Re: [drakelist] AC3 Bias
Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: I think we're saying the same thing. Using two new tubes and each one of the bad tubes to make the trio, I found that all three of the bad tubes produced the same effect when each was used in combination with the two good tubes. I kept exchanging one of the bad tubes in the trio and each time got the same oscillating. This occurred in both transceivers. The only time it functioned properly was when I combined the two new tubes with an older tube which was weak. Then both radios performed normally. The strange thing is that after I peak the rf tune control for max s-level or noise, when tuning the transmitter I find that the current and output peaks at a completely different spot when I turn the rf tune control to peak current. And current peaks when output peaks, of course, but it won't dip. The xmtr gain control will not reduce output or current once the load and plate controls are maxed. And rotating between sidebands makes no difference after the transmitter is tunes either. Both USB and LSB produce high output. I can't say what I'm looking for, but logic suggests that the PS must have something to do with this. Last night I checked at the PS connector and found normal bias and high voltages (although I get the feeling that the high side is a bit high), and I found less than .6 volts of AC at any of the three sides. I've got three new finals on the way, but I have no expectation that those will act any differently. I did check bias voltages with the transceiver on and found them normal at all three final sockets. I'll keep looking. And thanks for your continued comments. Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Monday, December 03, 2007 10:37 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - OK. I'm not sure you did what I asked. Let me try again... I believe I understood you to say that the 20M thing happens in _both_ transmitters when the good tubes are installed. What I want to try is with two of the good tubes and one of the bad tubes, what happens. If the problem is still there, then leave the two good tubes and put in a _different_ one of the bad tubes. If the problem still exists, again leave the two good tubes and put in the third of the bad tubes. What we're trying to determine is if just ONE of the bad tubes is really bad, or if the situation is that two good tubes with any one of the bad tubes results in the oscillation or whatever we're seeing. You also need to check the neutralization with each tube change. It doesn't have to be perfect, but you should be able to get close enough on 20M pretty easily. The voltages you are seeing are certainly in the ballpark. The HV will go to 700 VDC with no load, and should be around 650V at full load of ~350 mA. There's also the possibility that one of the good tubes is actually the problem, but I can't think what it might be! :-) Do you have one other good 6JB6 that you could rotate through the good trio? I thought Gary might have a clue, but I agree that it's unlikely that both transceivers have a bad capacitor resulting in the same symptoms with one set of tubes and not the other. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: The saga continues. This is getting weird. Back to the unable to control the transmit, and unable to dip the plate current on 20 meters thing. Using your advice I used two new tubes and one of the oscillating tubes on both radios (TR-4 and a TR-4C). The same symptom is there using two new tubes and one of the oscillating tubes. The only way I can get this thing to act normally is if I include one older tube that is only producing about 50% output. Any combination of three strong tubes in either radio causes the problem. Now, the only other thing in common is the power supply I just recapped and redioded. No load bias voltage is approx -45-85, low volt is 257, and high volt is a little over 700. At the PA feedthrough under the chassis when idling the voltage was also around 720. Could too high a voltage on the high section cause these tubes to oscillate on higher frequencies? I can't believe that I've got two transceivers that suddenly caught the same bug. WAY too much of a coincidence. Tonight I plan to open the PS and check everything I did, but if you have any thoughts I'd sure like to hear them. And to Gary P...thanks for the comments. I tried
Re: [drakelist] AC3 Bias
Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom - OK. Just wanted to be sure we were on the same page! :-) It sure sounds like an oscillation in the PA stage. The only thing that comes to mind is when you have one of the older tubes in there, the maximum plate current is lower. The fact that the max current and output peak at a different point than the max receive certainly indicates that the final is at a different frequency than the receiver. This is also reinforced by the fact that the Pi-net won't resonate, indicating that the frequency is outside the range of the 20M band position. Have you tried to find the oscillation frequency by listening on a general coverage receiver? Knowing that frequency might give us a clue. The fact that the RF TUNE control loses control of the output indicates that the PA is oscillating without drive, once it is kicked off by some initial drive. This is a weird one. Anyone else has an idea, speak up!! I thought Gary was on to something until we found that the problem follows the tubes in two separate transceivers. It's looking more and more like the problem is with the GOOD tube(s)! 