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#1
Posted to rec.audio.tubes
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Kenwood KW55
Anybody out there work on one of these?
http://www.mcmlv.org/Archive/HiFi/KW55U.pdf I'm about to endeavor restoring one of these, and I'm wondering if anybody did any mods to this receiver. Omer |
#2
Posted to rec.audio.tubes
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Kenwood KW55
On Jul 10, 4:31*pm, "Omer S" wrote:
Anybody out there work on one of these? http://www.mcmlv.org/Archive/HiFi/KW55U.pdf I'm about to endeavor restoring one of these, and I'm wondering *if anybody did any mods to this receiver. Omer Hi Omer, I once completely restored and improved a tubed Kenwood AM-FM receiver in about 1996. A guy here bought one made for the Japanese market which had TWO am turners because in Japan when stereo FM began they simply put one channel in AM because they only had one mono channel at first. There were tuning eyes for the tuning because you couldn't tune the AM channel for FM stereo by ear with both channels running. The second AM tuner was provided so that once the FM was tuned on favourite you could leave it untouched then switch to listen to the AM only station. It was all very complex and had no MPX converter for producing FM stereo from a multiplexed carrier with supressed 38kHz sub-carrier. I installed a kit which had a chip for MPX, and removed the extra AM tuner used for a "stereo" FM channel. The spare sockets were used to make a much better AM detector for the AM section which my customer wanted to be equal to the best available with 9kHz of audio bandwidth, 9kHz whistle filter, and low THD/IMD. Your Kenwood is a different circuit to the one I worked on. But if you look at Pioneer, Fisher and Kenwood, there is much similarity in the AM and FM front ends, IF strips, and diode detectors. Yours has tubed MPX decoding but has the usual diode matrix for L&R channel production. Some of these things work well, and some work very badly, and to align these sets and keep them aligned and running as quietly as possible is a labour of love. Previous to my work on a cutomer's AM/FM Kenwood, I had bought A Trio receiver with single ended audio output stages with a single 6BQ5 each. I gave up trying to get the MPX decoder to work properly after a week of ****ing around. I visited local university and college library archives where I found old books with the theory and basic circuits which were followed by many makers including Kenwood for the days before tubes were retired. I copied and studied many pages from books and applied what I began to understand when I redesigned the Trio. I got rid of the audio output stages off the Trio and made the chassis just for AM and FM stereo. The MPX decoder is somewhat a critical item if you want good imaging, low noise, and low distortions. In the days of tubed MPX to comply with the GE-Zenith format of stereo multiplexing, the first MPX decoders propsed and recomended by RCA used 3 tubes, and Quad made add on addaptors with just 3 germanium transistors. Usually such primitive MPX circuits offer increased noise and distortion when using stereo compared to just having mono function. In all my experience since 14 years ago I have found most generic MPX decoder chips work better than any arrangement of tubes and tuned coils and diode matrixes. I built what is probably my final version of an MPX decoder based on the Quad transistor circuit knowing that with tubes instead of crummy germanium transistors there was a chance I could get better dynamic range performance and less distortion. But the noise is still worse than most chip sets. It sure sounds well though. The filtering of the switching noise and remnants of 38kHz and filtering of the 19kHz pilot tone take real expertise. Many tubed AM-FM sets can have wonderful looking schematics which withstand visual scrutiny but when you make the measurements which you NEED to make with this sort of gear they just don't always measure very well at all. Measuring AM/FM radio performance requires that you KNOW how and why each tube and R and C and L has been used where it is. When I set out to learn enough to service any AM receiver dumped in my bench for a fix, I was alone. I did know a few fellows in the local amateur radio group. I used to go to their swap meets and sales days and found them to be a nice lot but when I mentioned FM, one guy retorted, "****, FM?, Hell, we are always tryin to keep the frequency stable, and not let it vary". I didn't get anywhere with my questions about FM radio of any type. Some amateurs were using narrow band FM, but none here did. Also the Army used narrow band FM at around citizen radio 27MHz. It was secure in 1955, and I have a beuatifully made transceiver from that time, needed a jeep and two guys to get it around. But now is all entirely a digital