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help building oscillator
I don't know of any good way to troubleshoot an oscillator without an
oscilloscope and signal generator, except to measure all the voltages. However, the voltages can be right and the oscillator still not oscillate. Or you can find that some voltages are different simply because it's not oscillating. Perhaps someone else has some tips that can help. Your problems with oscillators are possibly caused by poor component choices. Capacitors should be low loss types, like mica, NPO ceramic, or polystyrene plastic, and should be physically small so they don't have much inductance. NPO ceramic has the lowest temperature coefficient so is the best choice for stability. If you have a poorly made inductor, the temperature drift of a polystyrene capacitor might more-or-less cancel it, but that's a poor way to get stability. Inductors also have to be low loss, and that's probably the answer to your question about the toroid. There are just one or two types of ferrite that might be usable for an oscillator inductor, but they're uncommon so you're unlikely to have them in your junk box. All other kinds of ferrite cores are too lossy for this use. You should use only powdered iron cores. In my experience, the best is type 6, which is colored yellow and typically with a glossy finish. Type 7, colored white, should be similar, although it's much less common. I wouldn't use any other type of toroid core except these two types of powdered iron. Ferrite is a good core choice for the RF choke, although you might have trouble with core saturation if you're putting a fair amount of current through it. Chances are you don't really need a choke with a value this high, although you might in some applications. Don't apologize for the questions! We all had to start somewhere, and are glad to see you learning. And you never learn much from circuits that work -- it's the ones that don't which are educational. Roy Lewallen, W7EL Jock Cooper wrote: Hi everyone, I've been interested in radios and how they work since I was a kid -- I had a crystal radio kit and a RF-2200. Recently I started building radios and have been having a blast. Please keep in mind that I am very newbie. After scouring the web for am radio circuit schematics I found quite a few and successfully built several of them (I'm using breadboards). Then I wanted to get a little more sophisticated so I got some NE602s and started to work on a superhet ish type design -- basically cobbling together bits from other circuits. I had some decent success using the NE602 internal oscillator (although getting the oscillator to track the rf input is tricky), then I wanted to use an external oscillator. I tried numerous circuits and numerous types (Hartley, Colpitts etc) I had found but I couldn't get any of them to start up. Finally somehow I got one working reliably: it was a JFET Clapp with a JFET buffer amp. I hooked up my Elenco LCR to it to measure the frequency. I was using a 3 gang 10-500pf air cap and a coil I wound onto a pill bottle. The oscillator was running from 980 khz to 2050 or so. I fed it into the NE602 pin 6. Only problem was (again) I had a hard time getting the main tuning (also on the 3 gang cap) to track the same range (well the same minus 455). I guess I had plenty of stray capacitance or something that was mucking up my calculations. So at say 540 they would line up but tuning to the other end they'd be off by a few 100 (!) khz. So I decided to build a second oscillator to help align the antenna tuning tank. I pulled out one of those little 'mini' breadboards and basically dupicated the working oscillator circuit on it. Only this time no startup. I also moved it to a second breadboard but still no good. Now I'm at a loss as to how to proceed. How do you diagnose a bad oscillator circuit? I thought about measuring the voltages all various points on the working osc while it was running and comparing those, not sure how much that will tell me. Also, a related question. I tried winding a toroid with the same inductance as my pill bottle coil, and hooking it in place of the pill bottle coil in the working oscillator. But it wouldn't start then. Any clue why this might be the case?? Also I have seen some circuits that call for a 1mH RF choke.. Can I wind a toroid for this? I don't have any chokes that high. thanks for any help and apologies for any dumb questions, Jock Cooper --- http://www.fractal-recursions.com |
I don't know of any good way to troubleshoot an oscillator without an