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: I think we're saying the same thing. Using two new tubes and each one of the bad tubes to make the trio, I found that all three of the bad tubes produced the same effect when each was used in combination with the two good tubes. I kept exchanging one of the bad tubes in the trio and each time got the same oscillating. This occurred in both transceivers. The only time it functioned properly was when I combined the two new tubes with an older tube which was weak. Then both radios performed normally. The strange thing is that after I peak the rf tune control for max s-level or noise, when tuning the transmitter I find that the current and output peaks at a completely different spot when I turn the rf tune control to peak current. And current peaks when output peaks, of course, but it won't dip. The xmtr gain control will not reduce output or current once the load and plate controls are maxed. And rotating between sidebands makes no difference after the transmitter is tunes either. Both USB and LSB produce high output. I can't say what I'm looking for, but logic suggests that the PS must have something to do with this. Last night I checked at the PS connector and found normal bias and high voltages (although I get the feeling that the high side is a bit high), and I found less than .6 volts of AC at any of the three sides. I've got three new finals on the way, but I have no expectation that those will act any differently. I did check bias voltages with the transceiver on and found them normal at all three final sockets. I'll keep looking. And thanks for your continued comments. Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Monday, December 03, 2007 10:37 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - OK. I'm not sure you did what I asked. Let me try again... I believe I understood you to say that the 20M thing happens in _both_ transmitters when the good tubes are installed. What I want to try is with two of the good tubes and one of the bad tubes, what happens. If the problem is still there, then leave the two good tubes and put in a _different_ one of the bad tubes. If the problem still exists, again leave the two good tubes and put in the third of the bad tubes. What we're trying to determine is if just ONE of the bad tubes is really bad, or if the situation is that two good tubes with any one of the bad tubes results in the oscillation or whatever we're seeing. You also need to check the neutralization with each tube change. It doesn't have to be perfect, but you should be able to get close enough on 20M pretty easily. The voltages you are seeing are certainly in the ballpark. The HV will go to 700 VDC with no load, and should be around 650V at full load of ~350 mA. There's also the possibility that one of the good tubes is actually the problem, but I can't think what it might be! :-) Do you have one other good 6JB6 that you could rotate through the good trio? I thought Gary might have a clue, but I agree that it's unlikely that both transceivers have a bad capacitor resulting in the same symptoms with one set of tubes and not the other. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement
Re: [drakelist] AC3 Bias
Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: The saga continues. This is getting weird. Back to the unable to control the transmit, and unable to dip the plate current on 20 meters thing. Using your advice I used two new tubes and one of the oscillating tubes on both radios (TR-4 and a TR-4C). The same symptom is there using two new tubes and one of the oscillating tubes. The only way I can get this thing to act normally is if I include one older tube that is only producing about 50% output. Any combination of three strong tubes in either radio causes the problem. Now, the only other thing in common is the power supply I just recapped and redioded. No load bias voltage is approx -45-85, low volt is 257, and high volt is a little over 700. At the PA feedthrough under the chassis when idling the voltage was also around 720. Could too high a voltage on the high section cause these tubes to oscillate on higher frequencies? I can't believe that I've got two transceivers that suddenly caught the same bug. WAY too much of a coincidence. Tonight I plan to open the PS and check everything I did, but if you have any thoughts I'd sure like to hear them. And to Gary P...thanks for the comments. I tried replacing the driver tube with a new one, and measured the capacitance of the capacitors you mentioned, and they were good. The new tube had no change. Thanks guys. - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Sunday, December 02, 2007 1:15 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom - It sounds like you have an oscillation in the final caused by a defect in one of the tubes. It's a little puzzling why it only shows on 20M, but probably has to do with the neutralization circuitry and the defective tube. The fact that the plate current won't dip implies that the frequency of the oscillation is outside the range of the pi-net when set for 20M. You might be able to find it with an external general coverage receiver, but it's really not that important. I would expect one of the defective trio of tubes to show some color in the plate, since that one is probably drawing the excess plate current. If not, what I would do is take one of the tubes in the good trio, and swap it with one of the tubes in the bad trio. If the oscillation is still evident, replace the tube removed from the bad trio and try another one until you find which one is the cause. If the two tubes in the bad trio are otherwise in good shape, you can probably get a good new one and it will be a close enough match. You can check for match by putting in one tube at a time and noting the plate current at idle. It should be about 33 mA for one tube. More accurate would be to measure the voltage across the 15 ohm cathode resistor of the socket that the one tube is plugged into. Swap in each tube into the same socket and note the plate current of each _without adjusting the bias voltage_ and select the three that are closest in current. Hopefully they will be within a couple - three mA of each other. This is not a real matching, but will be close enough for all practical purposes. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- John: Well, I tried swapping finals. In fact I swapped with my TR-4 finals, which are weak. The finals that were in the TR-4C that was acting up do the same thing in the TR-4, and vice versa. When in the TR-4 the tubes produce very good outputs on all bands except 20, just like they do in the TR-4C. Obviously the problem is the Sylvania 6JB6's, or at least one of them. 20 meters simply won't dip on either radio when it has those tubes and output stays very high even when reducing xmit gain to full CCW. Can someone explain that to me? Do I sense that the only fix is new finals? I hate to give up such a strong trio. Tom - Original Message - From: Thomas Wright [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Saturday, December 01, 2007 2:44 PM Subject: Re: [drakelist] AC3 Bias Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- I assume you mean finals. I have a set but they're weak. - Original Message - From: john [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Saturday, December 01, 2007 1:43 PM Subject: Re: [drakelist] AC3 Bias john [EMAIL PROTECTED] made an utterance to the drakelist gang
