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#41
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I admire your improvements but it seems that since most all toob sets are collecibles, changing the circuitry in any way would cause them to be no longer "stock from the factory", i.e. they'd be modified. And if you did modify a collectible radio without telling the buyer, it would be grounds for calling you a dishonest seller. True to some extent, though it would be an easily reversed mod. The real reason manufacturers didn't install this circuit was that it would cost one resistor and one capacitor extra, plus the labor of wiring it in. In a consumer product, every fraction of a penny you can shave becomes significant after millions of units made. And most people didn't seem to mind the extra distortion. And as you said, there's probably not much point in it for most cheaper radios.'''''''''''''''''''''''''''''''''''''''''' ''''' I have to agree with the sentiments here about collectables, but one reason I don't bother collecting many AM radios is their poor sound, and to improve them, you have to modify them, and I do just that if there is no other way to repair them, and if an owner is happy about it. Hopefully they'll run well for another 50 years, and the next repairist will have to make similar decisions I have. This same technique should help the AM section of a tuner sound better, in those receivers where AM was an afterthought ("Oh, just throw in a 6BE6, make that 6BA6 FM IF amp also do the AM IF, and use the diodes of that 6AV6 over there..."). |
#42
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John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: In article , wrote: Frank Dresser wrote: I think the fact that the synchrodyne never became at all popular as an AM radio detector in the tube era means something. If I recall correctly, I read that synchrodyne detectors would howl until they sync'd. Also, the phasing would have to be perfect to get good demodulation from normal double sideband AM. Now appearing on alt.binaries.pictures.radio Syncrhodyne three tube receiver from the April 1951 edition of Radio News. Looks more like the article came from Radio Electronics than from Radio News, no way of confirming the April 1951 date. The synchrodyne schematic never appeared on ABPR when I looked out for it. Too bad, it's an Australian design from "down under". Not all the postings of binaries are able to fit the pouches of the trained kangaroo couriers who take binaries south, from the north, to where they originated. So the ABPR news group I see from my povety stricken ISP often lacks the illuminations of the american views. But this article gives the Australian view, being written by one John W. Straede, who was a "Lecturer in electronics and electro-acoustics, Melbourne Technical College, Australia." There have been some bright Oz stars in the world of electronics over the years. I have since recieved a copy of the synchrodyne from someone privately. It basically uses two tubes to make the tuner, followed by a 6V6 audio output and a there was a rectifier tube and has similarities to other synchrodynes of the era, although it has a variable FB control to the snyc tube oscilator-detector. Its similar to another circuit I have. I will have to scan and post a few for ppl soon. These synchrodynes were slightly simpler circuits than a superhet for the same number of tubes. Patrick Turner. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#43
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In article , Robert Casey
wrote: Looks like an envelope detector can perform well. See my post over in a.b.p.radio with diagrams. Below is the text without the diagrams: After reading Brian's web page on modulation acceptance http://n2.net/k6sti/ma.htm I decided to try some simulations of the AA5 envelope detector and got results similar to Brian's page before any mods. Then I decided to try using that second diode 12AV6's and 12SQ7s come with. That second diode will do the AVC function, thus leaving the first diode doing only the audio envelope detection. As Brian described, the AVC filter cap tends to back bias the envelope detector in convential AA5 circuits, causing clipping at low valleys of the modulation. Moving the AVC to its own diode gets rid of that problem. Also reducing the envelope detector resistance value to 150K from 500K also helps (you will lose about 3dB of audio, which isn't that much). And with the AVC now separate this will only mildly affect the AVC (by about 1dB). And when you move the AVC pick-off cap to the plate of the IF tube, this drop goes away. This was observed in a real radio I tried this mod in. There are a couple of other interesting effects that taking the AGC voltage from the primary of the last IFT has. At first as we tune toward the center of the pass band the voltage across the primary of the IFT increases as expected, but then as we get close to the center the voltage starts dropping, reaching a minimum when the carrier is centered in the pass band. The AGC voltage follows and the effect