oscilloscope and signal generator, except to measure all the voltages. However, the voltages can be right and the oscillator still not oscillate. Or you can find that some voltages are different simply because it's not oscillating. Perhaps someone else has some tips that can help. Your problems with oscillators are possibly caused by poor component choices. Capacitors should be low loss types, like mica, NPO ceramic, or polystyrene plastic, and should be physically small so they don't have much inductance. NPO ceramic has the lowest temperature coefficient so is the best choice for stability. If you have a poorly made inductor, the temperature drift of a polystyrene capacitor might more-or-less cancel it, but that's a poor way to get stability. Inductors also have to be low loss, and that's probably the answer to your question about the toroid. There are just one or two types of ferrite that might be usable for an oscillator inductor, but they're uncommon so you're unlikely to have them in your junk box. All other kinds of ferrite cores are too lossy for this use. You should use only powdered iron cores. In my experience, the best is type 6, which is colored yellow and typically with a glossy finish. Type 7, colored white, should be similar, although it's much less common. I wouldn't use any other type of toroid core except these two types of powdered iron. Ferrite is a good core choice for the RF choke, although you might have trouble with core saturation if you're putting a fair amount of current through it. Chances are you don't really need a choke with a value this high, although you might in some applications. Don't apologize for the questions! We all had to start somewhere, and are glad to see you learning. And you never learn much from circuits that work -- it's the ones that don't which are educational. Roy Lewallen, W7EL Jock Cooper wrote: Hi everyone, I've been interested in radios and how they work since I was a kid -- I had a crystal radio kit and a RF-2200. Recently I started building radios and have been having a blast. Please keep in mind that I am very newbie. After scouring the web for am radio circuit schematics I found quite a few and successfully built several of them (I'm using breadboards). Then I wanted to get a little more sophisticated so I got some NE602s and started to work on a superhet ish type design -- basically cobbling together bits from other circuits. I had some decent success using the NE602 internal oscillator (although getting the oscillator to track the rf input is tricky), then I wanted to use an external oscillator. I tried numerous circuits and numerous types (Hartley, Colpitts etc) I had found but I couldn't get any of them to start up. Finally somehow I got one working reliably: it was a JFET Clapp with a JFET buffer amp. I hooked up my Elenco LCR to it to measure the frequency. I was using a 3 gang 10-500pf air cap and a coil I wound onto a pill bottle. The oscillator was running from 980 khz to 2050 or so. I fed it into the NE602 pin 6. Only problem was (again) I had a hard time getting the main tuning (also on the 3 gang cap) to track the same range (well the same minus 455). I guess I had plenty of stray capacitance or something that was mucking up my calculations. So at say 540 they would line up but tuning to the other end they'd be off by a few 100 (!) khz. So I decided to build a second oscillator to help align the antenna tuning tank. I pulled out one of those little 'mini' breadboards and basically dupicated the working oscillator circuit on it. Only this time no startup. I also moved it to a second breadboard but still no good. Now I'm at a loss as to how to proceed. How do you diagnose a bad oscillator circuit? I thought about measuring the voltages all various points on the working osc while it was running and comparing those, not sure how much that will tell me. Also, a related question. I tried winding a toroid with the same inductance as my pill bottle coil, and hooking it in place of the pill bottle coil in the working oscillator. But it wouldn't start then. Any clue why this might be the case?? Also I have seen some circuits that call for a 1mH RF choke.. Can I wind a toroid for this? I don't have any chokes that high. thanks for any help and apologies for any dumb questions, Jock Cooper --- http://www.fractal-recursions.com |
"Roy Lewallen" wrote in message
... I don't know of any good way to troubleshoot an oscillator without an oscilloscope and signal generator, except to measure all the voltages. However, the voltages can be right and the oscillator still not oscillate. Or you can find that some voltages are different simply because it's not oscillating. Perhaps someone else has some tips that can help. Your problems with oscillators are possibly caused by poor component choices. Capacitors should be low loss types, like mica, NPO ceramic, or polystyrene plastic, and should be physically small so they don't have much inductance. NPO ceramic has the lowest temperature coefficient so is the best choice for stability. If you have a poorly made inductor, the temperature drift of a polystyrene capacitor might more-or-less cancel it, but that's a poor way to get stability. Inductors also have to be low