Re: [drakelist] AC3 Bias
Gary Poland [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom, The bad .001 capacitor was only discovered with circuit cooler and re heating it with a soldering iron tip ... it failed with a specific temperature. 73, Gary -- Submissions:[EMAIL PROTECTED] Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net --
Re: [drakelist] AC3 Bias
Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - OK. I'm not sure you did what I asked. Let me try again... I believe I understood you to say that the 20M thing happens in _both_ transmitters when the good tubes are installed. What I want to try is with two of the good tubes and one of the bad tubes, what happens. If the problem is still there, then leave the two good tubes and put in a _different_ one of the bad tubes. If the problem still exists, again leave the two good tubes and put in the third of the bad tubes. What we're trying to determine is if just ONE of the bad tubes is really bad, or if the situation is that two good tubes with any one of the bad tubes results in the oscillation or whatever we're seeing. You also need to check the neutralization with each tube change. It doesn't have to be perfect, but you should be able to get close enough on 20M pretty easily. The voltages you are seeing are certainly in the ballpark. The HV will go to 700 VDC with no load, and should be around 650V at full load of ~350 mA. There's also the possibility that one of the good tubes is actually the problem, but I can't think what it might be! :-) Do you have one other good 6JB6 that you could rotate through the good trio? I thought Gary might have a clue, but I agree that it's unlikely that both transceivers have a bad capacitor resulting in the same symptoms with one set of tubes and not the other. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: The saga continues. This is getting weird. Back to the unable to control the transmit, and unable to dip the plate current on 20 meters thing. Using your advice I used two new tubes and one of the oscillating tubes on both radios (TR-4 and a TR-4C). The same symptom is there using two new tubes and one of the oscillating tubes. The only way I can get this thing to act normally is if I include one older tube that is only producing about 50% output. Any combination of three strong tubes in either radio causes the problem. Now, the only other thing in common is the power supply I just recapped and redioded. No load bias voltage is approx -45-85, low volt is 257, and high volt is a little over 700. At the PA feedthrough under the chassis when idling the voltage was also around 720. Could too high a voltage on the high section cause these tubes to oscillate on higher frequencies? I can't believe that I've got two transceivers that suddenly caught the same bug. WAY too much of a coincidence. Tonight I plan to open the PS and check everything I did, but if you have any thoughts I'd sure like to hear them. And to Gary P...thanks for the comments. I tried replacing the driver tube with a new one, and measured the capacitance of the capacitors you mentioned, and they were good. The new tube had no change. Thanks guys. - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Sunday, December 02, 2007 1:15 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom - It sounds like you have an oscillation in the final caused by a defect in one of the tubes. It's a little puzzling why it only shows on 20M, but probably has to do with the neutralization circuitry and the defective tube. The fact that the plate current won't dip implies that the frequency of the oscillation is outside the range of the pi-net when set for 20M. You might be able to find it with an external general coverage receiver, but it's really not that important. I would expect one of the defective trio of tubes to show some color in the plate, since that one is probably drawing the excess plate current. If not, what I would do is take one of the tubes in the good trio, and swap it with one of the tubes in the bad trio. If the oscillation is still evident, replace the tube removed from the bad trio and try another one until you find which one is the cause. If the two tubes in the bad trio are otherwise in good shape, you can probably get a good new one and it will be a close enough match. You can check for match by putting in one tube at a time and noting the plate current at idle. It should be about 33 mA for one tube. More accurate would be to measure the voltage across the 15 ohm cathode resistor of the socket that the one tube is plugged into. Swap in each tube into the same socket and note the plate current of each _without adjusting the bias voltage_ and select the three that are closest in current. Hopefully
Re: [drakelist] AC3 Bias
Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- John: Well, I tried swapping finals. In fact I swapped with my TR-4 finals, which are weak. The finals that were in the TR-4C that was acting up do the same thing in the TR-4, and vice versa. When in the TR-4 the tubes produce very good outputs on all bands except 20, just like they do in the TR-4C. Obviously the problem is the Sylvania 6JB6's, or at least one of them. 20 meters simply won't dip on either radio when it has those tubes and output stays very high even when reducing xmit gain to full CCW. Can someone explain that to me? Do I sense that the only fix is new finals? I hate to give up such a strong trio. Tom - Original Message - From: Thomas Wright [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Saturday, December 01, 2007 2:44 PM Subject: Re: [drakelist] AC3 Bias Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- I assume you mean finals. I have a set but they're weak. - Original Message - From: john [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Saturday, December 01, 2007 1:43 PM Subject: Re: [drakelist] AC3 Bias john [EMAIL PROTECTED] made an utterance to the drakelist gang -- Sounds like the PA might be oscillating on 20M. Do you have another set of tubes to try, to see if behaves differently? John At 12:52 PM 12/1/2007, you wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: What I found taught me a lesson and has me confused. After rechecking the PS, and reconnecting to the TR4C, I realized I hadn't tried bias adjustment on anywhere other than 20m. The bias acts normally on all other bands, but on 14Mhz, the bias won't go below .25 amp, the rf tune control has no effect on output, the plate current won't dip, and when the mode switch is ccw, as on LSB, the PA output is still high. This has me totally confused. The rig acts correctly on all other bandseven gives me 140 watts on 28Mhz. Any ideas? Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, November 30, 2007 3:47 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom - OK. The specified load for the Bias supply is ~ 30k, so a load of this value would emulate the load of the transceiver. The bias goes to the PA via the _middle_ feedthrough on the side of the PA shield under the chassis. These feedthroughs are fragile and easily cracked if too much force is applied to the terminals. The other points in the transceiver that are tied to the Bias supply are all very high impedance, so a short on one of those is unlikely to cause as big a difference as you are seeing. Another unlikely fault would be a Grid-Cathode short in one of the final tubes. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: I measured the bias while disconnected from the transceiver, and when swinging the pot from stop to stop it seemed to vary the minus voltage linearly with no spikes, but of course the Fluke may dampen such a spike if it's a fast one. I'm thinking it may be one of the two resistors opening up under heat or current (6.8K and 10K). The pot measures correct resistance from one end to the other. When connected to the rig the current was either very low or very high, but when disconnected, the voltage seems vernier as it should. I thought that -52 volts or so may be too high a bias voltage, especially if that's as low as it will go. Thanks for you continued help. Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, November 30, 2007 3:13 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - I just re-read your message.. If the IDLE current is either Zero or 0.4A, with little adjustment, I'd suspect the potentiometer has a break or bad spot on the resistive element. That behavior indicates that the Bias voltage is either some high value, such as 70-80 Negative volts, cutting the final tubes off completely, or somewhere around 30-40 Negative volts, allowing some 400 ma of idle current, depending upon which side of the break the wiper
Re: [drakelist] AC3 Bias
Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom - It sounds like you have an oscillation in the final caused by a defect in one of the tubes. It's a little puzzling why it only shows on 20M, but probably has to do with the neutralization circuitry and the defective tube. The fact that the plate current won't dip implies that the frequency of the oscillation is outside the range of the pi-net when set for 20M. You might be able to find it with an external general coverage receiver, but it's really not that important. I would expect one of the defective trio of tubes to show some color in the plate, since that one is probably drawing the excess plate current. If not, what I would do is take one of the tubes in the good trio, and swap it with one of the tubes in the bad trio. If the oscillation is still evident, replace the tube removed from the bad trio and try another one until you find which one is the cause. If the two tubes in the bad trio are otherwise in good shape, you can probably get a good new one and it will be a close enough match. You can check for match by putting in one tube at a time and noting the plate current at idle. It should be about 33 mA for one tube. More accurate would be to measure the voltage across the 15 ohm cathode resistor of the socket that the one tube is plugged into. Swap in each tube into the same socket and note the plate current of each _without adjusting the bias voltage_ and select the three that are closest in current. Hopefully they will be within a couple - three mA of each other. This is not a real matching, but will be close enough for all practical purposes. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- John: Well, I tried swapping finals. In fact I swapped with my TR-4 finals, which are weak. The finals that were in the TR-4C that was acting up do the same thing in the TR-4, and vice versa. When in the TR-4 the tubes produce very good outputs on all bands except 20, just like they do in the TR-4C. Obviously the problem is the Sylvania 6JB6's, or at least one of them. 20 meters simply won't dip on either radio when it has those tubes and output stays very high even when reducing xmit gain to full CCW. Can someone explain that to me? Do I sense that the only fix is new finals? I hate to give up such a strong trio. Tom - Original Message - From: Thomas Wright [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Saturday, December 01, 2007 2:44 PM Subject: Re: [drakelist] AC3 Bias Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- I assume you mean finals. I have a set but they're weak. - Original Message - From: john [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Saturday, December 01, 2007 1:43 PM Subject: Re: [drakelist] AC3 Bias john [EMAIL PROTECTED] made an utterance to the drakelist gang -- Sounds like the PA might be oscillating on 20M. Do you have another set of tubes to try, to see if behaves differently? John -- Submissions:[EMAIL PROTECTED] Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net --