is to sharpen the apparent aural tuning characteristic because the decrease in AGC voltage when center tuned increases the gain at that point. If a tuning eye is fitted to this type of radio it is almost universally connected to the audio detector rather than the AGC detector, which is unfortunate from the AC loading point of view as the filter for the eye tube often presents an even worse AC load than the AGC circuit. For the longest time I couldn't figure out why the designers of most of these radios made this apparently dumb mistake in connecting the eye tube, but I eventually realized that if the eye tube was connected to the AGC line it would have an unnatural and non intuitive visual tuning characteristic, the dip at center tune might confuse the average consumer, although a radio nut would probably delight in the effect as an indicator of exact tuning. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#44
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If a tuning eye is fitted to this type of radio it is almost universally connected to the audio detector rather than the AGC detector, which is unfortunate from the AC loading point of view as the filter for the eye tube often presents an even worse AC load than the AGC circuit. Is that because of a resistor-capacitor filter between the audio detector and the grid of the eye tube? I would just feed the audio directly to the eye tube grid, and place a cap on the eye tube triode plate to do the filtering there. |
#45
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John Byrns wrote: In article , Robert Casey wrote: Looks like an envelope detector can perform well. See my post over in a.b.p.radio with diagrams. Below is the text without the diagrams: After reading Brian's web page on modulation acceptance http://n2.net/k6sti/ma.htm I decided to try some simulations of the AA5 envelope detector and got results similar to Brian's page before any mods. Then I decided to try using that second diode 12AV6's and 12SQ7s come with. That second diode will do the AVC function, thus leaving the first diode doing only the audio envelope detection. As Brian described, the AVC filter cap tends to back bias the envelope detector in convential AA5 circuits, causing clipping at low valleys of the modulation. Moving the AVC to its own diode gets rid of that problem. Also reducing the envelope detector resistance value to 150K from 500K also helps (you will lose about 3dB of audio, which isn't that much). And with the AVC now separate this will only mildly affect the AVC (by about 1dB). And when you move the AVC pick-off cap to the plate of the IF tube, this drop goes away. This was observed in a real radio I tried this mod in. There are a couple of other interesting effects that taking the AGC voltage from the primary of the last IFT has. At first as we tune toward the center of the pass band the voltage across the primary of the IFT increases as expected, but then as we get close to the center the voltage starts dropping, reaching a minimum when the carrier is centered in the pass band. The AGC voltage follows and the effect is to sharpen the apparent aural tuning characteristic because the decrease in AGC voltage when center tuned increases the gain at that point. If a tuning eye is fitted to this type of radio it is almost universally connected to the audio detector rather than the AGC detector, which is unfortunate from the AC loading point of view as the filter for the eye tube often presents an even worse AC load than the AGC circuit. For the longest time I couldn't figure out why the designers of most of these radios made this apparently dumb mistake in connecting the eye tube, but I eventually realized that if the eye tube was connected to the AGC line it would have an unnatural and non intuitive visual tuning characteristic, the dip at center tune might confuse the average consumer, although a radio nut would probably delight in the effect as an indicator of exact tuning. But there is rarely a dip in the tuning character of most AM sets. But I often derive the AGC voltage separately from a small cap&diode off the anode of the last IF amp, where more AGC voltage is available because the envelope amplitude is greater than at the secondary of the IFT. The IFTs can be aligned with a VTM attatched to the AGC voltage, and the tuning done to generate the highest AGC. There is usually only one peak, and if two tuning peaks are seen, its often because of mis-alignment of the IF, or the circuit has been configured for wide BW and has slightly over critically coupled IFTs, which rarely occurs in any ordinary domestic AM set. Some sets with variable IF selectivity had a switched tertiary on IFT1, so first you tuned with the eye for max peak, then switched the tertiary in for wide band audio, or for locals. Patrick Turner. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#46
