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In article , Uwe
writes: 12V, holy cow! If I go low voltage I keep it at around 5. I am building a controller board with basic stamps and recently fried an A/D chip by letting it have 12V, it was lethal. Heh heh heh...you low-voltage kids are all alike. Us vacuum veterans would not think twice about the +120 VDC (the "B+") in the old "All-American Five" cheap table model AM BC radios.... :-) By the way, talking about overload. The issue of crystals breaking has been mentioned a few times, but how do I measure what the crystal "sees" for current and what is tooo much??? Seriously, folks, the websites for International Crystal, Corning Frequency Control division, etc., all give specs on various sizes and cuts of their quartz crystal units. Few really "measure" the crystal drive levels since that can be done analytically...if one knows how to do this. If not, there are several hints on the various crystal unit websites for approximating that, such as typical circuits. The best approach at the beginning is to take advice from others on what works and what doesn't...such as the old, old FT-243 holder crystals can take more power dissipation (thicker slice of quartz) than most of the smaller HC-6 holder units. The newer SMD quartz crystal units have very low power specs and should not be used with most vacuum tube circuits because of that. Having been in the electron-pushing racket for better than a half century and bridging the tube and transistor eras, I've never experienced any quartz crystal physically "breaking." If a quartz crystal circuit stopped working, the quartz unit just sat there without a sound, same as it did when it worked OK. :-) Only the oscilloscope trace knew what was in the hearts of such circuits... Len Anderson retired (from regular hours) electronic engineer person |
By the way, talking about overload. The issue of crystals breaking has been mentioned a few times, but how do I measure what the crystal "sees" for current and what is tooo much??? It's not easy to measure crystal current, since it's RF current at the crystals frequency of oscillation. Most AC milliameters will stop working at about 10Khz, and the lead length and impedance will probably throw off your oscillator. One "trick" we used in the old days, which also sometimes saved the crystal from destruction, was to put a low current pilot lamp in series with the crystal. Seems to me it was a #49, but you'd best look it up. My recollection was that this was a 60ma bulb. In normal operation it should not glow visibly. Doug Moore KB9TMY |
By the way, talking about overload. The issue of crystals breaking has been mentioned a few times, but how do I measure what the crystal "sees" for current and what is tooo much??? It's not easy to measure crystal current, since it's RF current at the crystals frequency of oscillation. Most AC milliameters will stop working at about 10Khz, and the lead length and impedance will probably throw off your oscillator. One "trick" we used in the old days, which also sometimes saved the crystal from destruction, was to put a low current pilot lamp in series with the crystal. Seems to me it was a #49, but you'd best look it up. My recollection was that this was a 60ma bulb. In normal operation it should not glow visibly. Doug Moore KB9TMY |
In article ,
"Paul_Morphy" writes: " Uncle Peter" wrote in message news:vQnbc.9210$pM1.6556@lakeread06... I suggest he put all his radios on a shelf, and get a cell phone. No dangerous RF levels, no deadly voltages... NOT Gee, Unc, have a tough week? : I don't want him to give up radio, I just don't think an Ameco AC-1 is worth the trouble. But he does think it's worth the trouble. I didn't want one when I was 15 and they were new, and I wouldn't waste my time fiddling with one now. Nobody is saying *you* should. But your're saying Uwwe shouldn't. See the contradiction? 73 de Jim, N2EY |
In article ,
"Paul_Morphy" writes: " Uncle Peter" wrote in message news:vQnbc.9210$pM1.6556@lakeread06... I suggest he put all his radios on a shelf, and get a cell phone. No dangerous RF levels, no deadly voltages... NOT Gee, Unc, have a tough week? : I don't want him to give up radio, I just don't think an Ameco AC-1 is worth the trouble. But he does think it's worth the trouble. I didn't want one when I was 15 and they were new, and I wouldn't waste my time fiddling with one now. Nobody is saying *you* should. But your're saying Uwwe shouldn't. See the contradiction? 73 de Jim, N2EY |
In article , Uwe
writes: I will just have to fiddle a bit more with the pi network (since at the B+ voltages suggested here my plate current would be way high) and I will have to live with the chirp. Are you getting a "dip" in plate current? If not, the coil is probably too large or too small. Unless you get a real dip, the output network isn;t right. I have used a very similar transmitter with 350 volts on the plate, and the dip is clean and pronounced. Nobody has complained yet anyways and maybe the chirp is worse in my receiver than on the air, as someone here suggested. By the way I use a Icom R75 for a receiver and switch the antenna off during transmit-still plenty of a signal seems to get into the receiver anyways. It is quite possible that the receiver is being overloaded by the large signal and creating a chirp. 73 de Jim, N2EY |
In article , Uwe