world for the armed forces, and not a tube to be seen anywhere. Tubes were at about their limit when TV came along and with FM stereo. Once you have 20 tubes on a chassis all doing things that are high- falootin and not easily understood by an average man in the street, and you have a tendency for hot runninng bits and peices to malfunction in circuits designed by been counters then you find the Kenwoods et all will need to be serviced once a year. After tubes came discrete circuits using individual silicon bjts which suddenly became very reliable and cheep. The myriad of slug tuned LC coil units in cans suddenly became much smaller. This suited FM but usually made the sound much worse in AM. Then came the integrated circuit, and these mostly worked better than anything with discrete bjts or tubes. I have tried to find out exactly what is inside a typical MPX decoder chip which performs the stereo decoding function so I could emulate the circuit using several twin triodes, but I never found an exact schematic of what actually does the business. Whatever it is, it works better than a diode matrix, even though that is simpler. I recall there were things like a pair of cross coupled differential pairs. But exactly how they were arranged I never found out. Tim DeParavicini put out a schematic for a "tubed MPX decoder" but what it really contained was a 14 pin generic chip with a tubed output stage. It was anything but a fully tubed MPX decoder and one which had no diode matrix. Nobody I know has ever produced a fully tubed MPX decoder without diodes, even tube diodes. I have a few thoughts at my website.... http://www.turneraudio.com.au/am-fm-...ex-decoder.htm Farnarkling around with AM-FM radios can occupy a man for weeks, months or years. I spent lots of time on all that but rarely have any need to use my knowledge which is now a bit rusty. I am now not sure what a "quieting factor" is. But I do know one had to use a lot of very good limiting to make FM reception quiet. The "limiter" stages using 6AU6 are very important, and two such stages are best, although usually there is often only one. The limiter stage is an SE pentode gain stage which operates normally like any linear gain stage which has maybe 30dB over drive. It works in a state of being wildly over driven and any changes in IF frequency must not cause any output amplitude changes and where any amplitude changes to the IF exist, they are to be eliminated as much as possible along with any noise. FM is thus a quiet medium compared to AM. But you can make a heck of an effort to get an FM receiver to work and still find there is an annoying rustling noise from a receiver, and that the SNR is only a lousy 45dB. Its usually easy to get good mono with low noise from a tubed FM tuner, but more difficult if you add the MPX unit. Patrick Turner. |
#3
Posted to rec.audio.tubes
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Kenwood KW55
"Patrick Turner" wrote in message ... On Jul 10, 4:31 pm, "Omer S" wrote: Anybody out there work on one of these? http://www.mcmlv.org/Archive/HiFi/KW55U.pdf I'm about to endeavor restoring one of these, and I'm wondering if anybody did any mods to this receiver. Omer Hi Omer, I once completely restored and improved a tubed Kenwood AM-FM receiver in about 1996. A guy here bought one made for the Japanese market which had TWO am turners because in Japan when stereo FM began they simply put one channel in AM because they only had one mono channel at first. There were tuning eyes for the tuning because you couldn't tune the AM channel for FM stereo by ear with both channels running. The second AM tuner was provided so that once the FM was tuned on favourite you could leave it untouched then switch to listen to the AM only station. It was all very complex and had no MPX converter for producing FM stereo from a multiplexed carrier with supressed 38kHz sub-carrier. I installed a kit which had a chip for MPX, and removed the extra AM tuner used for a "stereo" FM channel. The spare sockets were used to make a much better AM detector for the AM section which my customer wanted to be equal to the best available with 9kHz of audio bandwidth, 9kHz whistle filter, and low THD/IMD. Your Kenwood is a different circuit to the one I worked on. But if you look at Pioneer, Fisher and Kenwood, there is much similarity in the AM and FM front ends, IF strips, and diode detectors. Yours has tubed MPX decoding but has the usual diode matrix for L&R channel production. Some of these things work well, and some work very badly, and to align these sets and keep them aligned and running as quietly as possible is a labour of love. Previous to my work on a cutomer's AM/FM Kenwood, I had bought A Trio receiver with single ended audio output stages with a single 6BQ5 each. I gave up trying to get the MPX decoder to work properly after a week of ****ing around. I