Re: [drakelist] AC3 Bias
Gary Poland [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom, I have had the same problem in the past with two TR-4C's. All bands ok except 20 meters. One was fixed buy changing the 12BY7A driver and the other by replacing C51, a .001 disc capacitor at the tube socket for the 12BY7A. I also have a note written to check C59, a 865 pf silver mica capacitor. I don't remember if I have ever changed C59, it may be a suggestion I got from Drake at one time. 73, Gary W8PU http://home.cinci.rr.com/w8pu -- Submissions:[EMAIL PROTECTED] Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net --
Re: [drakelist] AC3 Bias
Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: What I found taught me a lesson and has me confused. After rechecking the PS, and reconnecting to the TR4C, I realized I hadn't tried bias adjustment on anywhere other than 20m. The bias acts normally on all other bands, but on 14Mhz, the bias won't go below .25 amp, the rf tune control has no effect on output, the plate current won't dip, and when the mode switch is ccw, as on LSB, the PA output is still high. This has me totally confused. The rig acts correctly on all other bandseven gives me 140 watts on 28Mhz. Any ideas? Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, November 30, 2007 3:47 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom - OK. The specified load for the Bias supply is ~ 30k, so a load of this value would emulate the load of the transceiver. The bias goes to the PA via the _middle_ feedthrough on the side of the PA shield under the chassis. These feedthroughs are fragile and easily cracked if too much force is applied to the terminals. The other points in the transceiver that are tied to the Bias supply are all very high impedance, so a short on one of those is unlikely to cause as big a difference as you are seeing. Another unlikely fault would be a Grid-Cathode short in one of the final tubes. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: I measured the bias while disconnected from the transceiver, and when swinging the pot from stop to stop it seemed to vary the minus voltage linearly with no spikes, but of course the Fluke may dampen such a spike if it's a fast one. I'm thinking it may be one of the two resistors opening up under heat or current (6.8K and 10K). The pot measures correct resistance from one end to the other. When connected to the rig the current was either very low or very high, but when disconnected, the voltage seems vernier as it should. I thought that -52 volts or so may be too high a bias voltage, especially if that's as low as it will go. Thanks for you continued help. Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, November 30, 2007 3:13 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - I just re-read your message.. If the IDLE current is either Zero or 0.4A, with little adjustment, I'd suspect the potentiometer has a break or bad spot on the resistive element. That behavior indicates that the Bias voltage is either some high value, such as 70-80 Negative volts, cutting the final tubes off completely, or somewhere around 30-40 Negative volts, allowing some 400 ma of idle current, depending upon which side of the break the wiper is on. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Maybe you guys can direct me a little. My AC3 has been showing signs of bad caps, especially in the bias section ( one of the bias caps measured at 700MFD). I replaced all the caps in all sections, using 33MFD 160 volt caps in the bias section, and while I was there did the diodes. After reassembly the bias was adjustable from 41 or so to 80 volts. TR-4C was running well. Today after operating for an hour or so, the transmitter started to show very high plate current, so I checked the bias and found that it was either zero or above .4 amps, without much adjustment between the two, and of course when higher the xmit relay was engaging. Reducing xmit gain would not reduce the plate current. I measured the voltage at the plug and the bias is now only adjustable from 52 to 94 volts, which I think is too high. There's not much else to replace in the AC3 except the bias pot and two resistors, both of which measure within tolerance statically. Any thoughts? -- Submissions:[EMAIL PROTECTED] Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net -- -- Submissions:[EMAIL PROTECTED] Unsubscribe
Re: [drakelist] AC3 Bias
john [EMAIL PROTECTED] made an utterance to the drakelist gang -- Sounds like the PA might be oscillating on 20M. Do you have another set of tubes to try, to see if behaves differently? John At 12:52 PM 12/1/2007, you wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: What I found taught me a lesson and has me confused. After rechecking the PS, and reconnecting to the TR4C, I realized I hadn't tried bias adjustment on anywhere other than 20m. The bias acts normally on all other bands, but on 14Mhz, the bias won't go below .25 amp, the rf tune control has no effect on output, the plate current won't dip, and when the mode switch is ccw, as on LSB, the PA output is still high. This has me totally confused. The rig acts correctly on all other bandseven gives me 140 watts on 28Mhz. Any ideas? Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, November 30, 2007 3:47 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom - OK. The specified load for the Bias supply is ~ 30k, so a load of this value would emulate the load of the transceiver. The bias goes to the PA via the _middle_ feedthrough on the side of the PA shield under the chassis. These feedthroughs are fragile and easily cracked if too much force is applied to the terminals. The other points in the transceiver that are tied to the Bias supply are all very high impedance, so a short on one of those is unlikely to cause as big a difference as you are seeing. Another unlikely fault would be a Grid-Cathode short in one of the final tubes. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: I measured the bias while disconnected from the transceiver, and when swinging the pot from stop to stop it seemed to vary the minus voltage linearly with no spikes, but of course the Fluke may dampen such a spike if it's a fast one. I'm thinking it may be one of the two resistors opening up under heat or current (6.8K and 10K). The pot measures correct resistance from one end to the other. When connected to the rig the current was either very low or very high, but when disconnected, the voltage seems vernier as it should. I thought that -52 volts or so may be too high a bias voltage, especially if that's as low as it will go. Thanks for you continued help. Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, November 30, 2007 3:13 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - I just re-read your message.. If the IDLE current is either Zero or 0.4A, with little adjustment, I'd suspect the potentiometer has a break or bad spot on the resistive element. That behavior indicates that the Bias voltage is either some high value, such as 70-80 Negative volts, cutting the final tubes off completely, or somewhere around 30-40 Negative volts, allowing some 400 ma of idle current, depending upon which side of the break the wiper is on. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Maybe you guys can direct me a little. My AC3 has been showing signs of bad caps, especially in the bias section ( one of the bias caps measured at 700MFD). I replaced all the caps in all sections, using 33MFD 160 volt caps in the bias section, and while I was there did the diodes. After reassembly the bias was adjustable from 41 or so to 80 volts. TR-4C was running well. Today after operating for an hour or so, the transmitter started to show very high plate current, so I checked the bias and found that it was either zero or above .4 amps, without much adjustment between the two, and of course when higher the xmit relay was engaging. Reducing xmit gain would not reduce the plate current. I measured the voltage at the plug and the bias is now only adjustable from 52 to 94 volts, which I think is too high. There's not much else to replace in the AC3 except the bias pot and two resistors, both of which measure within tolerance statically. Any thoughts? -- Submissions:[EMAIL PROTECTED] Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED
Re: [drakelist] AC3 Bias
Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- I assume you mean finals. I have a set but they're weak. - Original Message - From: john [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Saturday, December 01, 2007 1:43 PM Subject: Re: [drakelist] AC3 Bias john [EMAIL PROTECTED] made an utterance to the drakelist gang -- Sounds like the PA might be oscillating on 20M. Do you have another set of tubes to try, to see if behaves differently? John At 12:52 PM 12/1/2007, you wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: What I found taught me a lesson and has me confused. After rechecking the PS, and reconnecting to the TR4C, I realized I hadn't tried bias adjustment on anywhere other than 20m. The bias acts normally on all other bands, but on 14Mhz, the bias won't go below .25 amp, the rf tune control has no effect on output, the plate current won't dip, and when the mode switch is ccw, as on LSB, the PA output is still high. This has me totally confused. The rig acts correctly on all other bandseven gives me 140 watts on 28Mhz. Any ideas? Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, November 30, 2007 3:47 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom - OK. The specified load for the Bias supply is ~ 30k, so a load of this value would emulate the load of the transceiver. The bias goes to the PA via the _middle_ feedthrough on the side of the PA shield under the chassis. These feedthroughs are fragile and easily cracked if too much force is applied to the terminals. The other points in the transceiver that are tied to the Bias supply are all very high impedance, so a short on one of those is unlikely to cause as big a difference as you are seeing. Another unlikely fault would be a Grid-Cathode short in one of the final tubes. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: I measured the bias while disconnected from the transceiver, and when swinging the pot from stop to stop it seemed to vary the minus voltage linearly with no spikes, but of course the Fluke may dampen such a spike if it's a fast one. I'm thinking it may be one of the two resistors opening up under heat or current (6.8K and 10K). The pot measures correct resistance from one end to the other. When connected to the rig the current was either very low or very high, but when disconnected, the voltage seems vernier as it should. I thought that -52 volts or so may be too high a bias voltage, especially if that's as low as it will go. Thanks for you continued help. Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, November 30, 2007 3:13 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - I just re-read your message.. If the IDLE current is either Zero or 0.4A, with little adjustment, I'd suspect the potentiometer has a break or bad spot on the resistive element. That behavior indicates that the Bias voltage is either some high value, such as 70-80 Negative volts, cutting the final tubes off completely, or somewhere around 30-40 Negative volts, allowing some 400 ma of idle current, depending upon which side of the break the wiper is on. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Maybe you guys can direct me a little. My AC3 has been showing signs of bad caps, especially in the bias section ( one of the bias caps measured at 700MFD). I replaced all the caps in all sections, using 33MFD 160 volt caps in the bias section, and while I was there did the diodes. After reassembly the bias was adjustable from 41 or so to 80 volts. TR-4C was running well. Today after operating for an hour or so, the transmitter started to show very high plate current, so I checked the bias and found that it was either zero or above .4 amps, without much adjustment between the two, and of course when higher the xmit relay was engaging. Reducing xmit gain would not reduce the plate current. I measured the voltage at the plug and the bias is now only adjustable from 52 to 94 volts, which I think is too high
