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RACAL RA 6790 GM Receiver
One's up for auction on Ebay right now:
http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WDVW Has anyone owned one of these receivers? I'm not bidding on it, but I'm curious about how it would stack up against one of the better commercial receivers, like an R8B or 7030+. Steve |
On Sat, 23 Apr 2005 20:49:48 GMT, David wrote:
On 23 Apr 2005 13:22:06 -0700, wrote: One's up for auction on Ebay right now: http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WDVW Has anyone owned one of these receivers? I'm not bidding on it, but I'm curious about how it would stack up against one of the better commercial receivers, like an R8B or 7030+. Steve Per Bob, better than the AOR, not as good as an R8. http://www.sherweng.com/table.html Misread the chart. Both radis are rated above the Racal. |
On Sat, 23 Apr 2005 21:13:49 GMT, David wrote:
On Sat, 23 Apr 2005 20:49:48 GMT, David wrote: On 23 Apr 2005 13:22:06 -0700, wrote: One's up for auction on Ebay right now: http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WDVW Has anyone owned one of these receivers? I'm not bidding on it, but I'm curious about how it would stack up against one of the better commercial receivers, like an R8B or 7030+. Steve Per Bob, better than the AOR, not as good as an R8. http://www.sherweng.com/table.html Misread the chart. Both radis are rated above the Racal. Here's a more complete review: http://www.eham.net/reviews/detail/488 |
There is no shortage of people who get some exotic widget then drone on
about how good it is. I heard the keyboards wear out on those Racals. Just image the hassle getting parts. Pass. In fact, I passed on that radio years ago when some Russian was peddling them at Livermore. Something like $600 and he didn't even throw in some vodka for contract cleaning. |
dxAce wrote: wrote: One's up for auction on Ebay right now: http://cgi.ebay.com/ws/eBayISAPI.dll...tem=3D576 83= 29816&rd=3D1&ssPageName=3DWDVW Has anyone owned one of these receivers? I'm not bidding on it, but I'm curious about how it would stack up against one of the better commercial receivers, like an R8B or 7030+. Never had one, never will. Not even necessary for DX'ing... dxAce Michigan USA I swear by, not at, Drake receivers.=A9 Drake R7, R8, R8A and R8B http://www.iserv.net/~n8kdv/dxpage.htm I had one several years ago, worked ok, nothing to write home about. I agree, the Drake R8 series is better as are many other receivers. Guy Atkins, a well known mw dx'er sold his and kept a Kiwa modified Icom R75. Les |
Les is correct. After much comparison on the tropical bands and foreign MW
DXing, I found the modded R-75 essentially equal to a fine example of a RA6790GM (with various new parts, and tweaked, serviced, and refurbished by the Racal guru Gary Wingerd). At one point I installed a very desireable upgrade to the front end roofing filters, by changing them from a 20 kHz bandwidth to a 10 kHz bandwidth, using two 40.455 MHz modules from a Cubic R-3030 RF board. This made the Racal better at tough MW split-frequency DXing with strong signals nearby in frequency. Still, the R-75 was so much easier to operate-- smaller, lighter, cooler running, and has the advantage of working off 12vdc for power when needed at DXpeditions. In addition, the Twin PBT control in the R-75 is a great tool for DXing-- a feature sorely missing in the Racal. The RA6790GM model fit its commercial/military monitoring purpose well, at a reasonable cost in its day for organizations and governments. It was never intended to meet the specific needs of DXers. One example of the awkwardness of the RA6790GM for DXing is in the choice of filters for SSB. To use anything other than the default SSB filter (normally 3.0 kHz), you need to switch to CW mode, adjust the BFO offset to achieve natural speech, and then choose one of the other installed bandwidths. This is VERY inconvenient during changing DX reception conditions, particularly during dawn enhancement of signals when you need a flexible, quick-to-adjust receiver. My R-75 has these Kiwa mods: Synchronous AM module, Kiwa CLF-D2K filter (no longer made), and the Kiwa audio upgrade. I also have two 1.8 kHz INRAD filters in the 2nd IF , ECSS volume mod, Dr. Phil's high fidelity audio upgrade, and a number of other smaller mods. The radio doesn't need these mods to perform very well, but I to tinker with my gear... The R-75 is now my secondary radio. The main and best performing rig in the shack now is a ICOM IC-756Pro transceiver. In all but one DXing situation over the last 4 or 5 months, the 756Pro has noticeably outperformed the R-75. There is not a huge difference, but big enough that I can often times make out more words and IDs in a foreign language on a static-riddled, weak station on the 756Pro compared to the R-75. However, the R-75 is the best value in a new receiver these days (while they last...I wonder how many R-75's Universal Radio still has in their dwindling stock?) Guy Atkins Puyallup, WA USA "Les" wrote in message ups.com... SNIP I had one several years ago, worked ok, nothing to write home about. I agree, the Drake R8 series is better as are many other receivers. Guy Atkins, a well known mw dx'er sold his and kept a Kiwa modified Icom R75. Les |
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"Has anyone owned one of these receivers? I'm not bidding on it, but
I'm curious about how it would stack up against one of the better commercial receivers, like an R8B or 7030+." Well, remember the Racal is a commercial grade receiver designed for specific applications ranging from laboratory work to fixed frequency monitoring, etc. It was not built as a hobbyist radio. There are several interesting reviews that go into detail about it's strong points and several limitations: http://www.eham.net/reviews/detail/488 On the other hand, the R8B and 7030, R75, etc., are consumer grade receivers for serious hobbyists. All three are known for for having numerous very useful features that allow the user to manipulate the signal in a variety of ways. Unlike the Racal they all have memories, a feature that is basic to swling. Fully expect that you would hear the same signals on the Racal as you would with the three consumer receivers. The Racal does look the serious receiver with the rack mount case, handles and austere design. |
Pete G on this group raves about how great it is.