visited local university and college library archives where I found old books with the theory and basic circuits which were followed by many makers including Kenwood for the days before tubes were retired. I copied and studied many pages from books and applied what I began to understand when I redesigned the Trio. I got rid of the audio output stages off the Trio and made the chassis just for AM and FM stereo. The MPX decoder is somewhat a critical item if you want good imaging, low noise, and low distortions. In the days of tubed MPX to comply with the GE-Zenith format of stereo multiplexing, the first MPX decoders propsed and recomended by RCA used 3 tubes, and Quad made add on addaptors with just 3 germanium transistors. Usually such primitive MPX circuits offer increased noise and distortion when using stereo compared to just having mono function. In all my experience since 14 years ago I have found most generic MPX decoder chips work better than any arrangement of tubes and tuned coils and diode matrixes. I built what is probably my final version of an MPX decoder based on the Quad transistor circuit knowing that with tubes instead of crummy germanium transistors there was a chance I could get better dynamic range performance and less distortion. But the noise is still worse than most chip sets. It sure sounds well though. The filtering of the switching noise and remnants of 38kHz and filtering of the 19kHz pilot tone take real expertise. Many tubed AM-FM sets can have wonderful looking schematics which withstand visual scrutiny but when you make the measurements which you NEED to make with this sort of gear they just don't always measure very well at all. Measuring AM/FM radio performance requires that you KNOW how and why each tube and R and C and L has been used where it is. When I set out to learn enough to service any AM receiver dumped in my bench for a fix, I was alone. I did know a few fellows in the local amateur radio group. I used to go to their swap meets and sales days and found them to be a nice lot but when I mentioned FM, one guy retorted, "****, FM?, Hell, we are always tryin to keep the frequency stable, and not let it vary". I didn't get anywhere with my questions about FM radio of any type. Some amateurs were using narrow band FM, but none here did. Also the Army used narrow band FM at around citizen radio 27MHz. It was secure in 1955, and I have a beuatifully made transceiver from that time, needed a jeep and two guys to get it around. But now is all entirely a digital world for the armed forces, and not a tube to be seen anywhere. Tubes were at about their limit when TV came along and with FM stereo. Once you have 20 tubes on a chassis all doing things that are high- falootin and not easily understood by an average man in the street, and you have a tendency for hot runninng bits and peices to malfunction in circuits designed by been counters then you find the Kenwoods et all will need to be serviced once a year. After tubes came discrete circuits using individual silicon bjts which suddenly became very reliable and cheep. The myriad of slug tuned LC coil units in cans suddenly became much smaller. This suited FM but usually made the sound much worse in AM. Then came the integrated circuit, and these mostly worked better than anything with discrete bjts or tubes. I have tried to find out exactly what is inside a typical MPX decoder chip which performs the stereo decoding function so I could emulate the circuit using several twin triodes, but I never found an exact schematic of what actually does the business. Whatever it is, it works better than a diode matrix, even though that is simpler. I recall there were things like a pair of cross coupled differential pairs. But exactly how they were arranged I never found out. Tim DeParavicini put out a schematic for a "tubed MPX decoder" but what it really contained was a 14 pin generic chip with a tubed output stage. It was anything but a fully tubed MPX decoder and one which had no diode matrix. Nobody I know has ever produced a fully tubed MPX decoder without diodes, even tube diodes. I have a few thoughts at my website.... http://www.turneraudio.com.au/am-fm-...ex-decoder.htm Farnarkling around with AM-FM radios can occupy a man for weeks, months or years. I spent lots of time on all that but rarely have any need to use my knowledge which is now a bit rusty. I am now not sure what a "quieting factor" is. But I do know one had to use a lot of very good limiting to make FM reception quiet. The "limiter" stages using 6AU6 are very important, and two such stages are best, although usually there is often only one. The limiter stage is an SE pentode gain stage which operates normally like any linear gain stage which has maybe 30dB over drive. It works in a state of being wildly over driven and any changes in IF frequency must not cause any output amplitude