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Robert Casey wrote: If a tuning eye is fitted to this type of radio it is almost universally connected to the audio detector rather than the AGC detector, which is unfortunate from the AC loading point of view as the filter for the eye tube often presents an even worse AC load than the AGC circuit. Is that because of a resistor-capacitor filter between the audio detector and the grid of the eye tube? I would just feed the audio directly to the eye tube grid, and place a cap on the eye tube triode plate to do the filtering there. The loading effects are removed when a CF buffer is connected to the last IFT sec, and the diode detection and eye drive voltage is derived from the low impedance cathode of the CF. Patrick Turner. |
#47
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I have to agree with the sentiments here about collectables, but one reason I don't bother collecting many AM radios is their poor sound, and to improve them, you have to modify them, and I do just that if there is no other way to repair them, and if an owner is happy about it. Hopefully they'll run well for another 50 years, and the next repairist will have to make similar decisions I have. Which leads me to the question, will there be any AM stations to listen to in fifty years? And, after reading and looking over Randy's excellent story about the 5 KW AM transmitter, I wonder, will all postings like his just be a memory in fifty years? If so, the future generation will be missing out on a great deal of old, but really cool technology. Regards to all, Tom |
#48
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t.hoehler wrote:
someone else wrote (attributes lost): I have to agree with the sentiments here about collectables, but one reason I don't bother collecting many AM radios is their poor sound, and to improve them, you have to modify them, and I do just that if there is no other way to repair them, and if an owner is happy about it. Hopefully they'll run well for another 50 years, and the next repairist will have to make similar decisions I have. Which leads me to the question, will there be any AM stations to listen to in fifty years? And, after reading and looking over Randy's excellent story about the 5 KW AM transmitter, I wonder, will all postings like his just be a memory in fifty years? If so, the future generation will be missing out on a great deal of old, but really cool technology. Good question. It's hard to say where the MW BCB band will be in 50 years. Will there still be commercial stations? Will they broadcast in digital rather than AM? My son is very interested in getting the SSTRAN AM transmitter and transmit music and alternative programming he is now broadcasting on the Internet. He'd simply have an audio out of his server's sound card to go to the transmitter. Maybe in 50 years the band will be covered by more low-power broadcasting like what my son is interested in? (he's intrigued with setting up transmitters every mile in the neighborhood.) Who knows... Jon |
#49
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In article , Robert Casey
wrote: If a tuning eye is fitted to this type of radio it is almost universally connected to the audio detector rather than the AGC detector, which is unfortunate from the AC loading point of view as the filter for the eye tube often presents an even worse AC load than the AGC circuit. Is that because of a resistor-capacitor filter between the audio detector and the grid of the eye tube? Yes I would just feed the audio directly to the eye tube grid, and place a cap on the eye tube triode plate to do the filtering there. If you feed the audio directly to the eye tube grid, and place a cap on the eye tube triode plate you would probably get serious flickering of the eye angle with modulation, as the charge and discharge time constants would be quite different resulting rectification of the audio. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#50
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In article , Patrick Turner
wrote: John Byrns wrote: There are a couple of other interesting effects that taking the AGC voltage from the primary of the last IFT has. At first as we tune toward the center of the pass band the voltage across the primary of the IFT increases as expected, but then as we get close to the center the voltage starts dropping, reaching a minimum when the carrier is centered in the pass band. The AGC voltage follows and the effect is to sharpen the apparent aural tuning characteristic because the decrease in AGC voltage when center tuned increases the gain at that point. If a tuning eye is fitted to this type of radio it is almost universally connected to the audio detector rather than the AGC detector, which is unfortunate from the AC loading point of view as the filter for the eye tube often presents an even worse AC load than the AGC circuit. For the longest time I couldn't figure out why the designers of most of these radios made this apparently dumb mistake in connecting the eye tube, but I eventually realized that if the eye tube was connected to the AGC line it would have an unnatural and non intuitive visual tuning characteristic, the dip at center tune might confuse the average consumer, although a radio nut would probably delight