however it does not mean it is fun to use. commercial receivers are not always fun to use. I have a Drake maritime 5000 dollar receiver, I find it no fun at all to use compared to the satellit 800 or a modded DX-394 or similar fun to use receiver. wrote in message ups.com... One's up for auction on Ebay right now: http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WDVW Has anyone owned one of these receivers? I'm not bidding on it, but I'm curious about how it would stack up against one of the better commercial receivers, like an R8B or 7030+. Steve |
"mike maghakian" wrote in message ... Pete G on this group raves about how great it is. however it does not mean it is fun to use. commercial receivers are not always fun to use. I have a Drake maritime 5000 dollar receiver, I find it no fun at all to use compared to the satellit 800 or a modded DX-394 or similar fun to use receiver. The 6790 is a good receiver........if you want all of the "creature comforts", this isn't the receiver to use. It is my favorite as far as performance, but it is so DEEEEEEEEEEEEEEEEEEEEEEPPPPPPPPPPPPPPPP! We are talking about over 20 inches of depth. The gain of the I.F. system is higher than any of my other receivers. The MDS measures less than .1uV over the whole tuning range, which is in line with just about every other receiver being sold today. My favorite receiver? My Lowe HF-150. I also have the HF-225, but it doesn't compare with the 150 for MW reception, because of the lossy duplexer that it uses on the MW range (why they don't use a bandpass filter for this range, I don't know). My 2nd favorite is my NRD-515, followed by the Palstar R30, Yaesu FRG-100, Icom R75, AOR7030, SW-8, and all of the others (I lost count). Many folks are concerned about getting ahold of parts for the 6790, but the same could be said about the Cubic, WJ, Collins, and all of the other premium receivers. The main advantage of the super high end receivers is that most of them can be used for shipboard operation, where they will be co-located with high power transmitters. An example is the Collins HF-2054 or the 851S-1 receiver............these receivers can handle 100V of RF at the antenna input without damage to the receiver, whether it is powered on or turned off. Is this kind of robustness needed for the home receiver. Probably not. The ESD protection that these receivers afford would be helpful, as evidenced by the number of Lowe receivers that I have replaced the 1st mixer in recently. Somebody made a comment about how cool the Icom R-75 runs..........I bet that you haven't attempted to touch that internal 5V regulator. There have been some failures with this regulator recently. The failure mechanism is caused by the fact that Icom chose to use an 18V supply to power the radio. Dave Zantos has modified his power supply by adding a pre-regulator that brings the voltage down to 13.5V before it enters the radio. My advice? Either modify that Icom power brick or replace it with a regulated 13.5V power supply. Looking inside of that supply, you will find spots on the circuit board for adding bypass caps across each rectifier diode. Add .01uF caps in these places and it will prevent the diodes from rectifying RF. Another good mod for this power supply is to add a 1uF NP cap from each leg of the SECONDARY of the power transformer to ground. This snubs the switching noise from the rectifier diodes. I have heard mention on this NG that purchasing a good regulated supply gets rid of these emissions........this is not really the case. Any power supply can be made quiet by using the aforementioned techniques. The AOR7030s power supply can also benefit from these mods. Pete |
Hi Pete,
I was the one who mentioned the cool-running R-75. I should have also mentioned I *only* power it by a 13.5vdc Astron supply. I've never used the ICOM power brick. Guy "Pete KE9OA" wrote in message ... "mike maghakian" wrote in message ... Pete G on this group raves about how great it is. however it does not mean it is fun to use. commercial receivers are not always fun to use. I have a Drake maritime 5000 dollar receiver, I find it no fun at all to use compared to the satellit 800 or a modded DX-394 or similar fun to use receiver. The 6790 is a good receiver........if you want all of the "creature comforts", this isn't the receiver to use. It is my favorite as far as performance, but it is so DEEEEEEEEEEEEEEEEEEEEEEPPPPPPPPPPPPPPPP! We are talking about over 20 inches of depth. The gain of the I.F. system is higher than any of my other receivers. The MDS measures less than .1uV over the whole tuning range, which is in line with just about every other receiver being sold today. My favorite receiver? My Lowe HF-150. I also have the HF-225, but it doesn't compare with the 150 for MW reception, because of the lossy duplexer that it uses on the MW range (why they don't use a bandpass filter for this range, I don't know). My 2nd favorite is my NRD-515, followed by the Palstar R30, Yaesu FRG-100, Icom R75, AOR7030, SW-8, and all of the others (I lost count). Many folks are concerned about getting ahold of parts for the 6790, but the same could be said about the Cubic, WJ, Collins, and all of the other premium receivers. The main advantage of the super high end receivers is that most of them can be used for shipboard operation, where they will be co-located with high power transmitters. An example is the Collins HF-2054 or the 851S-1 receiver............these receivers can handle 100V of RF at the antenna input without damage to the receiver, whether it is powered on or turned off. Is this kind of robustness needed for the home receiver. Probably not. The ESD protection that these receivers afford would be helpful, as evidenced by the number of Lowe receivers that I have replaced the 1st mixer in recently. Somebody made a comment about how cool the Icom R-75 runs..........I bet that you haven't attempted to touch that internal 5V regulator. There have been some failures with this regulator recently. The failure mechanism is caused by the fact that Icom chose to use an 18V supply to power the radio. Dave Zantos has modified his power supply by adding a pre-regulator that brings the voltage down to 13.5V before it enters the radio. My advice? Either modify that Icom power brick or replace it with a regulated 13.5V power supply. Looking inside of that supply, you will find spots on the circuit board for adding bypass caps across each rectifier diode. Add .01uF caps in these places and it will prevent the diodes from rectifying RF. Another good mod for this power supply is to add a 1uF NP cap from each leg of the SECONDARY of the power transformer to ground. This snubs the switching noise from the rectifier diodes. I have heard mention on this NG that purchasing a good regulated supply gets rid of these emissions........this is not really the case. Any power supply can be made quiet by using the aforementioned techniques. The AOR7030s power supply can also benefit from these mods. Pete |