loss, and that's probably the answer to your question about the toroid. There are just one or two types of ferrite that might be usable for an oscillator inductor, but they're uncommon so you're unlikely to have them in your junk box. All other kinds of ferrite cores are too lossy for this use. You should use only powdered iron cores. In my experience, the best is type 6, which is colored yellow and typically with a glossy finish. Type 7, colored white, should be similar, although it's much less common. I wouldn't use any other type of toroid core except these two types of powdered iron. Ferrite is a good core choice for the RF choke, although you might have trouble with core saturation if you're putting a fair amount of current through it. Chances are you don't really need a choke with a value this high, although you might in some applications. Don't apologize for the questions! We all had to start somewhere, and are glad to see you learning. And you never learn much from circuits that work -- it's the ones that don't which are educational. Roy Lewallen, W7EL Jock Cooper wrote: Hi everyone, I've been interested in radios and how they work since I was a kid -- I had a crystal radio kit and a RF-2200. Recently I started building radios and have been having a blast. Please keep in mind that I am very newbie. After scouring the web for am radio circuit schematics I found quite a few and successfully built several of them (I'm using breadboards). Then I wanted to get a little more sophisticated so I got some NE602s and started to work on a superhet ish type design -- basically cobbling together bits from other circuits. I had some decent success using the NE602 internal oscillator (although getting the oscillator to track the rf input is tricky), then I wanted to use an external oscillator. I tried numerous circuits and numerous types (Hartley, Colpitts etc) I had found but I couldn't get any of them to start up. Finally somehow I got one working reliably: it was a JFET Clapp with a JFET buffer amp. I hooked up my Elenco LCR to it to measure the frequency. I was using a 3 gang 10-500pf air cap and a coil I wound onto a pill bottle. The oscillator was running from 980 khz to 2050 or so. I fed it into the NE602 pin 6. Only problem was (again) I had a hard time getting the main tuning (also on the 3 gang cap) to track the same range (well the same minus 455). I guess I had plenty of stray capacitance or something that was mucking up my calculations. So at say 540 they would line up but tuning to the other end they'd be off by a few 100 (!) khz. So I decided to build a second oscillator to help align the antenna tuning tank. I pulled out one of those little 'mini' breadboards and basically dupicated the working oscillator circuit on it. Only this time no startup. I also moved it to a second breadboard but still no good. Now I'm at a loss as to how to proceed. How do you diagnose a bad oscillator circuit? I thought about measuring the voltages all various points on the working osc while it was running and comparing those, not sure how much that will tell me. Also, a related question. I tried winding a toroid with the same inductance as my pill bottle coil, and hooking it in place of the pill bottle coil in the working oscillator. But it wouldn't start then. Any clue why this might be the case?? Also I have seen some circuits that call for a 1mH RF choke.. Can I wind a toroid for this? I don't have any chokes that high. thanks for any help and apologies for any dumb questions, Jock Cooper --- http://www.fractal-recursions.com Yup to everything that Roy said, and I'll add a couple comments of my own. Depending on the type of breadboard that you're using, the capacitance between the rows of contacts can upset RF tuning. At the relatively low frequencies that you're using, that shouldn't be having that much of an effect,but you can't rule it out entirely. I assume, that since you're trying to build a superhet radio, that you're tuning the oscillator and RF with a multi-section variable capacitor. I'll bet that you're using a variable with equal capacitance on each section. Wrong!! Since the frequencies that the oscillator needs to tune is different from the RF tuning, then you need to select a variable with a high capacity section for the RF side, and a lower capacity section for tuning the oscillator. Take a look inside an old tube superhet radio and look at the tuning capacitor. Notice that one section has very large plates on the rotor and the other section has plates that are about 1/2 to 2/3 the size of the large (high capacity) section. That's probably why the oscillator isn't tracking the RF tuning. Now, to get the oscillator to run reliably, you may need to separate the connections on the breadboard a little more. Instead of connecting somponents to adjacent rows, separate them by two rows. That will reduce the capacitance somewhat. Are you using a suggested circuit from the data sheet of the NE602? If so, be sure you use suggested types of components, especially capacitors. They can be notoriously problematic if they are the wrong type for the application. -- Tweetldee Tweetldee at att dot net (Just subsitute the appropriate characters in the address) Time is what keeps everything from happening all at once. |