changes and where any amplitude changes to the IF exist, they are to be eliminated as much as possible along with any noise. FM is thus a quiet medium compared to AM. But you can make a heck of an effort to get an FM receiver to work and still find there is an annoying rustling noise from a receiver, and that the SNR is only a lousy 45dB. Its usually easy to get good mono with low noise from a tubed FM tuner, but more difficult if you add the MPX unit. Patrick Turner. Hi Patrick, Fantastic and excellent work! Although I didn't get that far yet with the KW55, I will keep your web site as a referance for later. Right now, I like to replace the usual suspects, lytics, paper caps, and carbon comp resistors. The diodes seem to be OK (FR1K), but I'm thinking of seeing how Schottsky diodes work. I don't know if you came across these before or used them, but what is your opinion of using FRED's instead of the standard 1N5408 diodes? http://ixdev.ixys.com/DataSheet/DSEI12-12A.pdf http://www.allelectronics.com/make-a...T-E3/45/1.html Any mods you'd suggest for the power amp section, resistors between the EL84's? thanks Omer |
#4
Posted to rec.audio.tubes
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Kenwood KW55
On Jul 12, 9:30*am, "Omer S" wrote:
"Patrick Turner" wrote in message ... On Jul 10, 4:31 pm, "Omer S" wrote: Anybody out there work on one of these? http://www.mcmlv.org/Archive/HiFi/KW55U.pdf I'm about to endeavor restoring one of these, and I'm wondering if anybody did any mods to this receiver. Omer Hi Omer, I once completely restored and improved a tubed Kenwood AM-FM receiver in about 1996. A guy here bought one made for the Japanese market which had TWO am turners because in Japan when stereo FM began they simply put one channel in AM because they only had one mono channel at first. There were tuning eyes for the tuning because you couldn't tune the AM channel for FM stereo by ear with both channels running. The second AM tuner was provided so that once the FM was tuned on favourite you could leave it untouched then switch to listen to the AM only station. It was all very complex and had no MPX converter for producing FM stereo from a multiplexed carrier with supressed 38kHz sub-carrier. I installed a kit which had a chip for MPX, and removed the extra AM tuner used for a "stereo" FM channel. The spare sockets were used to make a much better AM detector for the AM section which my customer wanted to be equal to the best available with 9kHz of audio bandwidth, 9kHz whistle filter, and low THD/IMD. Your Kenwood is a different circuit to the one I worked on. But if you look at Pioneer, Fisher and Kenwood, there is much similarity in the AM and FM front ends, IF strips, and diode detectors. Yours has tubed MPX decoding but has the usual diode matrix for L&R channel production. Some of these things work well, and some work very badly, and to align these sets and keep them aligned and running as quietly as possible is a labour of love. Previous to my work on a cutomer's AM/FM Kenwood, I had bought A Trio receiver with single ended audio output stages with a single 6BQ5 each. I gave up trying to get the MPX decoder to work properly after a week of ****ing around. I visited local university and college library archives where I found old books with the theory and basic circuits which were followed by many makers including Kenwood for the days before tubes were retired. I copied and studied many pages from books and applied what I began to understand when I redesigned the Trio. I got rid of the audio output stages off the Trio and made the chassis just for AM and FM stereo. The MPX decoder is somewhat a critical item if you want good imaging, low noise, and low distortions. In the days of tubed MPX to comply with the GE-Zenith format of stereo multiplexing, the first MPX decoders propsed and recomended by RCA used 3 tubes, and Quad made add on addaptors with just 3 germanium transistors. Usually such primitive MPX circuits offer increased noise and distortion when using stereo compared to just having mono function. In all my experience since 14 years ago I have found most generic MPX decoder chips work better than any arrangement of tubes and tuned coils and diode matrixes. I built what is probably my final version of an MPX decoder based on the Quad transistor circuit knowing that with tubes instead of crummy germanium transistors there was a chance I could get better dynamic range performance and less distortion. But the noise is still worse than most chip sets. It sure sounds well though. The filtering of the switching noise and remnants of 38kHz and filtering of the 19kHz pilot tone take real expertise. Many tubed AM-FM sets can have wonderful looking schematics which withstand visual scrutiny but when you make the measurements which you NEED to make with this sort of gear they just don't always measure very