in the effect as an indicator of exact tuning. But there is rarely a dip in the tuning character of most AM sets. That was my point and is the reason sets with a tuning eye virtually always connect the tuning eye to the audio detector, not to the AGC rectifier. But I often derive the AGC voltage separately from a small cap&diode off the anode of the last IF amp, where more AGC voltage is available because the envelope amplitude is greater than at the secondary of the IFT. The IFTs can be aligned with a VTM attatched to the AGC voltage, and the tuning done to generate the highest AGC. Good luck with this approach, it is likely to result in misalignment, this sort of set should be aligned either with a modulated oscillator and audio VTVM on the output, or by measuring the detector voltage. An interesting experiment on a set that takes the AGC from the IFT primary is to short the secondary of the IFT with a clip lead, you will observe an increase in AGC voltage when the secondary is shorted, this is the same effect that causes the dip in primary voltage when properly tuned. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#51
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"t.hoehler" wrote: I have to agree with the sentiments here about collectables, but one reason I don't bother collecting many AM radios is their poor sound, and to improve them, you have to modify them, and I do just that if there is no other way to repair them, and if an owner is happy about it. Hopefully they'll run well for another 50 years, and the next repairist will have to make similar decisions I have. Which leads me to the question, will there be any AM stations to listen to in fifty years? Yes, unless the Got of Triodes changes the laws of physics, and dissallows RF propagation at the BCB. And, after reading and looking over Randy's excellent story about the 5 KW AM transmitter, I wonder, will all postings like his just be a memory in fifty years? In 50 years going back to recent postings about AM radios will be difficult. If so, the future generation will be missing out on a great deal of old, but really cool technology. Someone somewhere will be working from first principles, and designing and building AM radios without any assistance from our postings, or RDH4. What they miss out on is their worry, not mine. The 4 year old now will have potentially a lot to look forward to, cures for cancer, maybe a longer lifespan, slimming pills that work without side effects to allow huge supersize macs to be eaten without getting fat, or guilty about it, cars that use hydrogen, perhaps cheaper energy, and lower winter heating bills. He'll still have to deal with BS and face death and taxes. But in 10,054, thing will be *very* different, and we humans might have genetically engineered themselves into a vastly different form indeed, and complete with AM radio implants at birth from a test tube, or a cloning dish, and the ability to eat rubbish from the vast landfill deposits which will have nearly covered the planet entirely. But infinite development is impossible with finite resources. Methinks it might be just dusty in 100,054. So if we will be gone by then, why are we here now? Cool might be rare word to describe the world in 2054, when temperatures will have climbed alarmingly, and mainly because desperate western nation people have bought even larger air-conditioning units. The future may occur and be able to be considered, if we don't poison ourselves and all the other species first. The current rate of species extinctions is much faster than at any other time in the history of life on our bloomin dear little blue planet. Is life making the same mistakes as us on the millions of other bloomin little blue planets in the universe? Do they listen to rock and roll and talkback shows to cool the soul too? Patrick Turner. Regards to all, Tom |
#52
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John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: There are a couple of other interesting effects that taking the AGC voltage from the primary of the last IFT has. At first as we tune toward the center of the pass band the voltage across the primary of the IFT increases as expected, but then as we get close to the center the voltage starts dropping, reaching a minimum when the carrier is centered in the pass band. The AGC voltage follows and the effect is to sharpen the apparent aural tuning characteristic because the decrease in AGC voltage when center tuned increases the gain at that point. If a tuning eye is fitted to this type of radio it is almost universally connected to the audio detector rather than the AGC detector, which is unfortunate from the AC loading point of view as the filter for the eye tube often presents an even worse AC load than the AGC circuit. For the longest time I couldn't figure out why the designers of most of these radios made this apparently dumb mistake in connecting the eye tube, but I eventually realized that if the eye tube was connected to the AGC line it would have an unnatural and non intuitive visual tuning characteristic, the