifted out of tolerance and need replacement. The funny thing is that they drift up in frequency, so most times the radio will tune right up to the the transmitter, you just will not get the full range and performance from the system. The other thing is that on pulse, and digital systems, you cannot check frequency of the Tx without turning the modulation off somehow, which is not usually practical for the owner to do, so the easiest thing is to remove the crystal and test it on a Xtal checker.

All Day Dan

My Feedback: (5)

jayman, Did you do any destructive physical analysis, in other words, take some apart to see what has caused the drift? If the transmitter crystals are drifting, shouldn't the receiver crystals be drifting too? Maybe the IF will stay the same if you match them up. Dan.

fizzwater2

My Feedback: (61)

were they drifting downward in frequency?

jaymen

They drift up in frequency, both the Tx and Rx crystals. I confirmed with some of my fellow HAM radio operators about this and they said it is quite common for crystals to drift up in frequency, that is normal. Aging is the root cause, not usage, or damage. The quality of the plateed on electrodes on the crystal, and it's internal stresses contribute to this. Funny thin is, even new old stock one drift too, so it's not due to usage, they go bad on the shelf.

I did take the cover off of one, and there was no apperent defects that you could see visually.

In the past, I have been lucky enough that the Tx and Rx crystals have sometimes both drifted by about the same amount, so they will work together, but many times you change just just one crystal, like when making up a spare receiver for a second airborne. This is where you can get into trouble because if the Tx Xtal is off the new receiver Xtal will be on frequency and cause tuning problems. If you change the Tx Xtal, then you need to change the old Xtal in the other receivers as well that were used with that Tx, and re-tune.

From now on, I'm changing them out as a set to avoid any problems, and advising people to put new Xtals in any other receivers they have that they use with that particular transmitter.

Typically, if a crystal is good for 10PPM drift at it's fundemental frequency, it should only be out by less than 300 Hz at 27MHz.

fizzwater2

My Feedback: (61)

I visited a crystal manufacturer once, walked through the whole process of making crystals with them.

If a crystal drifts down in frequency, I was told that it's because minor impurities inside the can or housing for the crystal eventually settle on the crystal blank itself, dragging the frequency down.

Upward usually meant the crystal was losing mass, typically by overdriving a crystal so that plating atoms were dislodged from the surface of the blank.

We were talking about crystals in TXCO's, etc, which were quite a bit more stable than the typical RC transmitter / receiver crystals. Some were even in glass packages because the environment around the blank was more inert, reduced drift of the crystal.

It was an interesting trip.

jaymen

Well it is not surprising given what you said about overdriving. Typically, vintage transitorized circuits were made with the bare minimum of components to keep the cost down, and this meant that instead of adding an extra transistor stage to buffer and amplify the oscillator signal, instead the oscilliator stage was biased as hot as it could, usually overdriving the crystal, to get the required signal.

I once worked for a company named Efratom, where we made precision 10 MHz frequency standards, and we cut, ground, plated and encapsulated crystals, some in glass cases. These processes were done in a clean room, and vacuum chambers. The cutting, x-ray, grinding and polishing was done in a seperate building, to isolate the detritus created from contaminating the clean room. Typically, to obtain stability, crystals are run in a heated and temperature controlled oven to reduce drift.

We still have Cal Crystal labs here, and they have made custom 10 meter crystals for me for my HAM rigs, but nowdays, the cost is about $50.00 for a custom crystal.

fizzwater2

My Feedback: (61)

It's been a long time ago, but I think the place I visited was "Colorado Crystal" - they're still around.

Part of the process is pretty messy, for sure - slicing and grinding the crystals, etc. They were the same way - separate clean room facilities for assembly, etc.

It is an interesting thing to see - take a chunk of rock. slice it, polish it, plate it, and make a crystal!

Dave Harmon

Jay......
Cal Crystals are still around???
Yikes!

I used to buy from them back in the 60's when they were in Torrance.....their stuff wasn't very good then so I started using International Xtal in OK City.
IC charges $22 for typical ham xtals as of a month ago. They have the specs of most of the OEM and can make an accurate cut that works.
For xtals that are a bit high I have been pulling them with a few pf across it...works good.
I haven't made any home brew R/C stuff for a long time....Burlisle says I need to get ahold of Ed and let him see my stuff.
Some of it is so rare than no one has seen it but ME!
HAW....
Best

grotto2

My Feedback: (2)

Jay-
Have you found the crystal package to matter?
My OS transmitter (small crystal) is running 2.4kHz high, but my F&M transmitter (big crystal) is 600 Hz low.
Either way, somebody's gonna be replaced.
-Ron

aerowoof

does this apply to crystals on all bands or just the 27 mhz band?