Ok, that makes sense..............on 13.5V, the Icom runs great. I still
have to do the audio mods to mine one of these days. It is a super performer, RF wise. Right now, I am revamping a McKay Dymek DR-33C. I just measured the LO injection from the synthesizer. At +17dBm, I would expect much better IM performance. LW is laden with multiple MW signals. I am going to replace that whole kludge of a 1st mixer with a Mini-Circuits TAK-3H. This device has an IP3 of +29dBm. The 2nd mixer is getting a Siliconix E431..............this device has an IP3 of +30dBm. It should be a very good receiver when I get through with it. I used the TAK-3H when I upgraded my KWM-380. With a wide open front end on MW / LW, no IMD products were noted. These mods should improve the MDS of the DR-33C. I will post my results. Pete "Guy Atkins" wrote in message ... Hi Pete, I was the one who mentioned the cool-running R-75. I should have also mentioned I *only* power it by a 13.5vdc Astron supply. I've never used the ICOM power brick. Guy "Pete KE9OA" wrote in message ... "mike maghakian" wrote in message ... Pete G on this group raves about how great it is. however it does not mean it is fun to use. commercial receivers are not always fun to use. I have a Drake maritime 5000 dollar receiver, I find it no fun at all to use compared to the satellit 800 or a modded DX-394 or similar fun to use receiver. The 6790 is a good receiver........if you want all of the "creature comforts", this isn't the receiver to use. It is my favorite as far as performance, but it is so DEEEEEEEEEEEEEEEEEEEEEEPPPPPPPPPPPPPPPP! We are talking about over 20 inches of depth. The gain of the I.F. system is higher than any of my other receivers. The MDS measures less than .1uV over the whole tuning range, which is in line with just about every other receiver being sold today. My favorite receiver? My Lowe HF-150. I also have the HF-225, but it doesn't compare with the 150 for MW reception, because of the lossy duplexer that it uses on the MW range (why they don't use a bandpass filter for this range, I don't know). My 2nd favorite is my NRD-515, followed by the Palstar R30, Yaesu FRG-100, Icom R75, AOR7030, SW-8, and all of the others (I lost count). Many folks are concerned about getting ahold of parts for the 6790, but the same could be said about the Cubic, WJ, Collins, and all of the other premium receivers. The main advantage of the super high end receivers is that most of them can be used for shipboard operation, where they will be co-located with high power transmitters. An example is the Collins HF-2054 or the 851S-1 receiver............these receivers can handle 100V of RF at the antenna input without damage to the receiver, whether it is powered on or turned off. Is this kind of robustness needed for the home receiver. Probably not. The ESD protection that these receivers afford would be helpful, as evidenced by the number of Lowe receivers that I have replaced the 1st mixer in recently. Somebody made a comment about how cool the Icom R-75 runs..........I bet that you haven't attempted to touch that internal 5V regulator. There have been some failures with this regulator recently. The failure mechanism is caused by the fact that Icom chose to use an 18V supply to power the radio. Dave Zantos has modified his power supply by adding a pre-regulator that brings the voltage down to 13.5V before it enters the radio. My advice? Either modify that Icom power brick or replace it with a regulated 13.5V power supply. Looking inside of that supply, you will find spots on the circuit board for adding bypass caps across each rectifier diode. Add .01uF caps in these places and it will prevent the diodes from rectifying RF. Another good mod for this power supply is to add a 1uF NP cap from each leg of the SECONDARY of the power transformer to ground. This snubs the switching noise from the rectifier diodes. I have heard mention on this NG that purchasing a good regulated supply gets rid of these emissions........this is not really the case. Any power supply can be made quiet by using the aforementioned techniques. The AOR7030s power supply can also benefit from these mods. Pete |
In article
, "Pete KE9OA" wrote: Snip Somebody made a comment about how cool the Icom R-75 runs..........I bet that you haven't attempted to touch that internal 5V regulator. There have been some failures with this regulator recently. The failure mechanism is caused by the fact that Icom chose to use an 18V supply to power the radio. Dave Zantos has modified his power supply by adding a pre-regulator that brings the voltage down to 13.5V before it enters the radio. My advice? Either modify that Icom power brick or replace it with a regulated 13.5V power supply. Looking inside of that supply, you will find spots on the circuit board for adding bypass caps across each rectifier diode. Add .01uF caps in these places and it will prevent the diodes from rectifying RF. Another good mod for this power supply is to add a 1uF NP cap from each leg of the SECONDARY of the power transformer to ground. This snubs the switching noise from the rectifier diodes. I have heard mention on this NG that purchasing a good regulated supply gets rid of these emissions........this is not really the case. Any power supply can be made quiet by using the aforementioned techniques. The AOR7030s power supply can also benefit from these mods. You are right about the 5V regulator. These are series regulator elements and the higher the input voltage the more power they must dissipate to regulate the voltage down to the lower voltage. People don't seem to get the diode rectifier concept here though. The diode in the power supply passes current when the voltage polarity is in the right direction and blocks it in the reverse. This