"Roy Lewallen" wrote in message
... I don't know of any good way to troubleshoot an oscillator without an oscilloscope and signal generator, except to measure all the voltages. However, the voltages can be right and the oscillator still not oscillate. Or you can find that some voltages are different simply because it's not oscillating. Perhaps someone else has some tips that can help. Your problems with oscillators are possibly caused by poor component choices. Capacitors should be low loss types, like mica, NPO ceramic, or polystyrene plastic, and should be physically small so they don't have much inductance. NPO ceramic has the lowest temperature coefficient so is the best choice for stability. If you have a poorly made inductor, the temperature drift of a polystyrene capacitor might more-or-less cancel it, but that's a poor way to get stability. Inductors also have to be low loss, and that's probably the answer to your question about the toroid. There are just one or two types of ferrite that might be usable for an oscillator inductor, but they're uncommon so you're unlikely to have them in your junk box. All other kinds of ferrite cores are too lossy for this use. You should use only powdered iron cores. In my experience, the best is type 6, which is colored yellow and typically with a glossy finish. Type 7, colored white, should be similar, although it's much less common. I wouldn't use any other type of toroid core except these two types of powdered iron. Ferrite is a good core choice for the RF choke, although you might have trouble with core saturation if you're putting a fair amount of current through it. Chances are you don't really need a choke with a value this high, although you might in some applications. Don't apologize for the questions! We all had to start somewhere, and are glad to see you learning. And you never learn much from circuits that work -- it's the ones that don't which are educational. Roy Lewallen, W7EL Jock Cooper wrote: Hi everyone, I've been interested in radios and how they work since I was a kid -- I had a crystal radio kit and a RF-2200. Recently I started building radios and have been having a blast. Please keep in mind that I am very newbie. After scouring the web for am radio circuit schematics I found quite a few and successfully built several of them (I'm using breadboards). Then I wanted to get a little more sophisticated so I got some NE602s and started to work on a superhet ish type design -- basically cobbling together bits from other circuits. I had some decent success using the NE602 internal oscillator (although getting the oscillator to track the rf input is tricky), then I wanted to use an external oscillator. I tried numerous circuits and numerous types (Hartley, Colpitts etc) I had found but I couldn't get any of them to start up. Finally somehow I got one working reliably: it was a JFET Clapp with a JFET buffer amp. I hooked up my Elenco LCR to it to measure the frequency. I was using a 3 gang 10-500pf air cap and a coil I wound onto a pill bottle. The oscillator was running from 980 khz to 2050 or so. I fed it into the NE602 pin 6. Only problem was (again) I had a hard time getting the main tuning (also on the 3 gang cap) to track the same range (well the same minus 455). I guess I had plenty of stray capacitance or something that was mucking up my calculations. So at say 540 they would line up but tuning to the other end they'd be off by a few 100 (!) khz. So I decided to build a second oscillator to help align the antenna tuning tank. I pulled out one of those little 'mini' breadboards and basically dupicated the working oscillator circuit on it. Only this time no startup. I also moved it to a second breadboard but still no good. Now I'm at a loss as to how to proceed. How do you diagnose a bad oscillator circuit? I thought about measuring the voltages all various points on the working osc while it was running and comparing those, not sure how much that will tell me. Also, a related question. I tried winding a toroid with the same inductance as my pill bottle coil, and hooking it in place of the pill bottle coil in the working oscillator. But it wouldn't start then. Any clue why this might be the case?? Also I have seen some circuits that call for a 1mH RF choke.. Can I wind a toroid for this? I don't have any chokes that high. thanks for any help and apologies for any dumb questions, Jock Cooper --- http://www.fractal-recursions.com Yup to everything that Roy said, and I'll add a couple comments of my