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help with homebrew receiver!
Hi:
I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. Any chance? Thanks in advance, Tom KCØGXX |
"TC Dufresne" wrote in
: Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. Any chance? Thanks in advance, Tom KCØGXX Gotta be a short somewhere. Filter caps ain't supposed to get hot, and yours is over rated to begin with. |
"TC Dufresne" wrote in
: Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. Any chance? Thanks in advance, Tom KCØGXX Gotta be a short somewhere. Filter caps ain't supposed to get hot, and yours is over rated to begin with. |
Tom: The hot cap has two likely possibilities: either the cap is shorted internally (an ohmeter check will show leakage) or you have bad diode(s) so AC is being applied to the cap. An ohmeter will show the diodes as bad. Of course a wiring error could also be involved, but I am assuming you have double checked that. If the cap is running hot you probably don't have the correct voltage for the VFO, and correcting it may solve the other VFO problem. Roger K6XQ |
Tom: The hot cap has two likely possibilities: either the cap is shorted internally (an ohmeter check will show leakage) or you have bad diode(s) so AC is being applied to the cap. An ohmeter will show the diodes as bad. Of course a wiring error could also be involved, but I am assuming you have double checked that. If the cap is running hot you probably don't have the correct voltage for the VFO, and correcting it may solve the other VFO problem. Roger K6XQ |
"TC Dufresne" wrote in message ... Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. It's a good idea to build things in stages, testing each stage as you go. Try disconnecting everything from the power supply and sort that out first. The capacitor shouldn't get hot. 73, Leon -- Leon Heller, G1HSM http://www.geocities.com/leon_heller |
"TC Dufresne" wrote in message ... Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. It's a good idea to build things in stages, testing each stage as you go. Try disconnecting everything from the power supply and sort that out first. The capacitor shouldn't get hot. 73, Leon -- Leon Heller, G1HSM http://www.geocities.com/leon_heller |
Try contacting ARRL for corrections to the article. I built a DC rcvr from
about that time frame and it did not work. The ARRL sent corrections to the schematic and there were a lot!! |
Try contacting ARRL for corrections to the article. I built a DC rcvr from
about that time frame and it did not work. The ARRL sent corrections to the schematic and there were a lot!! |
"TC Dufresne" wrote in message
... Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! Electrolytic capacitors are polarized. Probably you reversed the cap and smoked it. The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. Start by looking at the voltage at the electrolytic cap, and see if you in fact have voltage or not. Any chance? Thanks in advance, Tom KCØGXX |
"TC Dufresne" wrote in message
... Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! Electrolytic capacitors are polarized. Probably you reversed the cap and smoked it. The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. Start by looking at the voltage at the electrolytic cap, and see if you in fact have voltage or not. Any chance? Thanks in advance, Tom KCØGXX |
"Carl R. Stevenson" wrote in message
... "TC Dufresne" wrote in message ... Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! Are you using a tantalum capacitor? If they are inserted with the wrong polarity they will get really hot ... sometime will actually burn. More likely explode, sending HARD ceramic fragments in ALL directions at HIGH velocity. |
"Carl R. Stevenson" wrote in message
... "TC Dufresne" wrote in message ... Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! Are you using a tantalum capacitor? If they are inserted with the wrong polarity they will get really hot ... sometime will actually burn. More likely explode, sending HARD ceramic fragments in ALL directions at HIGH velocity. |
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Active8 wrote:
In article , says... Tom: The hot cap has two likely possibilities: either the cap is shorted internally (an ohmeter check will show leakage) or you have bad diode(s) so AC is being applied to the cap. An ohmeter will show the diodes as bad. a backward diode will supply AC to the cap, also No, it won't. It will however supply the wrong polarity of DC, which could indeed cause these symptoms. I would think some other semiconductors might be running rather hot too if that were the case. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
Active8 wrote:
In article , says... Tom: The hot cap has two likely possibilities: either the cap is shorted internally (an ohmeter check will show leakage) or you have bad diode(s) so AC is being applied to the cap. An ohmeter will show the diodes as bad. a backward diode will supply AC to the cap, also No, it won't. It will however supply the wrong polarity of DC, which could indeed cause these symptoms. I would think some other semiconductors might be running rather hot too if that were the case. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
In article , lid
says... Active8 wrote: In article , says... Tom: The hot cap has two likely possibilities: either the cap is shorted internally (an ohmeter check will show leakage) or you have bad diode(s) so AC is being applied to the cap. An ohmeter will show the diodes as bad. a backward diode will supply AC to the cap, also No, it won't. oops. right. 1 diode backwards in a full wave (not a bridge) rectifier would short the xfrmr. It will however supply the wrong polarity of DC, which could indeed cause these symptoms. I would think some other semiconductors might be running rather hot too if that were the case. |
If you're using a diode bridge like most people do these days, and the
diode bridge output terminals (+ and -) are reversed, the filter capacitor would get hot and the receiver wouldn't work. As others have pointed out, an electrolytic capacitor with reverse voltage applied can catch fire or explode. Be careful when troubleshooting while the capacitor is unhappy. When you do find the problem, it would be a good idea to replace the capacitor. Roy Lewallen, W7EL Active8 wrote: In article , lid says... Active8 wrote: In article , says... Tom: The hot cap has two likely possibilities: either the cap is shorted internally (an ohmeter check will show leakage) or you have bad diode(s) so AC is being applied to the cap. An ohmeter will show the diodes as bad. a backward diode will supply AC to the cap, also No, it won't. oops. right. 1 diode backwards in a full wave (not a bridge) rectifier would short the xfrmr. It will however supply the wrong polarity of DC, which could indeed cause these symptoms. I would think some other semiconductors might be running rather hot too if that were the case. |
If you're using a diode bridge like most people do these days, and the
diode bridge output terminals (+ and -) are reversed, the filter capacitor would get hot and the receiver wouldn't work. As others have pointed out, an electrolytic capacitor with reverse voltage applied can catch fire or explode. Be careful when troubleshooting while the capacitor is unhappy. When you do find the problem, it would be a good idea to replace the capacitor. Roy Lewallen, W7EL Active8 wrote: In article , lid says... Active8 wrote: In article , says... Tom: The hot cap has two likely possibilities: either the cap is shorted internally (an ohmeter check will show leakage) or you have bad diode(s) so AC is being applied to the cap. An ohmeter will show the diodes as bad. a backward diode will supply AC to the cap, also No, it won't. oops. right. 1 diode backwards in a full wave (not a bridge) rectifier would short the xfrmr. It will however supply the wrong polarity of DC, which could indeed cause these symptoms. I would think some other semiconductors might be running rather hot too if that were the case. |
In article , says...
If you're using a diode bridge like most people do these days, and the diode bridge output terminals (+ and -) are reversed, the filter capacitor would get hot and the receiver wouldn't work. As others have pointed out, an electrolytic capacitor with reverse voltage applied can catch fire or explode. Be careful when troubleshooting while the capacitor is unhappy. When you do find the problem, it would be a good idea to replace the capacitor. i've had a couple explode, usually from overvoltage from a shorted series pass reg, i think. nice thing about (some? most?) electrolytics is that X that's scored into the top so it blows straight up. i'm glad my face wasn't over top of it, though. even with safety glasses, it's not something i'd like to experience. mike c Roy Lewallen, W7EL Active8 wrote: In article , lid says... Active8 wrote: In article , says... Tom: The hot cap has two likely possibilities: either the cap is shorted internally (an ohmeter check will show leakage) or you have bad diode(s) so AC is being applied to the cap. An ohmeter will show the diodes as bad. a backward diode will supply AC to the cap, also No, it won't. oops. right. 1 diode backwards in a full wave (not a bridge) rectifier would short the xfrmr. It will however supply the wrong polarity of DC, which could indeed cause these symptoms. I would think some other semiconductors might be running rather hot too if that were the case. |
In article , says...