is the rectification function. When the diode switched from on to off the circuit goes to high impedance. This results in a voltage spike that can damage the diode if the voltage goes above the PIV rating. PIV stands for Peak Inverse Voltage. It is this voltage spike every time the diode switches off that causes EMI/RFI depending on the path. It could be either or both but is usually mostly EMI. If it is mostly EMI, the usual case, then a common mode choke will block the majority of the diode switching noise on the cord to the radio. The power supply terminology to reduce the PIV voltage across the diode so as to not damage it is a "snubber" circuit. This is usually made of a cap and resistor across the diode. The cap/resistor time constant value is determined by the duration of the reverse spike it design to absorb. The resistor burns the power. If the spike is small and you want to suppress it for RF reasons only then it can be just a cap. The capacitor will circulate the current from the voltage spike around the diode, which is a small RF current loop, instead of allowing it to propagate away from the diode through the rest of the power supply circuit up the power cord and into your radio and make a buzz at 60 or 120 Hz. You can use a cap to the AC outlet ground on the secondary side of the transformer but it might not be the best thing to do as it or a pair on either side on the secondary will generate a continuous current down the AC mains ground lead at 60 Hz. It might be better to use one cap on the negative side of the DC output to ground in order to reduce this common mode switch noise. Alternatively you might try a cap on the positive output to ground in addition to the one one the negative side. Here you will only be sending the noise currents down the AC mains leads and not the 60Hz components. -- Telamon Ventura, California |
Telamon wrote:
In article , "Pete KE9OA" wrote: Snip Somebody made a comment about how cool the Icom R-75 runs..........I bet that you haven't attempted to touch that internal 5V regulator. There have been some failures with this regulator recently. The failure mechanism is caused by the fact that Icom chose to use an 18V supply to power the radio. Dave Zantos has modified his power supply by adding a pre-regulator that brings the voltage down to 13.5V before it enters the radio. My advice? Either modify that Icom power brick or replace it with a regulated 13.5V power supply. Looking inside of that supply, you will find spots on the circuit board for adding bypass caps across each rectifier diode. Add .01uF caps in these places and it will prevent the diodes from rectifying RF. Another good mod for this power supply is to add a 1uF NP cap from each leg of the SECONDARY of the power transformer to ground. This snubs the switching noise from the rectifier diodes. I have heard mention on this NG that purchasing a good regulated supply gets rid of these emissions........this is not really the case. Any power supply can be made quiet by using the aforementioned techniques. The AOR7030s power supply can also benefit from these mods. You are right about the 5V regulator. These are series regulator elements and the higher the input voltage the more power they must dissipate to regulate the voltage down to the lower voltage. People don't seem to get the diode rectifier concept here though. The diode in the power supply passes current when the voltage polarity is in the right direction and blocks it in the reverse. This is the rectification function. When the diode switched from on to off the circuit goes to high impedance. This results in a voltage spike that can damage the diode if the voltage goes above the PIV rating. A voltage spike occurs when interrupting power to an inductive load. There is no indictive load in what you are describing. PIV stands for Peak Inverse Voltage. It is this voltage spike every time the diode switches off that causes EMI/RFI depending on the path. It could be either or both but is usually mostly EMI. If it is mostly EMI, the usual case, then a common mode choke will block the majority of the diode switching noise on the cord to the radio. The power supply terminology to reduce the PIV voltage across the diode so as to not damage it is a "snubber" circuit. This is usually made of a cap and resistor across the diode. The cap/resistor time constant value is determined by the duration of the reverse spike it design to absorb. The resistor burns the power. A snubber circuit is used to protect a driver from the transient when switching power to an inductive load. See, http://focus.ti.com/lit/an/slup100/slup100.pdf The radio power supply is not that type of circuit. If the spike is small and you want to suppress it for RF reasons only then it can be just a cap. The capacitor will circulate the current from the voltage spike around the diode, which is a small RF current loop, instead of allowing it to propagate away from the diode through the rest of the power supply circuit up the power cord and into your radio and make a buzz at 60 or 120 Hz. The diode is acting as a mixer. It is combining the 60 Hz line voltages and the signals at the radio frequencies creating a signal at RF with a large 60Hz modulation. Adding a cap bypasses the diode at RF, significantly reducing the mixing action. You can use a cap to the AC outlet ground on the secondary side of the transformer but it might not be the best thing to do as it or a pair on either side on the secondary will generate a continuous current down the AC mains ground lead at 60 Hz. You need a complete circuit for current to flow. At 60 Hz, what is the rest of the circuit? It might be better to use one cap on the negative side of the DC output to ground in order to reduce this common mode switch noise. Alternatively you might try a cap on the positive output to ground in addition to the one one the negative side. Here you will only be sending the noise currents down the AC mains leads and not the 60Hz components. I suggest looking at the whole picture and look at what this does at 60 Hz and at RF frequencies. Later, Craigm |
In article , craigm