own. Depending on the type of breadboard that you're using, the capacitance between the rows of contacts can upset RF tuning. At the relatively low frequencies that you're using, that shouldn't be having that much of an effect,but you can't rule it out entirely. I assume, that since you're trying to build a superhet radio, that you're tuning the oscillator and RF with a multi-section variable capacitor. I'll bet that you're using a variable with equal capacitance on each section. Wrong!! Since the frequencies that the oscillator needs to tune is different from the RF tuning, then you need to select a variable with a high capacity section for the RF side, and a lower capacity section for tuning the oscillator. Take a look inside an old tube superhet radio and look at the tuning capacitor. Notice that one section has very large plates on the rotor and the other section has plates that are about 1/2 to 2/3 the size of the large (high capacity) section. That's probably why the oscillator isn't tracking the RF tuning. Now, to get the oscillator to run reliably, you may need to separate the connections on the breadboard a little more. Instead of connecting somponents to adjacent rows, separate them by two rows. That will reduce the capacitance somewhat. Are you using a suggested circuit from the data sheet of the NE602? If so, be sure you use suggested types of components, especially capacitors. They can be notoriously problematic if they are the wrong type for the application. -- Tweetldee Tweetldee at att dot net (Just subsitute the appropriate characters in the address) Time is what keeps everything from happening all at once. |
A general coverage receiver is a good piece of test eqpt. Tune the rx to
the approx frequency of one that works to get a good idea of how to couple and how loud it should be. CW or SSB is easier to use than AM. You can also couple the LO of a receiver to use it as a test LO or signal gen. good luck hank wd5jfr "Tweetldee" wrote in message ... "Roy Lewallen" wrote in message ... I don't know of any good way to troubleshoot an oscillator without an oscilloscope and signal generator, except to measure all the voltages. However, the voltages can be right and the oscillator still not oscillate. Or you can find that some voltages are different simply because it's not oscillating. Perhaps someone else has some tips that can help. Your problems with oscillators are possibly caused by poor component choices. Capacitors should be low loss types, like mica, NPO ceramic, or polystyrene plastic, and should be physically small so they don't have much inductance. NPO ceramic has the lowest temperature coefficient so is the best choice for stability. If you have a poorly made inductor, the temperature drift of a polystyrene capacitor might more-or-less cancel it, but that's a poor way to get stability. Inductors also have to be low loss, and that's probably the answer to your question about the toroid. There are just one or two types of ferrite that might be usable for an oscillator inductor, but they're uncommon so you're unlikely to have them in your junk box. All other kinds of ferrite cores are too lossy for this use. You should use only powdered iron cores. In my experience, the best is type 6, which is colored yellow and typically with a glossy finish. Type 7, colored white, should be similar, although it's much less common. I wouldn't use any other type of toroid core except these two types of powdered iron. Ferrite is a good core choice for the RF choke, although you might have trouble with core saturation if you're putting a fair amount of current through it. Chances are you don't really need a choke with a value this high, although you might in some applications. Don't apologize for the questions! We all had to start somewhere, and are glad to see you learning. And you never learn much from circuits that work -- it's the ones that don't which are educational. Roy Lewallen, W7EL Jock Cooper wrote: Hi everyone, I've been interested in radios and how they work since I was a kid -- I had a crystal radio kit and a RF-2200. Recently I started building radios and have been having a blast. Please keep in mind that I am very newbie. After scouring the web for am radio circuit schematics I found quite a few and successfully built several of them (I'm using breadboards). Then I wanted to get a little more sophisticated so I got some NE602s and started to work on a superhet ish type design -- basically cobbling together bits from other circuits. I had some decent success using the NE602 internal oscillator (although getting the oscillator to track the rf input is tricky), then I wanted to use an external oscillator. I tried numerous circuits and numerous types (Hartley, Colpitts etc) I had found but I couldn't get any of them to start up. Finally somehow I got one working reliably: it was a JFET Clapp with a JFET buffer amp. I