dip at center tune might confuse the average consumer, although a radio nut would probably delight in the effect as an indicator of exact tuning. But there is rarely a dip in the tuning character of most AM sets. That was my point and is the reason sets with a tuning eye virtually always connect the tuning eye to the audio detector, not to the AGC rectifier. It may not be critical where you connect the feed to the magic eye in many old radios. But I often derive the AGC voltage separately from a small cap&diode off the anode of the last IF amp, where more AGC voltage is available because the envelope amplitude is greater than at the secondary of the IFT. The IFTs can be aligned with a VTM attatched to the AGC voltage, and the tuning done to generate the highest AGC. Good luck with this approach, it is likely to result in misalignment, this sort of set should be aligned either with a modulated oscillator and audio VTVM on the output, or by measuring the detector voltage. It works, and its not neccesary to use a modulated carrier. A carrier of exactly the 455 kHz can even be cap coupled via 10 pF applied to the antenna terminal with the set tuned as low as it will go, and the IF tuned up starting on the last IF and moving forward, and repeated, to generate the highest AGC. Its also possible to shunt the AGC across the 0.047 uF cap at the tube grid connections, and keep the input signal low, and then tune for the highest AGC voltage or detector voltage. This method avoids connecting a lead via a cap to the anode of the mixer with a 455 kHz signal applied, since such a connection upsets the C of the first IF, and mistunes it. The shunted AGC allows the RF, mixer and IF tubes to operate with high gain so to stay away from over load, the input signal should be low. Using a modulated IF signal with 95% modulation with 100 Hz is a good idea, because as the IF amp becomes overloaded either with negligible bias, or with a lot of applied AGC, so to will the audio recovered. I like to tune up old radios with a CRO attached and a high % of modulation, and with AGC connected, because the set always works with it connected, and when things are right, including the adjustment of R loading for the diode detector, there should be at least 5vrms of audio recovered from the detector. This means about 28 volts peak to peak exist at the envelope entering the diode detector, and maybe twice that at the IF amp anode, if the IF transformer losses were 6 dB. If the IF amp is a typical 6BA6 etc, and has idle Ia = 3 mA, and Ea = 200v at the middle common AGC voltage, and ideal working point for a load of say 50 kohms, then the load line analysis reveals about 300 volts peak to peak is the maximum voltage swing. To cope with 100% modulation, the p-p modulation above and below the horizontal axix is 150v, and peak modulation = 75v, so there is 53 vrms theoretically available if the IF amp was fairly linear, which is won't be, ever, even if its a sharp cut off pentode and not the usual remote cut off type commonly used. At such high output voltages, the IF amp becomes quite distorted, and its clearly seen on the the audio recovered from the detector. Many old radios produce about 3 vrms average audio signal at their detectors, and the IF amp isn't working too hard, and the contribution to the set's thd by the IF tube isn't too high. An interesting experiment on a set that takes the AGC from the IFT primary is to short the secondary of the IFT with a clip lead, you will observe an increase in AGC voltage when the secondary is shorted, this is the same effect that causes the dip in primary voltage when properly tuned. I have not tried that, but common sense tells me that a shorted IFT sec will reduce the load seen by the IF amp severely, and reduce the envelope amplitute at the IFT pri, or the tube anode, and thus generate less AGC, despite the increase in GM of all the tubes subject to AGC control. What happens depends on the number of preceeding stages and the gain reaction to AGC. Patrick Turner. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#53
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In article , Patrick Turner
wrote: John Byrns wrote: In article , Patrick Turner wrote: But there is rarely a dip in the tuning character of most AM sets. That was my point and is the reason sets with a tuning eye virtually always connect the tuning eye to the audio detector, not to the AGC rectifier. It may not be critical where you connect the feed to the magic eye in many old radios. Then why is the eye tube always connected to the audio detector and not the AGC rectifier? Connecting it to the audio detector reduces the modulation acceptance, so I was surprised when I first noticed that the eye tube in a set I was looking at was connected to the audio detector, and not the AGC rectifier where it wouldn't reduce the modulation acceptance. I thought this was just a design blunder in the particular radio I was looking at, so I looked up as many schematics as I could find for eye tube radios with a separate AGC rectifier off the primary of the IFT, and I was surprised to