Re: [drakelist] AC3 Bias
Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: I measured the bias while disconnected from the transceiver, and when swinging the pot from stop to stop it seemed to vary the minus voltage linearly with no spikes, but of course the Fluke may dampen such a spike if it's a fast one. I'm thinking it may be one of the two resistors opening up under heat or current (6.8K and 10K). The pot measures correct resistance from one end to the other. When connected to the rig the current was either very low or very high, but when disconnected, the voltage seems vernier as it should. I thought that -52 volts or so may be too high a bias voltage, especially if that's as low as it will go. Thanks for you continued help. Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, November 30, 2007 3:13 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - I just re-read your message.. If the IDLE current is either Zero or 0.4A, with little adjustment, I'd suspect the potentiometer has a break or bad spot on the resistive element. That behavior indicates that the Bias voltage is either some high value, such as 70-80 Negative volts, cutting the final tubes off completely, or somewhere around 30-40 Negative volts, allowing some 400 ma of idle current, depending upon which side of the break the wiper is on. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Maybe you guys can direct me a little. My AC3 has been showing signs of bad caps, especially in the bias section ( one of the bias caps measured at 700MFD). I replaced all the caps in all sections, using 33MFD 160 volt caps in the bias section, and while I was there did the diodes. After reassembly the bias was adjustable from 41 or so to 80 volts. TR-4C was running well. Today after operating for an hour or so, the transmitter started to show very high plate current, so I checked the bias and found that it was either zero or above .4 amps, without much adjustment between the two, and of course when higher the xmit relay was engaging. Reducing xmit gain would not reduce the plate current. I measured the voltage at the plug and the bias is now only adjustable from 52 to 94 volts, which I think is too high. There's not much else to replace in the AC3 except the bias pot and two resistors, both of which measure within tolerance statically. Any thoughts? -- Submissions:[EMAIL PROTECTED] Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net -- -- Submissions:[EMAIL PROTECTED] Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net --
[drakelist] AC3 Bias
Maybe you guys can direct me a little. My AC3 has been showing signs of bad caps, especially in the bias section ( one of the bias caps measured at 700MFD). I replaced all the caps in all sections, using 33MFD 160 volt caps in the bias section, and while I was there did the diodes. After reassembly the bias was adjustable from 41 or so to 80 volts. TR-4C was running well. Today after operating for an hour or so, the transmitter started to show very high plate current, so I checked the bias and found that it was either zero or above .4 amps, without much adjustment between the two, and of course when higher the xmit relay was engaging. Reducing xmit gain would not reduce the plate current. I measured the voltage at the plug and the bias is now only adjustable from 52 to 94 volts, which I think is too high. There's not much else to replace in the AC3 except the bias pot and two resistors, both of which measure within tolerance statically. Any thoughts?
Re: [drakelist] AC3 Bias
Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - I just re-read your message.. If the IDLE current is either Zero or 0.4A, with little adjustment, I'd suspect the potentiometer has a break or bad spot on the resistive element. That behavior indicates that the Bias voltage is either some high value, such as 70-80 Negative volts, cutting the final tubes off completely, or somewhere around 30-40 Negative volts, allowing some 400 ma of idle current, depending upon which side of the break the wiper is on. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Maybe you guys can direct me a little. My AC3 has been showing signs of bad caps, especially in the bias section ( one of the bias caps measured at 700MFD). I replaced all the caps in all sections, using 33MFD 160 volt caps in the bias section, and while I was there did the diodes. After reassembly the bias was adjustable from 41 or so to 80 volts. TR-4C was running well. Today after operating for an hour or so, the transmitter started to show very high plate current, so I checked the bias and found that it was either zero or above .4 amps, without much adjustment between the two, and of course when higher the xmit relay was engaging. Reducing xmit gain would not reduce the plate current. I measured the voltage at the plug and the bias is now only adjustable from 52 to 94 volts, which I think is too high. There's not much else to replace in the AC3 except the bias pot and two resistors, both of which measure within tolerance statically. Any thoughts? -- Submissions:[EMAIL PROTECTED] Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net --
Re: [drakelist] AC3 Bias
Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - Yeah, there are mighty few AC-3 power supplies still running with their original electrolytics! First of all, the Final idle current is a DC circuit that requires nothing more than Plate, Screen and Filament voltage, and that the Cathode is at ground potential. These conditions will result in the Final tubes drawing literally Amps of plate current for as long as the power supply, its fuse, the tube plates and/or the wiring will last!! To control the idle current we apply a Negative voltage to the Grids of the final tubes, and somewhere around -50 to -60 VDC at the Grids, the final plate current will be around 0.1A for a trio of good tubes. If the Grid voltage is More negative, say -70 VDC, the idle current will _decrease_ and if the Grid voltage is Less negative, -40 VDC the idle current will _increase_ . So either range you are seeing on your Bias supply is sufficient to hold the idle current to 0.1A. Your