hooked up my Elenco LCR to it to measure the frequency. I was using a 3 gang 10-500pf air cap and a coil I wound onto a pill bottle. The oscillator was running from 980 khz to 2050 or so. I fed it into the NE602 pin 6. Only problem was (again) I had a hard time getting the main tuning (also on the 3 gang cap) to track the same range (well the same minus 455). I guess I had plenty of stray capacitance or something that was mucking up my calculations. So at say 540 they would line up but tuning to the other end they'd be off by a few 100 (!) khz. So I decided to build a second oscillator to help align the antenna tuning tank. I pulled out one of those little 'mini' breadboards and basically dupicated the working oscillator circuit on it. Only this time no startup. I also moved it to a second breadboard but still no good. Now I'm at a loss as to how to proceed. How do you diagnose a bad oscillator circuit? I thought about measuring the voltages all various points on the working osc while it was running and comparing those, not sure how much that will tell me. Also, a related question. I tried winding a toroid with the same inductance as my pill bottle coil, and hooking it in place of the pill bottle coil in the working oscillator. But it wouldn't start then. Any clue why this might be the case?? Also I have seen some circuits that call for a 1mH RF choke.. Can I wind a toroid for this? I don't have any chokes that high. thanks for any help and apologies for any dumb questions, Jock Cooper --- http://www.fractal-recursions.com Yup to everything that Roy said, and I'll add a couple comments of my own. Depending on the type of breadboard that you're using, the capacitance between the rows of contacts can upset RF tuning. At the relatively low frequencies that you're using, that shouldn't be having that much of an effect,but you can't rule it out entirely. I assume, that since you're trying to build a superhet radio, that you're tuning the oscillator and RF with a multi-section variable capacitor. I'll bet that you're using a variable with equal capacitance on each section. Wrong!! Since the frequencies that the oscillator needs to tune is different from the RF tuning, then you need to select a variable with a high capacity section for the RF side, and a lower capacity section for tuning the oscillator. Take a look inside an old tube superhet radio and look at the tuning capacitor. Notice that one section has very large plates on the rotor and the other section has plates that are about 1/2 to 2/3 the size of the large (high capacity) section. That's probably why the oscillator isn't tracking the RF tuning. Now, to get the oscillator to run reliably, you may need to separate the connections on the breadboard a little more. Instead of connecting somponents to adjacent rows, separate them by two rows. That will reduce the capacitance somewhat. Are you using a suggested circuit from the data sheet of the NE602? If so, be sure you use suggested types of components, especially capacitors. They can be notoriously problematic if they are the wrong type for the application. -- Tweetldee Tweetldee at att dot net (Just subsitute the appropriate characters in the address) Time is what keeps everything from happening all at once. |
A general coverage receiver is a good piece of test eqpt. Tune the rx to
the approx frequency of one that works to get a good idea of how to couple and how loud it should be. CW or SSB is easier to use than AM. You can also couple the LO of a receiver to use it as a test LO or signal gen. good luck hank wd5jfr "Tweetldee" wrote in message ... "Roy Lewallen" wrote in message ... I don't know of any good way to troubleshoot an oscillator without an oscilloscope and signal generator, except to measure all the voltages. However, the voltages can be right and the oscillator still not oscillate. Or you can find that some voltages are different simply because it's not oscillating. Perhaps someone else has some tips that can help. Your problems with oscillators are possibly caused by poor component choices. Capacitors should be low loss types, like mica, NPO ceramic, or polystyrene plastic, and should be physically small so they don't have much inductance. NPO ceramic has the lowest temperature coefficient so is the best choice for stability. If you have a poorly made inductor, the temperature drift of a polystyrene capacitor might more-or-less cancel it, but that's a poor way to get stability. Inductors also have to be low loss, and that's probably the answer to your question about the toroid. There are just one or two types of ferrite that might be usable for an oscillator inductor, but they're uncommon so you're unlikely to have them in your junk box. All other kinds of ferrite cores are too lossy for this use. You should use only powdered iron cores. In my experience, the best is type 6, which is colored yellow and typically