> Broken Ovenaire OSC 85-50 > J. L. Trantham jltran at worldnet.att.net > Sat Jul 4 02:07:48 UTC 2009
> I did not realize that this post would generate this much > discussion. > While I have a BEE and MSEE, I am a practicing Cardiac > Electrophysiologist and it has been some time since I thought > about such issues. I have learned a lot though from the > discussions and as medicine as we know it winds down, interests in > EE will grow. You probably don't realize how ironic it is that you are a Cardiac Electrophysiologist, and you have fixed a broken heart (in the sense that a crystal oscillator is the heartbeat of any circuit)? > I agree with Warren. It should be fixed before modified. However, > I could not find anything that was 'broken' with the possible > exception of the XTAL. You have done extremely well. No one is criticizing your observations or your solution. They work, and you can move on to greater things. But we are timenuts. We argue and discuss the most trivial-appearing topics, because underneath may lie much greater truths. > As I tried various things, I did notice from time to time some > reluctance to oscillate suggesting that the oscillator stage was > an issue. With no obvious failure and no hope of finding a > replacement XTAL, I had to choose other options. Again, you did good. The subject has left the operating table and is on his feet. > The real problem is size. There is no room for added components, > at least on the Oscillator Board which is inside a sealed oven. > There is only room for changing components. I would have liked to > try adding a resistor and capacitor to the emitter circuit of the > amplifier transistor on the Oscillator Board but there is no room. > I have pictures if anyone is interested. The Oscillator Board is > inside a metal can about 0.5x1.5x2.0 cm that has a heater coil > wrapped around the outside and an Oven Controller Board attached > to the outside. This package is surrounded by foam with the Output > Board on one face of the foam, mounted to one wall of the outside > metal package. The bottom of the metal package has 2 feed thru's, > plus a ground connection. One is 5 VDC and the other is the 10MHz > out. > While with better equipment one might be able to find a problem, > it was certainly not obvious to me. > Do XTAL's fail often? If so, by what mechanism? Slowly fade away? > Sudden death? I am not experienced enough to know this answer but > in all my years with various HAM gear, I do not recall the failure > of a single XTAL. Your question is a good one. Recall that many precision weigh scales deposit material on a crystal to measure the change in frequency. And other systems immerse the crystal in liquid to look for frequency changes. These show the basic piezo response is intrinsic to the material, and is quite robust. I think crystals have to be pretty badly abused before they quit entirely. I remember grinding a FT-247 crystal for 40 meters to move it up slightly and get off the European broadcasts when I was a teenager. My transmitter was a single 6AG7 with 600V on the plate, and about 4 watts into a long line antenna. The chirp from the sidetone was pretty bad, but the crystal seemed happy no matter what abuse I gave it. The biggest problem with crystal oscillators is overdriving the crystal. The acceleration at the surface is on the order of 4 to 5 million G, so it is easy to understand how overdriving can result in fracture. But the crystal is so badly damaged it won't work, or it oscillates at a different frequency. If you look at the oscillator circuits used during the war, you will find circuits using a 300 Watt 813 transmitting tube with a crystal in the grid. This is abuse beyond belief, and a modern crystal would simply fracture on the first key-down. This has happened many times in the past. Recall the BART failure in San Francisco some years ago where a train ran into another one and killed passengers. The complete analysis was never published, but it really looks like the crystal fractured from overdriving and ran at a much higher frequency. This upset the software timing algorithms enough so the train could not stop as programmed. People died from this mistake. I find these problems in my SPICE crystal analysis, at http://pstca.com/spice/xtal/clapp.htm Most of the circuits I analyze overdrive the crystal by a factor of 10 or more. I don't know why they survive. I don't know what could happen to a crystal to change the piezeoelectric response, which is an intrinsic part of the physics of the crystal. There are many other flaws that can happen to a crystal, but these usually occur at manufacture and probably would never reach the field. So it is likely there is some simple electronic failure in question. > However, none of these were in an oven. > Thanks again for all the useful information. > Joe > None the less, with a simple change of resistor, the gain in the > amplifier stage increased and, at least so far, problem solved. So we don't know what happened in your circuit. But you are still to be congratulated at getting it to work under the most difficult of circumstances. Regards, Mike _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.