writes: I will just have to fiddle a bit more with the pi network (since at the B+ voltages suggested here my plate current would be way high) and I will have to live with the chirp. Are you getting a "dip" in plate current? If not, the coil is probably too large or too small. Unless you get a real dip, the output network isn;t right. I have used a very similar transmitter with 350 volts on the plate, and the dip is clean and pronounced. Nobody has complained yet anyways and maybe the chirp is worse in my receiver than on the air, as someone here suggested. By the way I use a Icom R75 for a receiver and switch the antenna off during transmit-still plenty of a signal seems to get into the receiver anyways. It is quite possible that the receiver is being overloaded by the large signal and creating a chirp. 73 de Jim, N2EY |
in article , N2EY at
PAMNO wrote on 4/5/04 18:59: In article , Uwe writes: I will just have to fiddle a bit more with the pi network (since at the B+ voltages suggested here my plate current would be way high) and I will have to live with the chirp. Are you getting a "dip" in plate current? If not, the coil is probably too large or too small. Unless you get a real dip, the output network isn;t right. I have used a very similar transmitter with 350 volts on the plate, and the dip is clean and pronounced. Jim, the original docs I got for this tx call, at 40 m, for a 15 turn coil on the coil form provided with the kit, which I hear was 1.25" diameter. If I use the formula for air coils this turns out to be roughly a 22 microhenry coil. The coil which works best with my tx is 8 turns on a 1,125" ceramic core. To get guess work out of it I just bought and built a L/C meter and measured my coil to have 2.7 microhenry. So I am way off, but it works, sort of. The air caps are 36 to 420pf at the plate and 15 to 728pf at the antenna, so that seem right. All this happens with B+200V and 35 mA plate current. Older ARRL handbooks give typical values for pi network for 50 Ohm antenna loads and my values are in range for the caps but my coil is too small. The ouput voltage on my antenna measured with a scope is up to 75 volts peak to peak, with a 50 Ohm load that would mean I get more out of the tx than I put into it and I am not of the sort who says this might happen. So my conclusion is, and tell me if this sounds right, that I have an antenna which is far from 50 ohm resistive at 40m and that that makes everything weird. The dips in plate current are nearly imperceptible and they are not aided by my 250mA full scale meter. They may be 2 or 3 mA. I tune with the help of my scope. 73 Uwe |
"Uwe" wrote in message ... The dips in plate current are nearly imperceptible and they are not aided by my 250mA full scale meter. They may be 2 or 3 mA. I tune with the help of my scope. The pi network should be able to match a wide range of impedances but it would help to connect a known resistive load. Five, 2-watt, 270-ohm resistors in parallel would be close enough. If you're getting a dip in plate current the circuit is resonating somewhere, and you say you've worked people, so it's putting some rf on the band. See if you're getting two dips. A lot of those pi networks would resonate on the operating band with the plate tuning cap almost completely meshed, but there was enough range in the cap that it would also tune to the second harmonic. If your LC meter is right you may be dipping at a harmonic, not the fundamental. OTOH, if your calculation of what the original coil was is correct, the plate tuning capacitor should resonate when its value is about 20 pF -- for a 22-uH coil. That doesn't mesh with the range of your plate tuning cap. A 2.7 uH coil would resonate with the plate tuning cap at about 185 pF, which seems more reasonable. This is one of those rare occasions when a grid-dip meter is handy. Is there a ham club in your area? Someone may have one to lend. Meanwhile, hook it up to a dummy load and see what you get. There's something else you could try, but I don't know how well it would work. With the AC-1 unplugged you could connect your receiver antenna to the top of the plate tuning cap and adjust the plate tuning cap while listening for a peak in the noise level. That would tell you the circuit was resonating at 7 MHz. If NG, try 20 meters and 10 meters. In the olden days I had a 6BE6 connected inside my Viking Valiant, such that it turned on and bridged the receiver antenna input when transmitted rf appeared at the grid. When the key was up, rf from the antenna passed into the receiver. This allowed for full break-in CW, and I could dip the plate tuning cap just by listening to the noise level. Real handy when moving around the band in a contest. You would probably pop the front end of a solid-state receiver doing this, so don't try it. The circuit was in an old Radio Handbook, which was edited by Bill Orr, W6SAI. That's what made me think that you could try this with your receiver, but exercise appropriate caution. You know, I think you can get coil forms to fit your rig from Antique Radio Supply, and also maybe Ocean State Electronics (oselectronics.com). You could even make one for 30 meters. Ocean State has a power transformer in their catalog that may do for a power supply for your rig, too. Or look for an old tube-type hi-fi receiver in a thrift shop or at a tag sale. If you're going to do, you may as well do it! : 73, "PM" |
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