wrote: Telamon wrote: In article , "Pete KE9OA" wrote: Snip Somebody made a comment about how cool the Icom R-75 runs..........I bet that you haven't attempted to touch that internal 5V regulator. There have been some failures with this regulator recently. The failure mechanism is caused by the fact that Icom chose to use an 18V supply to power the radio. Dave Zantos has modified his power supply by adding a pre-regulator that brings the voltage down to 13.5V before it enters the radio. My advice? Either modify that Icom power brick or replace it with a regulated 13.5V power supply. Looking inside of that supply, you will find spots on the circuit board for adding bypass caps across each rectifier diode. Add .01uF caps in these places and it will prevent the diodes from rectifying RF. Another good mod for this power supply is to add a 1uF NP cap from each leg of the SECONDARY of the power transformer to ground. This snubs the switching noise from the rectifier diodes. I have heard mention on this NG that purchasing a good regulated supply gets rid of these emissions........this is not really the case. Any power supply can be made quiet by using the aforementioned techniques. The AOR7030s power supply can also benefit from these mods. You are right about the 5V regulator. These are series regulator elements and the higher the input voltage the more power they must dissipate to regulate the voltage down to the lower voltage. People don't seem to get the diode rectifier concept here though. The diode in the power supply passes current when the voltage polarity is in the right direction and blocks it in the reverse. This is the rectification function. When the diode switched from on to off the circuit goes to high impedance. This results in a voltage spike that can damage the diode if the voltage goes above the PIV rating. A voltage spike occurs when interrupting power to an inductive load. There is no indictive load in what you are describing. PIV stands for Peak Inverse Voltage. It is this voltage spike every time the diode switches off that causes EMI/RFI depending on the path. It could be either or both but is usually mostly EMI. If it is mostly EMI, the usual case, then a common mode choke will block the majority of the diode switching noise on the cord to the radio. The power supply terminology to reduce the PIV voltage across the diode so as to not damage it is a "snubber" circuit. This is usually made of a cap and resistor across the diode. The cap/resistor time constant value is determined by the duration of the reverse spike it design to absorb. The resistor burns the power. A snubber circuit is used to protect a driver from the transient when switching power to an inductive load. See, http://focus.ti.com/lit/an/slup100/slup100.pdf The radio power supply is not that type of circuit. The snubber protects the rectifier diode from the transformer and series inductors in the power supply LC output filtering. There is a reason why most diodes have a PIV much higher than the voltage in the circuits they operate in. Ever wonder why? If the spike is small and you want to suppress it for RF reasons only then it can be just a cap. The capacitor will circulate the current from the voltage spike around the diode, which is a small RF current loop, instead of allowing it to propagate away from the diode through the rest of the power supply circuit up the power cord and into your radio and make a buzz at 60 or 120 Hz. The diode is acting as a mixer. It is combining the 60 Hz line voltages and the signals at the radio frequencies creating a signal at RF with a large 60Hz modulation. Adding a cap bypasses the diode at RF, significantly reducing the mixing action. Thanks for writing that. The reason I have written this now overly long thread is to explain the reason for by passing and how it works and why the thinking that the diode is switching RF ground on and off is generating the noise is rubbish. You should connect an oscilloscope to a power supply rectifier diode in operation. If you would do that you will see a large voltage spike based on the inductance of the circuit and how fast the diode switches. Faster diodes are more efficient but the spike voltage will increase with the faster switch time so faster diodes will need a higher PIV rating. A fairly slow diode switching at 60 or 120 Hz depending on the rectifier circuit in a low current supply may not develop a very high PIV so a capacitor by itself may do the job. Low current supplies likely have slower diodes because the heat they dissipate results from the product of the switch time and current going through them. Larger supplies will have more current through the diodes and so that they don't burn up they have to switch faster. Bigger supplies have bigger inductors and faster diodes with larger PIV as a result. Switching more power means you need a snubber RC across the diode instead of just a capacitor. You can use a cap to the AC outlet ground on the secondary side of the transformer but it might not be the best thing to do as it or a pair on either side on the secondary will generate a continuous current down the AC mains ground lead at 60 Hz. You need a complete circuit for current to flow. At 60 Hz, what is the rest of the circuit? One side of the capacitor is grounded through the AC mains ground lead and on the other side is alternating voltage at 60 Hz. It might be better to use one cap on the negative side of the DC output to ground in order to reduce this common mode switch noise. Alternatively you might try a cap on the positive output to ground in addition to the one one the negative side. Here you will only be sending the noise currents down the AC mains leads and not the 60Hz components. I suggest looking at the whole picture and look at what this does at 60 Hz and at RF frequencies. I did and have. It is common practice to use a small value capacitor to ground on or near the power supply outputs where any common mode noise from switching transients is coupled to ground. You can do this at the transformer secondary but why generate the 60 Hz current if you don't have to do that? The object is to conduct noise currents to ground not 60 Hz mains supply. -- Telamon Ventura, California |
I remember...........you have come up with some pretty good ideas about this