well at all. Measuring AM/FM radio performance requires that you KNOW how and why each tube and R and C and L has been used where it is. When I set out to learn enough to service any AM receiver dumped in my bench for a fix, I was alone. I did know a few fellows in the local amateur radio group. I used to go to their swap meets and sales days and found them to be a nice lot but when I mentioned FM, one guy retorted, "****, FM?, Hell, we are always tryin to keep the frequency stable, and not let it vary". I didn't get anywhere with my questions about FM radio of any type. Some amateurs were using narrow band FM, but none here did. Also the Army used narrow band FM at around citizen radio 27MHz. It was secure in 1955, and I have a beuatifully made transceiver from that time, needed a jeep and two guys to get it around. But now is all entirely a digital world for the armed forces, and not a tube to be seen anywhere. Tubes were at about their limit when TV came along and with FM stereo. Once you have 20 tubes on a chassis all doing things that are high- falootin and not easily understood by an average man in the street, and you have a tendency for hot runninng bits and peices to malfunction in circuits designed by been counters then you find the Kenwoods et all will need to be serviced once a year. After tubes came discrete circuits using individual silicon bjts which suddenly became very reliable and cheep. The myriad of slug tuned LC coil units in cans suddenly became much smaller. This suited FM but usually made the sound much worse in AM. Then came the integrated circuit, and these mostly worked better than anything with discrete bjts or tubes. I have tried to find out exactly what is inside a typical MPX decoder chip which performs the stereo decoding function so I could emulate the circuit using several twin triodes, but I never found an exact schematic of what actually does the business. Whatever it is, it works better than a diode matrix, even though that is simpler. I recall there were things like a pair of cross coupled differential pairs. But exactly how they were arranged I never found out. Tim DeParavicini put out a schematic for a "tubed MPX decoder" but what it really contained was a 14 pin generic chip with a tubed output stage. It was anything but a fully tubed MPX decoder and one which had no diode matrix. Nobody I know has ever produced a fully tubed MPX decoder without diodes, even tube diodes. I have a few thoughts at my website....http://www.turneraudio.com.au/am-fm-...ex-decoder.htm Farnarkling around with AM-FM radios can occupy a man for weeks, months or years. I spent lots of time on all that but rarely have any need to use my knowledge which is now a bit rusty. I am now not sure what a "quieting factor" is. But I do know one had to use a lot of very good limiting to make FM reception quiet. The "limiter" stages using 6AU6 are very important, and two such stages are best, although usually there is often only one. The limiter stage is an SE pentode gain stage which operates normally like any linear gain stage which has maybe 30dB over drive. It works in a state of being wildly over driven and any changes in IF frequency must not cause any output amplitude changes and where any amplitude changes to the IF exist, they are to be eliminated as much as possible along with any noise. FM is thus a quiet medium compared to AM. But you can make a heck of an effort to get an FM receiver to work and still find there is an annoying rustling noise from a receiver, and that the SNR is only a lousy 45dB. Its usually easy to get good mono with low noise from a tubed FM tuner, but more difficult if you add the MPX unit. Patrick Turner. Hi Patrick, Fantastic and excellent work! Although I didn't get that far yet with the KW55, *I will keep your web site as a referance for later. Right now, I like to replace the usual suspects, lytics, paper caps, and carbon comp resistors. *The diodes seem to be OK (FR1K), *but I'm thinking of seeing how Schottsky diodes work. *I don't know if you came across these before or used them, but what is your opinion of using FRED's instead of the standard 1N5408 diodes? http://ixdev.ixys.com/DataSheet/DSEI12-12A.pdf http://www.allelectronics.com/make-a.../15A-600V-ULTR... Hi Omer, The result of having a power supply on the same chassis as radio / audio amps can be diode switching noise which finds its way into the radio / audio circuits and you get a raspy sounding hum because its a train of pulses at 100Hz / 120Hz rate. Often fast switching diodes make utterly no difference to the problem because the switching excites the LC circuits which exist in the power tranny because of shunt capacitances and leakage inductances. One may find a typical resonant Fo at 70kHz, and the switching excites the tunes circuits to give burst of energy at 70kHz each time a diode turns on/off. Sometimes the