with a glossy finish. Type 7, colored white, should be similar, although it's much less common. I wouldn't use any other type of toroid core except these two types of powdered iron. Ferrite is a good core choice for the RF choke, although you might have trouble with core saturation if you're putting a fair amount of current through it. Chances are you don't really need a choke with a value this high, although you might in some applications. Don't apologize for the questions! We all had to start somewhere, and are glad to see you learning. And you never learn much from circuits that work -- it's the ones that don't which are educational. Roy Lewallen, W7EL Jock Cooper wrote: Hi everyone, I've been interested in radios and how they work since I was a kid -- I had a crystal radio kit and a RF-2200. Recently I started building radios and have been having a blast. Please keep in mind that I am very newbie. After scouring the web for am radio circuit schematics I found quite a few and successfully built several of them (I'm using breadboards). Then I wanted to get a little more sophisticated so I got some NE602s and started to work on a superhet ish type design -- basically cobbling together bits from other circuits. I had some decent success using the NE602 internal oscillator (although getting the oscillator to track the rf input is tricky), then I wanted to use an external oscillator. I tried numerous circuits and numerous types (Hartley, Colpitts etc) I had found but I couldn't get any of them to start up. Finally somehow I got one working reliably: it was a JFET Clapp with a JFET buffer amp. I hooked up my Elenco LCR to it to measure the frequency. I was using a 3 gang 10-500pf air cap and a coil I wound onto a pill bottle. The oscillator was running from 980 khz to 2050 or so. I fed it into the NE602 pin 6. Only problem was (again) I had a hard time getting the main tuning (also on the 3 gang cap) to track the same range (well the same minus 455). I guess I had plenty of stray capacitance or something that was mucking up my calculations. So at say 540 they would line up but tuning to the other end they'd be off by a few 100 (!) khz. So I decided to build a second oscillator to help align the antenna tuning tank. I pulled out one of those little 'mini' breadboards and basically dupicated the working oscillator circuit on it. Only this time no startup. I also moved it to a second breadboard but still no good. Now I'm at a loss as to how to proceed. How do you diagnose a bad oscillator circuit? I thought about measuring the voltages all various points on the working osc while it was running and comparing those, not sure how much that will tell me. Also, a related question. I tried winding a toroid with the same inductance as my pill bottle coil, and hooking it in place of the pill bottle coil in the working oscillator. But it wouldn't start then. Any clue why this might be the case?? Also I have seen some circuits that call for a 1mH RF choke.. Can I wind a toroid for this? I don't have any chokes that high. thanks for any help and apologies for any dumb questions, Jock Cooper --- http://www.fractal-recursions.com Yup to everything that Roy said, and I'll add a couple comments of my own. Depending on the type of breadboard that you're using, the capacitance between the rows of contacts can upset RF tuning. At the relatively low frequencies that you're using, that shouldn't be having that much of an effect,but you can't rule it out entirely. I assume, that since you're trying to build a superhet radio, that you're tuning the oscillator and RF with a multi-section variable capacitor. I'll bet that you're using a variable with equal capacitance on each section. Wrong!! Since the frequencies that the oscillator needs to tune is different from the RF tuning, then you need to select a variable with a high capacity section for the RF side, and a lower capacity section for tuning the oscillator. Take a look inside an old tube superhet radio and look at the tuning capacitor. Notice that one section has very large plates on the rotor and the other section has plates that are about 1/2 to 2/3 the size of the large (high capacity) section. That's probably why the oscillator isn't tracking the RF tuning. Now, to get the oscillator to run reliably, you may need to separate the connections on the breadboard a little more. Instead of connecting somponents to adjacent rows, separate them by two rows. That will reduce the capacitance somewhat. Are you using a suggested circuit from the data sheet of the NE602? If so, be sure you use suggested types of components, especially capacitors. They can be notoriously problematic if they are the wrong type for the application. -- Tweetldee Tweetldee at att dot net (Just subsitute the appropriate characters in the address) Time is what keeps everything from happening all at once. |
Jock Cooper wrote in message ...