problem is that your Bias supply is dropping to zero volts, which results in maximum Final Plate current. You don't say where you were checking the Bias voltage when it was at Zero, but I suspect you have either a bad solder joint or broken wire in the Bias supply. You can work on the supply with it disconnected from the transceiver, by shorting Pins 1 and 2 of the connector together, which applies AC to the power supply. Don't forget that there is still 300 and 700 VDC floating around on the other pins of that connector when it is ON, and for a brief period after it is turned off until the bleeder resistors can drop that voltage back near zero!! It's also possible that an intermittent open of either R6 or R7 is occurring, or that the pot wiper is losing contact with the resistive element. An open in either of these elements will take the Bias voltage to zero. An open in R5 will take the Bias voltage to maximum. A pot cleaner/lube such as CAIG CaiLube can often clean a dirty pot, and sometimes just rotating the pot through it's range several times with the power off will clear it. Occasionally a pot element will open up where the wiper has been sitting for a while, and the circuit will operate as long as the wiper is bridging the break. From what you wrote, I'd suspect a problem with R5 and/or its connections. I'd suggest carefully examining the Bias supply wiring, and then with a meter across the output while disconnected from the TR-4 push and prod the connections and components will an insulated stick. Again, be aware that there are serious, dangerous voltages floating around in there any time the power supply is on. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Maybe you guys can direct me a little. My AC3 has been showing signs of bad caps, especially in the bias section ( one of the bias caps measured at 700MFD). I replaced all the caps in all sections, using 33MFD 160 volt caps in the bias section, and while I was there did the diodes. After reassembly the bias was adjustable from 41 or so to 80 volts. TR-4C was running well. Today after operating for an hour or so, the transmitter started to show very high plate current, so I checked the bias and found that it was either zero or above .4 amps, without much adjustment between the two, and of course when higher the xmit relay was engaging. Reducing xmit gain would not reduce the plate current. I measured the voltage at the plug and the bias is now only adjustable from 52 to 94 volts, which I think is too high. There's not much else to replace in the AC3 except the bias pot and two resistors, both of which measure within tolerance statically. Any thoughts? No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.503 / Virus Database: 269.16.10/1160 - Release Date: 11/29/2007 8:32 PM -- Submissions:[EMAIL PROTECTED] Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net --
Re: [drakelist] AC3 Bias
Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Tom - OK. The specified load for the Bias supply is ~ 30k, so a load of this value would emulate the load of the transceiver. The bias goes to the PA via the _middle_ feedthrough on the side of the PA shield under the chassis. These feedthroughs are fragile and easily cracked if too much force is applied to the terminals. The other points in the transceiver that are tied to the Bias supply are all very high impedance, so a short on one of those is unlikely to cause as big a difference as you are seeing. Another unlikely fault would be a Grid-Cathode short in one of the final tubes. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Thomas Wright [EMAIL PROTECTED] made an utterance to the drakelist gang -- Garey: I measured the bias while disconnected from the transceiver, and when swinging the pot from stop to stop it seemed to vary the minus voltage linearly with no spikes, but of course the Fluke may dampen such a spike if it's a fast one. I'm thinking it may be one of the two resistors opening up under heat or current (6.8K and 10K). The pot measures correct resistance from one end to the other. When connected to the rig the current was either very low or very high, but when disconnected, the voltage seems vernier as it should. I thought that -52 volts or so may be too high a bias voltage, especially if that's as low as it will go. Thanks for you continued help. Tom - Original Message - From: Garey Barrell [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, November 30, 2007 3:13 PM Subject: Re: [drakelist] AC3 Bias Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Thomas - I just re-read your message.. If the IDLE current is either Zero or 0.4A, with little adjustment, I'd suspect the potentiometer has a break or bad spot on the resistive element. That behavior indicates that the Bias voltage is either some high value, such as 70-80 Negative volts, cutting the final tubes off completely, or somewhere around 30-40 Negative volts, allowing some 400 ma of idle current, depending upon which side of the break the wiper is on. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B C-Line Service Supplement CDs www.k4oah.com Thomas Wright wrote: Maybe you guys can direct me a little. My AC3 has been showing signs of bad caps, especially in the bias section ( one of the bias caps measured at 700MFD). I replaced all the caps in all sections, using 33MFD 160 volt caps in the bias section, and while I was there did the diodes. After reassembly the bias was adjustable from 41 or so to 80 volts. TR-4C was running well. Today after operating for an hour or so, the transmitter started to show very high plate current, so I checked the bias and found that it was either zero or above .4 amps, without much adjustment between the two, and of course when higher the xmit relay was engaging. Reducing xmit gain would not reduce the plate current. I measured the voltage at the plug and the bias is now only adjustable from 52 to 94 volts, which I think is too high. There's not much else to replace in the AC3 except the bias pot and two resistors, both of which measure within tolerance statically. Any thoughts? -- Submissions:[EMAIL PROTECTED] Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net --