If you're using a diode bridge like most people do these days, and the diode bridge output terminals (+ and -) are reversed, the filter capacitor would get hot and the receiver wouldn't work. As others have pointed out, an electrolytic capacitor with reverse voltage applied can catch fire or explode. Be careful when troubleshooting while the capacitor is unhappy. When you do find the problem, it would be a good idea to replace the capacitor. i've had a couple explode, usually from overvoltage from a shorted series pass reg, i think. nice thing about (some? most?) electrolytics is that X that's scored into the top so it blows straight up. i'm glad my face wasn't over top of it, though. even with safety glasses, it's not something i'd like to experience. mike c Roy Lewallen, W7EL Active8 wrote: In article , lid says... Active8 wrote: In article , says... Tom: The hot cap has two likely possibilities: either the cap is shorted internally (an ohmeter check will show leakage) or you have bad diode(s) so AC is being applied to the cap. An ohmeter will show the diodes as bad. a backward diode will supply AC to the cap, also No, it won't. oops. right. 1 diode backwards in a full wave (not a bridge) rectifier would short the xfrmr. It will however supply the wrong polarity of DC, which could indeed cause these symptoms. I would think some other semiconductors might be running rather hot too if that were the case. |
Everyone:
Thank you all for some great ideas! I will have more time tomorrow and see if I can make 'er go. This thing looks sooo easy, maybe that is why I am so frustrated with it!! I DO have schematics at my work, if anyone wants I could post tomorrow night......if it would help. best wishes Tom |
Everyone:
Thank you all for some great ideas! I will have more time tomorrow and see if I can make 'er go. This thing looks sooo easy, maybe that is why I am so frustrated with it!! I DO have schematics at my work, if anyone wants I could post tomorrow night......if it would help. best wishes Tom |
TC Dufresne wrote: Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. If you happened to insert the diodes in the fullwave rectifier reversed, you will have negative voltage at the input of your filter instead of positive voltage, and the input filter cap will conduct, and get hot (in a fullwave rectifier supply). If that happens you hope the filter cap DID short the supply to ground instead of passing the reverse polarized voltage on to other solid state devices! In time this will be rough on your transformer, too. You might want to recheck all the polarities of the diodes. Dick |
TC Dufresne wrote: Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. If you happened to insert the diodes in the fullwave rectifier reversed, you will have negative voltage at the input of your filter instead of positive voltage, and the input filter cap will conduct, and get hot (in a fullwave rectifier supply). If that happens you hope the filter cap DID short the supply to ground instead of passing the reverse polarized voltage on to other solid state devices! In time this will be rough on your transformer, too. You might want to recheck all the polarities of the diodes. Dick |
The only time I have seen caps get hot is when they are in backwards. If
there is too much ripple voltage, the cap can get warm. I would double check the wiring. I am not sure what method you used to make the VFO, so everything is up in the air. Pete Active8 wrote in message ... In article , says... "TC Dufresne" wrote in : Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. Any chance? Thanks in advance, Tom KCØGXX Gotta be a short somewhere. Filter caps ain't supposed to get hot, and yours is over rated to begin with. and when you run an electrolytic at a different WVDC than it's rated at, you can't be sure the capacitance is the same as marked on the can. but it should still filter and not get hot. you'd think something else would heat up if there was a short. should draw excessive current. unless it blew something and is now an open, then the cap shouldn't get hot any more. |
The only time I have seen caps get hot is when they are in backwards. If
there is too much ripple voltage, the cap can get warm. I would double check the wiring. I am not sure what method you used to make the VFO, so everything is up in the air. Pete Active8 wrote in message ... In article , says... "TC Dufresne" wrote in : Hi: I am having a problem with a "Simple Receiver for Beginners". It is from a design that was featured in an old (1981) ARRL handbook. It is a DC receiver, looks like alot of fun, but it isn't fun now. Here is the problem. 1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all, or on my Radio Shack freq counter. 2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?! The VFO is REALLY simple, it has one transistor and about 7 other parts. I can't seem to see what is wrong, and I have tried. I would sure appreciate some Elmer'ing here! ANY help would be much appreciated. I have the above mentioned tools at my command, a multimeter, and an old Heathkit "scope. (which I really don't know how to use. Any chance? Thanks in advance, Tom KCØGXX Gotta be a short somewhere. Filter caps ain't supposed to get hot, and yours is over rated to begin with. and when you run an electrolytic at a different WVDC than it's rated at, you can't be sure the capacitance is the same as marked on the can. but it should still filter and not get hot. you'd think something else would heat up if there was a short. should draw excessive current. unless it blew something and is now an open, then the cap shouldn't get hot any more. |
Everyone. I fixed the VFO. Its a good news/bad news thing.
Apparently my MPF102, the heart of the VFO, was either shorted out or not working. I removed it and replaced it with another. Good news..Cool! Now I have a good VFO in my receiver, I can hear it in my trusty DX-398. I adjust the RF coil so its low end (with maximum capacitance on my Hammerland 1-10pF variable capacitor-fully meshed) is around 7.00mHz. When I go to minimum capacitance (no meshed), I hear it only around 7.060mHz. According to specs, I should get a range from 7.00 to 7.2mHz. Why do I not get more range? As far as I know, all caps are up to spec as called for. Do I need to add/subtract capacitance somewhere? Thanks again, Tom |
Everyone. I fixed the VFO. Its a good news/bad news thing.