Hi everyone, ... How do you diagnose a bad oscillator circuit? Roy offered some good advice. No need for me to try to duplicate it! But I know there can be a lot of times you just don't see something that would be obvious to someone else who has more experience. If you can find someone locally to have a look at things, the answers might come a lot quicker. Failing that, if there's a place you could put up some pictures on the web, that might help. Are you using known-good circuits? Sounds like you probably are, given that you understand the differences among the various types of circuits. But it might be good to get your hands on a book with a bunch of circuit ideas in it, if you don't already have one (or more). Cheers, Tom |
Jock Cooper wrote in message ...
Hi everyone, ... How do you diagnose a bad oscillator circuit? Roy offered some good advice. No need for me to try to duplicate it! But I know there can be a lot of times you just don't see something that would be obvious to someone else who has more experience. If you can find someone locally to have a look at things, the answers might come a lot quicker. Failing that, if there's a place you could put up some pictures on the web, that might help. Are you using known-good circuits? Sounds like you probably are, given that you understand the differences among the various types of circuits. But it might be good to get your hands on a book with a bunch of circuit ideas in it, if you don't already have one (or more). Cheers, Tom |
I highly recommend _Experimental Methods in RF Design_, by Hayward,
Campbell, and Larkin, available from the ARRL or your favorite book seller. All the circuits in the book have actually been built with the component values shown, and have been designed by people who know what they're doing. My personal favorite oscillator is the one shown in fig. 4.4 on p. 4.3. It's a dead-simple Hartley, is very stable, and just about bulletproof. I've made versions from 2 to over 100 MHz, and they always oscillate. The authors say in the book that they've made versions from 1 MHz to 3 GHz. You'll also find equations so you can calculate the capacitors you need for a given tuning range, and a wealth of other information. Although it's a terrific bargain at the price, some people balk at spending the money for it. If you're one of those, check your local library. They might not have it yet -- if not, ask them to order it if they haven't already. Roy Lewallen, W7EL Tom Bruhns wrote: . . . Are you using known-good circuits? Sounds like you probably are, given that you understand the differences among the various types of circuits. But it might be good to get your hands on a book with a bunch of circuit ideas in it, if you don't already have one (or more). |
I highly recommend _Experimental Methods in RF Design_, by Hayward,
Campbell, and Larkin, available from the ARRL or your favorite book seller. All the circuits in the book have actually been built with the component values shown, and have been designed by people who know what they're doing. My personal favorite oscillator is the one shown in fig. 4.4 on p. 4.3. It's a dead-simple Hartley, is very stable, and just about bulletproof. I've made versions from 2 to over 100 MHz, and they always oscillate. The authors say in the book that they've made versions from 1 MHz to 3 GHz. You'll also find equations so you can calculate the capacitors you need for a given tuning range, and a wealth of other information. Although it's a terrific bargain at the price, some people balk at spending the money for it. If you're one of those, check your local library. They might not have it yet -- if not, ask them to order it if they haven't already. Roy Lewallen, W7EL Tom Bruhns wrote: . . . Are you using known-good circuits? Sounds like you probably are, given that you understand the differences among the various types of circuits. But it might be good to get your hands on a book with a bunch of circuit ideas in it, if you don't already have one (or more). |
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