find that they all were wired the same way. The only explanation that I can think of for this is that it is done so the eye tube will provide a sharper tuning indication, without a possible dip at the center. But I often derive the AGC voltage separately from a small cap&diode off the anode of the last IF amp, where more AGC voltage is available because the envelope amplitude is greater than at the secondary of the IFT. The IFTs can be aligned with a VTM attatched to the AGC voltage, and the tuning done to generate the highest AGC. Good luck with this approach, it is likely to result in misalignment, this sort of set should be aligned either with a modulated oscillator and audio VTVM on the output, or by measuring the detector voltage. It works, and its not neccesary to use a modulated carrier. A carrier of exactly the 455 kHz can even be cap coupled via 10 pF applied to the antenna terminal with the set tuned as low as it will go, and the IF tuned up starting on the last IF and moving forward, and repeated, to generate the highest AGC. Its also possible to shunt the AGC across the 0.047 uF cap at the tube grid connections, and keep the input signal low, and then tune for the highest AGC voltage or detector voltage. But this will cause miss tuning of the last IFT when the set uses a separate AGC rectifier connected to the primary of the transformer, as a result of the dip in the primary response. An interesting experiment on a set that takes the AGC from the IFT primary is to short the secondary of the IFT with a clip lead, you will observe an increase in AGC voltage when the secondary is shorted, this is the same effect that causes the dip in primary voltage when properly tuned. I have not tried that, but common sense tells me that a shorted IFT sec will reduce the load seen by the IF amp severely, and reduce the envelope amplitute at the IFT pri, or the tube anode, and thus generate less AGC, But this is one of those situations where "common sense" is wrong, and was the first amazing lesson I learned from an early mentor, a lesson I have never forgotten. Try it, connect the primary of an IF transformer to some kind of impedance measuring device like an RX meter and see what the primary impedance looks like with and without the secondary shorted. If the primary and secondary Qs are the same and if the coupling is critical the primary resistance will double when the secondary is shorted, if the coupling is not critical the primary resistance will change by a factor other than 2 X. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#54
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"John Byrns" wrote in message ... [snip] Then why is the eye tube always connected to the audio detector and not the AGC rectifier? [snip] The Radiotron Designer's Handbook 3 (p. 225) brings up this topic. Basically they say, in radios with simple AVC seperated from the audio detector, it's preferable to connect the eye tube to the AVC line. If the radio uses delayed AVC, it's preferable to connect the eye tube to the audio detector (through a decoupling network) if eye tube action is wanted on weak signals. Frank Dresser |
#55
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In article ,
"Frank Dresser" wrote: "John Byrns" wrote in message ... [snip] Then why is the eye tube always connected to the audio detector and not the AGC rectifier? [snip] The Radiotron Designer's Handbook 3 (p. 225) brings up this topic. Basically they say, in radios with simple AVC seperated from the audio detector, it's preferable to connect the eye tube to the AVC line. If the radio uses delayed AVC, it's preferable to connect the eye tube to the audio detector (through a decoupling network) if eye tube action is wanted on weak signals. Good point, I forgot about that aspect of the problem, which of course also applies to delayed AGC developed off the audio detector diode. I can't remember whether or not I was checking for delayed AGC when I was rummaging through the schematics of eye tube radios to see if I could find any where the eye tube was connected to the AGC line and the AGC rectifier was connected to the IFT primary. Delayed AGC seems relatively uncommon however. Does anyone know of any examples of eye tube radios that take the AGC from the IFT, without delay, and have the eye tube connected to the AGC line? Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#56
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Jeffrey D Angus writes:
Now appearing on alt.binaries.pictures.radio Syncrhodyne three tube receiver from the April 1951 edition of Radio News. Jeff Did anyone actually see this schematic? I can't find it. Steve. -- Steven D. Swift, , http://www.novatech-instr.com NOVATECH INSTRUMENTS, INC. P.O. Box 55997 206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA |
#57
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"Steven Swift" a écrit dans le message
Did anyone actually see this schematic? I can't find it. Right he http://techpreservation.dyndns.org/b...files/?C=N&O=A Synchro-1 and -2.jpg Syl |
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