subject in the past. Thanks! Pete "Telamon" wrote in message ... In article , "Pete KE9OA" wrote: Snip Somebody made a comment about how cool the Icom R-75 runs..........I bet that you haven't attempted to touch that internal 5V regulator. There have been some failures with this regulator recently. The failure mechanism is caused by the fact that Icom chose to use an 18V supply to power the radio. Dave Zantos has modified his power supply by adding a pre-regulator that brings the voltage down to 13.5V before it enters the radio. My advice? Either modify that Icom power brick or replace it with a regulated 13.5V power supply. Looking inside of that supply, you will find spots on the circuit board for adding bypass caps across each rectifier diode. Add .01uF caps in these places and it will prevent the diodes from rectifying RF. Another good mod for this power supply is to add a 1uF NP cap from each leg of the SECONDARY of the power transformer to ground. This snubs the switching noise from the rectifier diodes. I have heard mention on this NG that purchasing a good regulated supply gets rid of these emissions........this is not really the case. Any power supply can be made quiet by using the aforementioned techniques. The AOR7030s power supply can also benefit from these mods. You are right about the 5V regulator. These are series regulator elements and the higher the input voltage the more power they must dissipate to regulate the voltage down to the lower voltage. People don't seem to get the diode rectifier concept here though. The diode in the power supply passes current when the voltage polarity is in the right direction and blocks it in the reverse. This is the rectification function. When the diode switched from on to off the circuit goes to high impedance. This results in a voltage spike that can damage the diode if the voltage goes above the PIV rating. PIV stands for Peak Inverse Voltage. It is this voltage spike every time the diode switches off that causes EMI/RFI depending on the path. It could be either or both but is usually mostly EMI. If it is mostly EMI, the usual case, then a common mode choke will block the majority of the diode switching noise on the cord to the radio. The power supply terminology to reduce the PIV voltage across the diode so as to not damage it is a "snubber" circuit. This is usually made of a cap and resistor across the diode. The cap/resistor time constant value is determined by the duration of the reverse spike it design to absorb. The resistor burns the power. If the spike is small and you want to suppress it for RF reasons only then it can be just a cap. The capacitor will circulate the current from the voltage spike around the diode, which is a small RF current loop, instead of allowing it to propagate away from the diode through the rest of the power supply circuit up the power cord and into your radio and make a buzz at 60 or 120 Hz. You can use a cap to the AC outlet ground on the secondary side of the transformer but it might not be the best thing to do as it or a pair on either side on the secondary will generate a continuous current down the AC mains ground lead at 60 Hz. It might be better to use one cap on the negative side of the DC output to ground in order to reduce this common mode switch noise. Alternatively you might try a cap on the positive output to ground in addition to the one one the negative side. Here you will only be sending the noise currents down the AC mains leads and not the 60Hz components. -- Telamon Ventura, California |
The diode is acting as a mixer. It is combining the 60 Hz line voltages and the signals at the radio frequencies creating a signal at RF with a large 60Hz modulation. Adding a cap bypasses the diode at RF, significantly reducing the mixing action. Thanks for writing that. The reason I have written this now overly long thread is to explain the reason for by passing and how it works and why the thinking that the diode is switching RF ground on and off is generating the noise is rubbish. I didn't say anything about switching RF ground......................I said that the diode is radiating the noise. You should connect an oscilloscope to a power supply rectifier diode in operation. If you would do that you will see a large voltage spike based on the inductance of the circuit and how fast the diode switches. Faster diodes are more efficient but the spike voltage will increase with the faster switch time so faster diodes will need a higher PIV rating. A fairly slow diode switching at 60 or 120 Hz depending on the rectifier circuit in a low current supply may not develop a very high PIV so a capacitor by itself may do the job. Low current supplies likely have slower diodes because the heat they dissipate results from the product of the switch time and current going through them. Larger supplies will have more current through the diodes and so that they don't burn up they have to switch faster. Bigger supplies have bigger inductors and faster diodes with larger PIV as a result. The mechanism I am talking about isn't isolated to just high current supplies, unless you are talking about a 200mA supply as being high current. Switching more power means you need a snubber RC across the diode instead of just a capacitor. You can use a cap to the AC outlet ground on the secondary side of the transformer but it might not be the best thing to do as it or a pair on either side on the secondary will generate a continuous current down the AC mains ground lead at 60 Hz. At 60Hz, a 1uF cap has a considerable amount of capacitive reactance, so very little current would go through this loop. You need a complete circuit for current to flow. At 60 Hz, what is the rest of the circuit? One side of the capacitor is grounded through the AC mains ground lead and on the other side is alternating voltage at 60 Hz. It might be better to use one cap on the negative side of the DC output to ground in order to reduce this common mode switch noise. Alternatively you might try a cap on the positive output to ground in addition to the one one the negative side. Here you will only be sending the noise currents down the AC mains leads and not the 60Hz components. I suggest looking at the whole picture and look at what this does at 60 Hz and at RF frequencies. I did and have. It is common practice to use a small value capacitor to ground on or near the power supply outputs where any common mode noise from switching transients is coupled to ground. You can do this at the transformer secondary but why generate the 60 Hz current if you don't have to do that? The object is to conduct noise currents to ground not 60 Hz mains supply. -- Telamon Ventura, California |
No problem. I used to work on switching power supply designs.