Apparently my MPF102, the heart of the VFO, was either shorted out or not working. I removed it and replaced it with another. Good news..Cool! Now I have a good VFO in my receiver, I can hear it in my trusty DX-398. I adjust the RF coil so its low end (with maximum capacitance on my Hammerland 1-10pF variable capacitor-fully meshed) is around 7.00mHz. When I go to minimum capacitance (no meshed), I hear it only around 7.060mHz. According to specs, I should get a range from 7.00 to 7.2mHz. Why do I not get more range? As far as I know, all caps are up to spec as called for. Do I need to add/subtract capacitance somewhere? Thanks again, Tom |
"TC Dufresne" wrote in message ... Everyone. I fixed the VFO. Its a good news/bad news thing. Apparently my MPF102, the heart of the VFO, was either shorted out or not working. I removed it and replaced it with another. Good news..Cool! Now I have a good VFO in my receiver, I can hear it in my trusty DX-398. I adjust the RF coil so its low end (with maximum capacitance on my Hammerland 1-10pF variable capacitor-fully meshed) is around 7.00mHz. When I go to minimum capacitance (no meshed), I hear it only around 7.060mHz. According to specs, I should get a range from 7.00 to 7.2mHz. Why do I not get more range? As far as I know, all caps are up to spec as called for. Do I need to add/subtract capacitance somewhere? Tolerances. Try removing a turn from the coil (if it is one you wound) and/or adding some capacitance. Leon -- Leon Heller, G1HSM http://www.geocities.com/leon_heller |
"TC Dufresne" wrote in message ... Everyone. I fixed the VFO. Its a good news/bad news thing. Apparently my MPF102, the heart of the VFO, was either shorted out or not working. I removed it and replaced it with another. Good news..Cool! Now I have a good VFO in my receiver, I can hear it in my trusty DX-398. I adjust the RF coil so its low end (with maximum capacitance on my Hammerland 1-10pF variable capacitor-fully meshed) is around 7.00mHz. When I go to minimum capacitance (no meshed), I hear it only around 7.060mHz. According to specs, I should get a range from 7.00 to 7.2mHz. Why do I not get more range? As far as I know, all caps are up to spec as called for. Do I need to add/subtract capacitance somewhere? Tolerances. Try removing a turn from the coil (if it is one you wound) and/or adding some capacitance. Leon -- Leon Heller, G1HSM http://www.geocities.com/leon_heller |
"TC Dufresne" wrote in message
... Everyone. I fixed the VFO. Its a good news/bad news thing. Apparently my MPF102, the heart of the VFO, was either shorted out or not working. I removed it and replaced it with another. Good news..Cool! Now I have a good VFO in my receiver, I can hear it in my trusty DX-398. I adjust the RF coil so its low end (with maximum capacitance on my Hammerland 1-10pF variable capacitor-fully meshed) is around 7.00mHz. When I go to minimum capacitance (no meshed), I hear it only around 7.060mHz. According to specs, I should get a range from 7.00 to 7.2mHz. Why do I not get more range? As far as I know, all caps are up to spec as called for. Do I need to add/subtract capacitance somewhere? How much did you allow for stray circuit capacitance? |
"TC Dufresne" wrote in message
... Everyone. I fixed the VFO. Its a good news/bad news thing. Apparently my MPF102, the heart of the VFO, was either shorted out or not working. I removed it and replaced it with another. Good news..Cool! Now I have a good VFO in my receiver, I can hear it in my trusty DX-398. I adjust the RF coil so its low end (with maximum capacitance on my Hammerland 1-10pF variable capacitor-fully meshed) is around 7.00mHz. When I go to minimum capacitance (no meshed), I hear it only around 7.060mHz. According to specs, I should get a range from 7.00 to 7.2mHz. Why do I not get more range? As far as I know, all caps are up to spec as called for. Do I need to add/subtract capacitance somewhere? How much did you allow for stray circuit capacitance? |
TC Dufresne wrote:
Good news..Cool! Now I have a good VFO in my receiver, I can hear it in my trusty DX-398. I adjust the RF coil so its low end (with maximum capacitance on my Hammerland 1-10pF variable capacitor-fully meshed) is around 7.00mHz. When I go to minimum capacitance (no meshed), I hear it only around 7.060mHz. According to specs, I should get a range from 7.00 to 7.2mHz. You've probably got more parasitic capacitance in the circuit than that 10 pF represents. That means your capacitance range is more like 41-50 pF (figure picked out of the air) than 1-10 pF. Did the article suggest any special construction techniques to minimize this? -- The function of an asshole is to emit quantities of crap. Spammers do a very good job of that. However, I do object to my inbox being a spammer's toilet bowl. -- Walter Dnes |
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