simplest remedy is a high voltage rated 0.05uF across the HT winding of the PT. It does not load the winding at mains F but it shunts the RF. Location of wirinfg and layout can be critical and stray magnetic induction of switcting currents need to be tracked down the a circuit revised if necessary. Having an RCA input near the mains input wires is never good practice. So I never bother using fancy PS diodes and the IN5048 is fine, and in a fault condition it is rugged enough to blow a fuse rather than become a short circuit due to over voltage/current conditions. Any mods you'd suggest for the power amp section, resistors between the EL84's? Which resistors? But there is rather a lot of mods I do on old junk like this which always has facilities which are irrelevant today. I like to have each EL84 output tube with its own cathode biasing. Your Kenwood has the typical bean counter way of tying all the EL84 cathodes together and using the combined dc flow to heat phono amp heaters. All tubes except EL84 should have a separate DC heater supply. This can be +/- 12.6Vdc so that pairs of tubes can be seriesed and run from +12.6V and -12.6V so that the current is kept low from a generic 15V-0-15V transformer which will give +/- 20Vdc and then RC filtering and zener diode regulation can give perferctly clean dc heater power. The reverse/mono/stereo/L/R switch should be eliminated. All you need is stereo. V16a/16b should be able to be fully deleted. The output stage has EL84 in pentode mode. The speaker select switch should be removed and 4mm banana plug sockets installed and labelled Com, 4 ohms, 8 ohms, 16 ohms, on both channels. If you have "8 ohms" speakers, probably they will dip down to 5 ohms, so always try using them on the 4 ohm outlet which will give you 1/3 of the THD/IMD, twice the damping factor but slightly less maximum power than using the 8 ohm connection. Its because the output tubes "see" a higher value of RLa-a whch gives more class A power. Pda in each EL84 should be 10W, ie, Vdc between cathode and anode x tube dc current at idle = 10Watts. Of course all caps and carbon composition resistors should all be replaced with better quality. Much old junk has bad RCA sockets for inputs. Maybe you need a cd player input with matching for the higher levels for cd which are well above the normal output of the radio or phono sources. A resistance divider can be used. In the PSU the B+ to the OPT CT is unfiltered. This is standard crappy practice. The correct way is to either have a 10H choke for R301, 220R and take the OPT CT to top of C302. Alternatively, use a 470uF cap for C302 and take OPT CT to the + of the 470uF where the ripple voltage will be MUCH lower than it is now. I usually find good reasons to do many drastic mods which I don't have time to list. Patrick Turner. thanks Omer |
#5
Posted to rec.audio.tubes
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Kenwood KW55
(Sorry for top posting)
Thank you very much Patrick for your detailed suggestions, I think that my KW 55 is a later model, because I see a choke in there by the power supply. In the next few days, I'll have more info when I get in to the receiver. Again , many thanks. Omer "Patrick Turner" wrote in message ... On Jul 12, 9:30 am, "Omer S" wrote: "Patrick Turner" wrote in message ... On Jul 10, 4:31 pm, "Omer S" wrote: Anybody out there work on one of these? http://www.mcmlv.org/Archive/HiFi/KW55U.pdf I'm about to endeavor restoring one of these, and I'm wondering if anybody did any mods to this receiver. Omer Hi Omer, I once completely restored and improved a tubed Kenwood AM-FM receiver in about 1996. A guy here bought one made for the Japanese market which had TWO am turners because in Japan when stereo FM began they simply put one channel in AM because they only had one mono channel at first. There were tuning eyes for the tuning because you couldn't tune the AM channel for FM stereo by ear with both channels running. The second AM tuner was provided so that once the FM was tuned on favourite you could leave it untouched then switch to listen to the AM only station. It was all very complex and had no MPX converter for producing FM stereo from a multiplexed carrier with supressed 38kHz sub-carrier. I installed a kit which had a chip for MPX, and removed the extra AM tuner used for a "stereo" FM channel. The spare sockets were used to make a much better AM detector for the AM section which my customer wanted to be equal to the best available with 9kHz of audio bandwidth, 9kHz whistle filter, and low THD/IMD. Your Kenwood is a different circuit to the one I worked on. But if you look at Pioneer, Fisher and Kenwood, there is much similarity in the AM and FM front ends, IF strips, and diode detectors. Yours has tubed MPX decoding but has the usual diode matrix for L&R channel production. Some of these things work well, and some work very