In article , "Pete KE9OA" wrote: I remember...........you have come up with some pretty good ideas about this subject in the past. Thanks! Pete "Telamon" wrote in message .. . In article , "Pete KE9OA" wrote: Snip Somebody made a comment about how cool the Icom R-75 runs..........I bet that you haven't attempted to touch that internal 5V regulator. There have been some failures with this regulator recently. The failure mechanism is caused by the fact that Icom chose to use an 18V supply to power the radio. Dave Zantos has modified his power supply by adding a pre-regulator that brings the voltage down to 13.5V before it enters the radio. My advice? Either modify that Icom power brick or replace it with a regulated 13.5V power supply. Looking inside of that supply, you will find spots on the circuit board for adding bypass caps across each rectifier diode. Add .01uF caps in these places and it will prevent the diodes from rectifying RF. Another good mod for this power supply is to add a 1uF NP cap from each leg of the SECONDARY of the power transformer to ground. This snubs the switching noise from the rectifier diodes. I have heard mention on this NG that purchasing a good regulated supply gets rid of these emissions........this is not really the case. Any power supply can be made quiet by using the aforementioned techniques. The AOR7030s power supply can also benefit from these mods. You are right about the 5V regulator. These are series regulator elements and the higher the input voltage the more power they must dissipate to regulate the voltage down to the lower voltage. People don't seem to get the diode rectifier concept here though. The diode in the power supply passes current when the voltage polarity is in the right direction and blocks it in the reverse. This is the rectification function. When the diode switched from on to off the circuit goes to high impedance. This results in a voltage spike that can damage the diode if the voltage goes above the PIV rating. PIV stands for Peak Inverse Voltage. It is this voltage spike every time the diode switches off that causes EMI/RFI depending on the path. It could be either or both but is usually mostly EMI. If it is mostly EMI, the usual case, then a common mode choke will block the majority of the diode switching noise on the cord to the radio. The power supply terminology to reduce the PIV voltage across the diode so as to not damage it is a "snubber" circuit. This is usually made of a cap and resistor across the diode. The cap/resistor time constant value is determined by the duration of the reverse spike it design to absorb. The resistor burns the power. If the spike is small and you want to suppress it for RF reasons only then it can be just a cap. The capacitor will circulate the current from the voltage spike around the diode, which is a small RF current loop, instead of allowing it to propagate away from the diode through the rest of the power supply circuit up the power cord and into your radio and make a buzz at 60 or 120 Hz. You can use a cap to the AC outlet ground on the secondary side of the transformer but it might not be the best thing to do as it or a pair on either side on the secondary will generate a continuous current down the AC mains ground lead at 60 Hz. It might be better to use one cap on the negative side of the DC output to ground in order to reduce this common mode switch noise. Alternatively you might try a cap on the positive output to ground in addition to the one one the negative side. Here you will only be sending the noise currents down the AC mains leads and not the 60Hz components. -- Telamon Ventura, California -- Telamon Ventura, California |
In article ,
"Pete KE9OA" wrote: The diode is acting as a mixer. It is combining the 60 Hz line voltages and the signals at the radio frequencies creating a signal at RF with a large 60Hz modulation. Adding a cap bypasses the diode at RF, significantly reducing the mixing action. Thanks for writing that. The reason I have written this now overly long thread is to explain the reason for by passing and how it works and why the thinking that the diode is switching RF ground on and off is generating the noise is rubbish. I didn't say anything about switching RF ground......................I said that the diode is radiating the noise. Yeah, that was someone else responding to my response to your post. I hope that makes sense. You should connect an oscilloscope to a power supply rectifier diode in operation. If you would do that you will see a large voltage spike based on the inductance of the circuit and how fast the diode switches. Faster diodes are more efficient but the spike voltage will increase with the faster switch time so faster diodes will need a higher PIV rating. A fairly slow diode switching at 60 or 120 Hz depending on the rectifier circuit in a low current supply may not develop a very high PIV so a capacitor by itself may do the job. Low current supplies likely have slower diodes because the heat they dissipate results from the product of the switch time and current going through them. Larger supplies will have more current through the diodes and so that they don't burn up they have to switch faster. Bigger supplies have bigger inductors and faster diodes with larger PIV as a result. The mechanism I am talking about isn't isolated to just high current supplies, unless you are talking about a 200mA supply as being high current. Not 200mA. Things don't get interesting until you are above a few amps. Switching more power means you need a snubber RC across the diode instead of just a capacitor. You can use a cap to the AC outlet ground on the secondary side of the transformer but it might not be the best thing to do as it or a pair on either side on the secondary will generate a continuous current down the AC mains ground lead at 60 Hz. At 60Hz, a 1uF cap has a considerable amount of capacitive reactance, so very little current would go through this loop. Well, that depends on the secondary voltage. A few tens of volts would generate current in the milli amp range, which is not a lot but it's not necessary. You need a complete circuit for current to flow. At 60 Hz, what is the rest of the circuit? One side of the capacitor is grounded through the AC mains ground lead and on the other side is alternating voltage at 60 Hz. It might be better to use one cap on the negative side of the DC output to ground in order to reduce this common mode switch noise. Alternatively you might try a cap on the positive output to ground in addition to the one one the negative side. Here you will only be sending the noise currents down the AC mains leads and not the 60Hz components. I suggest looking at the whole picture and look at what this does at 60 Hz and at RF frequencies. I did and have. It is common practice to use a small value capacitor to ground on or near the power supply outputs where any common mode noise from switching transients is coupled to ground. You can do this at the transformer secondary but why generate the 60 Hz current if you don't have to do that? The object is to conduct noise currents to ground not 60 Hz mains supply. -- Telamon Ventura, California -- Telamon Ventura, California |
How have you measured the noise from the AR7030's soap on a rope
supply? The power dissipation in a linear regular is something that you engineer the product to handle. Running hot isn't good, but hot to the touch isn't nearly as bad as you think. It gets impractical to keep adding heat sink after heat sink to a product, or worse yet a fan that untimately will fail. |
Switching power supplies, AKA switch mode power supplies, suck.
The active switch device is certain to fail at some point. I have lenear supplies that are 30+ years old, and except for replacing the filter caps caps at 15~20 year intervals will work until lightning gets them. I had to attend a school on repairing switchmodes and the Sony techs warned us that they are very sensitive to even minor changes in device parameters. Gain,leakage everthing changes wth age and that is not good. As to repair, the replacement active devices must match exactly. Their advice was repair was difficult and the repaired supply would never be as trustworthy as a new one. They also showed us how the switching transitor junction acts radiates ultrasonics that will eventually cause the junction to fail. They had a nifty ultrasonic transducer that downconverted "audio" up to several MHz to down to 50~10,000Hz audio. I owned a JBL professional audio amp with a switchmode that would eat swithcing power transistors every couple of months. I got a friend to mill the head off the transisotr case and the junction was shattered like glass. We sacrifcied a new transistor and the junction looked like those in a text book. Management insisted that we repair the switchmode supplies and sure enough, no matter what parts we changed, they all failed in less then a year. Part of the problem was we had no way to pick truely matched parts. Thjose little dabs of color on the parts show oddites like turn on/turn off time etc. Diode turn on had to match transistor turn on etc. A big goat rope and lots of fun. Switchmodes are dirty as hell, and any attempt to round those nice harmonic generating square waves only made them run hotter and fail sooner. Some of the low noise equipemnt has the switchers in double shielded cases with fancy feed throughs for power. And they are still dirtier then a good linear supply. Terry |
Thanks for that Terry.
I was recently in the market for a new power supply and straight off thought of one of the lightweight portable switchmode power supplies. Thank goodness my good friend Guy Atkins warned me off them, as you do now, so I bought instead a high quality linear (old fashioned) power supply that is doing sterling service here in this QTH right now. Powered up with this I got a nice catch yesterday morning at sunrise (0500) - WTOP, Washington DC, 1500 Khz, 8,000 miles from here. Have not logged a U.S. station on 1500 Khz before (from any location), so was quite thrilled with this new catch. The U.S. stations come in so rarely here over the mountains that it is quite an occasion. Had the opportunity to fiddle with the Icom 756 a bit, and found that I probably would not have been able to ID and record this weak catch without the 756's outstanding NR facility. -- John Plimmer, Montagu, Western Cape Province, South Africa South 33 d 47 m 32 s, East 20 d 07 m 32 s Icom IC-756 PRO III with MW mods RX Drake R8B, SW8 & ERGO software Sony 7600D GE SRIII BW XCR 30, Braun T1000, Sangean 818 & 803A. Hallicrafters SX-100, Eddystone 940 GE circa 50's radiogram Antenna's RF Systems DX 1 Pro, Datong AD-270 Kiwa MW Loop http://www.dxing.info/about/dxers/plimmer.dx wrote in message oups.com... Switching power supplies, AKA switch mode power supplies, suck. The active switch device is certain to fail at some point. I have lenear supplies that are 30+ years old, and except for replacing the filter caps caps at 15~20 year intervals will work until lightning gets them. I had to attend a school on repairing switchmodes and the Sony techs warned us that they are very sensitive to even minor changes in device parameters. Gain,leakage everthing changes wth age and that is not good. As to repair, the replacement active devices must match exactly. Their advice was repair was difficult and the repaired supply would never be as trustworthy as a new one. They also showed us how the switching transitor junction acts radiates ultrasonics that will eventually cause the junction to fail. They had a nifty ultrasonic transducer that downconverted "audio" up to several MHz to down to 50~10,000Hz audio. I owned a JBL professional audio amp with a switchmode that would eat swithcing power transistors every couple of months. I got a friend to mill the head off the transisotr case and the junction was shattered like glass. We sacrifcied a new transistor and the junction looked like those in a text book. Management insisted that we repair the switchmode supplies and sure enough, no matter what parts we changed, they all failed in less then a year. Part of the problem was we had no way to pick truely matched parts. Thjose little dabs of color on the parts show oddites like turn on/turn off time etc. Diode turn on had to match transistor turn on etc. A big goat rope and lots of fun. Switchmodes are dirty as hell, and any attempt to round those nice harmonic generating square waves only made them run hotter and fail sooner. Some of the low noise equipemnt has the switchers in double shielded cases with fancy feed throughs for power. And they are still dirtier then a good linear supply. Terry |
My preference is for linear supplies, but today's switchers are not all
that bad. I'm baffled about your comment on the ultrasonics causing the transistor junction to fail. Most power devices fail due to hot spot (really the weak points in a reverse bias breakdown) or electromigration. Modern switchers use power fets, so there is no junction in the bipolar sense, unless you count the fast recovery diodes. The real problem with switchers was the poor level of protection circuitry. [I'm lumping start up and short circuit protection together, though start up can really be a problem.] It takes a great deal of overhead to make a rugged switcher. Discrete designs couldn't add all that "just in case" hardware than is common in good switcher controller IC made today. For my own use, I still build/buy linear supplies. I don't care if they are big and heavy. But think how big a 400 watt supply would be in a PC! One problem with switchers is the noise they generate on the line itself. You need to filter both the output and the input on a switcher. For radio use, the line noise can radiate and cause QRM. |
I realize that the inquiry was over six years ago; I do not like to be negative about anything, but I thought I could post my experience with the Racal 6790; I have owned the rig for about three years; on-the-air time has been no more than an hour or two (literally); during that short tenture, I was impressed with the receiver; yes, no modern bells and whistes, but it seemed not to get in the way of the signal; that being said, I would not buy the receiver again; the first time I fired the rcvr up, it worked for about an hour and then I heard a pop and a little smoke came from the chassis; off to New England it was sent; it cost me a few hundred dollars for the repair (and not much less for the shipping as the rig is about 2' xs 2' square); got the receiver back and it worked for another hour and then I kept getting an odd error message on the screen that the manual didn't identify; make sure you download the first few pages of the manual as you will need it for the receiver's self-diagnosis system (push the AM and LOCK membrane switches simultaneously for such); my Harris 590A, Icom R-9000, Racal 17, and R-390 are comparable in performance and have run for years without so much of a hick-up; It is surprising the number of boat anchor people I talk to that include the 6970 in their collections that adds, "with issues". I see two listed today on QTH.COM for sale "as is". IMHO, I don't mean to bash anyone or anything, I just thought my two cents was worth giving to those still mulling this receiver. Thank you for the bandwidth, Phil
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