badly, and to align these sets and keep them aligned and running as quietly as possible is a labour of love. Previous to my work on a cutomer's AM/FM Kenwood, I had bought A Trio receiver with single ended audio output stages with a single 6BQ5 each. I gave up trying to get the MPX decoder to work properly after a week of ****ing around. I visited local university and college library archives where I found old books with the theory and basic circuits which were followed by many makers including Kenwood for the days before tubes were retired. I copied and studied many pages from books and applied what I began to understand when I redesigned the Trio. I got rid of the audio output stages off the Trio and made the chassis just for AM and FM stereo. The MPX decoder is somewhat a critical item if you want good imaging, low noise, and low distortions. In the days of tubed MPX to comply with the GE-Zenith format of stereo multiplexing, the first MPX decoders propsed and recomended by RCA used 3 tubes, and Quad made add on addaptors with just 3 germanium transistors. Usually such primitive MPX circuits offer increased noise and distortion when using stereo compared to just having mono function. In all my experience since 14 years ago I have found most generic MPX decoder chips work better than any arrangement of tubes and tuned coils and diode matrixes. I built what is probably my final version of an MPX decoder based on the Quad transistor circuit knowing that with tubes instead of crummy germanium transistors there was a chance I could get better dynamic range performance and less distortion. But the noise is still worse than most chip sets. It sure sounds well though. The filtering of the switching noise and remnants of 38kHz and filtering of the 19kHz pilot tone take real expertise. Many tubed AM-FM sets can have wonderful looking schematics which withstand visual scrutiny but when you make the measurements which you NEED to make with this sort of gear they just don't always measure very well at all. Measuring AM/FM radio performance requires that you KNOW how and why each tube and R and C and L has been used where it is. When I set out to learn enough to service any AM receiver dumped in my bench for a fix, I was alone. I did know a few fellows in the local amateur radio group. I used to go to their swap meets and sales days and found them to be a nice lot but when I mentioned FM, one guy retorted, "****, FM?, Hell, we are always tryin to keep the frequency stable, and not let it vary". I didn't get anywhere with my questions about FM radio of any type. Some amateurs were using narrow band FM, but none here did. Also the Army used narrow band FM at around citizen radio 27MHz. It was secure in 1955, and I have a beuatifully made transceiver from that time, needed a jeep and two guys to get it around. But now is all entirely a digital world for the armed forces, and not a tube to be seen anywhere. Tubes were at about their limit when TV came along and with FM stereo. Once you have 20 tubes on a chassis all doing things that are high- falootin and not easily understood by an average man in the street, and you have a tendency for hot runninng bits and peices to malfunction in circuits designed by been counters then you find the Kenwoods et all will need to be serviced once a year. After tubes came discrete circuits using individual silicon bjts which suddenly became very reliable and cheep. The myriad of slug tuned LC coil units in cans suddenly became much smaller. This suited FM but usually made the sound much worse in AM. Then came the integrated circuit, and these mostly worked better than anything with discrete bjts or tubes. I have tried to find out exactly what is inside a typical MPX decoder chip which performs the stereo decoding function so I could emulate the circuit using several twin triodes, but I never found an exact schematic of what actually does the business. Whatever it is, it works better than a diode matrix, even though that is simpler. I recall there were things like a pair of cross coupled differential pairs. But exactly how they were arranged I never found out. Tim DeParavicini put out a schematic for a "tubed MPX decoder" but what it really contained was a 14 pin generic chip with a tubed output stage. It was anything but a fully tubed MPX decoder and one which had no diode matrix. Nobody I know has ever produced a fully tubed MPX decoder without diodes, even tube diodes. I have a few thoughts at my website....http://www.turneraudio.com.au/am-fm-...ex-decoder.htm Farnarkling around with AM-FM radios can occupy a man for weeks, months or years. I spent lots of time on all that but rarely have any need to use my knowledge which is now a bit rusty. I am now not sure what a "quieting factor" is. But I do know one had to use a lot of very good limiting to make FM reception quiet. The "limiter" stages using 6AU6 are very important, and two such stages are best, although usually there is often only one. The limiter stage is an SE pentode gain stage which operates normally like any linear gain stage which has maybe 30dB over drive. It works in a state of being wildly over driven and any changes in IF frequency must not cause any output amplitude changes and where any amplitude changes to the IF exist, they are to be eliminated as much as possible along with any noise. FM is thus a quiet medium compared to AM. But you can make a heck of an effort to get an FM receiver to work and still find there is an annoying rustling noise from a receiver, and that the SNR is only a lousy 45dB. Its usually easy to get good mono with low noise from a tubed FM tuner, but more difficult if you add the MPX unit. Patrick Turner. Hi Patrick, Fantastic and excellent work! Although I didn't get that far yet with the KW55, I will keep your web site as a referance for later. Right now, I like to replace the usual suspects, lytics, paper caps, and carbon comp resistors. The diodes seem to be OK (FR1K), but I'm thinking of seeing how Schottsky diodes work. I don't know if you came across these before or used them, but what is your opinion of using FRED's instead of the standard 1N5408 diodes? http://ixdev.ixys.com/DataSheet/DSEI12-12A.pdf http://www.allelectronics.com/make-a.../15A-600V-ULTR... Hi Omer, The result of having a power supply on the same chassis as radio / audio amps can be diode switching noise which finds its way into the radio / audio circuits and you get a raspy sounding hum because its a train of pulses at 100Hz / 120Hz rate. Often fast switching diodes make utterly no difference to the problem because the switching excites the LC circuits which exist in the power tranny because of shunt capacitances and leakage inductances. One may find a typical resonant Fo at 70kHz, and the switching excites the tunes circuits to give burst of energy at 70kHz each time a diode turns on/off. Sometimes the simplest remedy is a high voltage rated 0.05uF across the HT winding of the PT. It does not load the winding at mains F but it shunts the RF. Location of wirinfg and layout can be critical and stray magnetic induction of switcting currents need to be tracked down the a circuit revised if necessary. Having an RCA input near the mains input wires is never good practice. So I never bother using fancy PS diodes and the IN5048 is fine, and in a fault condition it is rugged enough to blow a fuse rather than become a short circuit due to over voltage/current conditions. Any mods you'd suggest for the power amp section, resistors between the EL84's? Which resistors? But there is rather a lot of mods I do on old junk like this which always has facilities which are irrelevant today. I like to have each EL84 output tube with its own cathode biasing. Your Kenwood has the typical bean counter way of tying all the EL84 cathodes together and using the combined dc flow to heat phono amp heaters. All tubes except EL84 should have a separate DC heater supply. This can be +/- 12.6Vdc so that pairs of tubes can be seriesed and run from +12.6V and -12.6V so that the current is kept low from a generic 15V-0-15V transformer which will give +/- 20Vdc and then RC filtering and zener diode regulation can give perferctly clean dc heater power. The reverse/mono/stereo/L/R switch should be eliminated. All you need is stereo. V16a/16b should be able to be fully deleted. The output stage has EL84 in pentode mode. The speaker select switch should be removed and 4mm banana plug sockets installed and labelled Com, 4 ohms, 8 ohms, 16 ohms, on both channels. If you have "8 ohms" speakers, probably they will dip down to 5 ohms, so always try using them on the 4 ohm outlet which will give you 1/3 of the THD/IMD, twice the damping factor but slightly less maximum power than using the 8 ohm connection. Its because the output tubes "see" a higher value of RLa-a whch gives more class A power. Pda in each EL84 should be 10W, ie, Vdc between cathode and anode x tube dc current at idle = 10Watts. Of course all caps and carbon composition resistors should all be replaced with better quality. Much old junk has bad RCA sockets for inputs. Maybe you need a cd player input with matching for the higher levels for cd which are well above the normal output of the radio or phono sources. A resistance divider can be used. In the PSU the B+ to the OPT CT is unfiltered. This is standard crappy practice. The correct way is to either have a 10H choke for R301, 220R and take the OPT CT to top of C302. Alternatively, use a 470uF cap for C302 and take OPT CT to the + of the 470uF where the ripple voltage will be MUCH lower than it is now. I usually find good reasons to do many drastic mods which I don't have time to list. Patrick Turner. thanks Omer |
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