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Post by mark4 on Jul 15, 2017 15:49:24 GMT -5
I have a Tram D201A fixed bias version. What I noticed was a added yellow wire that goes to the very bottom right side of the post on the mode switch. This wire goes to the terminal strip to a spare terminal not used on other versions. Then a 100K 1/2 watt resistor attaches at this point to a trace on the audio board behind the added blue pot.
Can someone fill me in. Every fixed bias version I have seen has this added wire to the mode switch. And I have never heard anything about it.
When returning the unit to the original bias circuit does this remain connected?
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Post by Sandbagger on Jul 15, 2017 20:41:20 GMT -5
I have a Tram D201A fixed bias version. What I noticed was a added yellow wire that goes to the very bottom right side of the post on the mode switch. This wire goes to the terminal strip to a spare terminal not used on other versions. Then a 100K 1/2 watt resistor attaches at this point to a trace on the audio board behind the added blue pot. Can someone fill me in. Every fixed bias version I have seen has this added wire to the mode switch. And I have never heard anything about it. When returning the unit to the original bias circuit does this remain connected? That wire most likely provided the negative grid bias voltage to keep the tube from "cherry-ing" while operating. They did away with the cathode resistors (located on the BA board) that provided the self bias that was employed in previous models. Not sure why Tram changed over. Based on tube data notes, the grid bias scheme will result in slightly more power output than the self bias method. But the self bias design was a lot more stable. If you are converting back to self bias, make sure to add the cathode resistors back onto the BA board, and never use a BA board for the fixed bias version in an older radio.
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Post by 2600 on Jul 16, 2017 0:06:47 GMT -5
Well, dang! I know I posted a rundown of this issue with the title "Danger Will Robinson, that D201 is gonna EXPLODE!" Meant to warn against using the fixed-bias version of the BA board (the jumper-wire version) in a radio that lacks the fixed bias mod. Had pics of the differences. But it must have been posted on the defunct CB Tricks forum. Can't find my copy on a hard drive here. Dang! Found the pics, though. Here's the fixed-bias version of the BA board. This is the only yellow wire that figures into that discussion. It connects the wiper of the audio tube's bias control to the grid resistor of the audio tube. Best long-term strategy is to remove the yellow wire and solder a wire jumper across the cut in this foil pad. The fixed-bias version of the BA will now work properly in that radio. I consider it better because it's more stable. The fixed-bias setup is just less reliable in the long run. But a yellow wire to the mode selector? Not so sure about that. I'll have a peek inside a D201A and see if I can make sense of that issue. This is our solution to BA board problems. Uses wirewound resistors for the audio tube's bias. They won't drift with age like the factory-original resistors. 73
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Post by mark4 on Jul 16, 2017 8:26:52 GMT -5
Got it. I'm fully aware of not installing the fixed bias BA board in a non fixed bias radio. But it's ok to use the self bias BA board in a fixed bias radio. Never had a issue with doing that.
Curious on your thoughts about that.
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Post by Sandbagger on Jul 16, 2017 8:49:22 GMT -5
Got it. I'm fully aware of not installing the fixed bias BA board in a non fixed bias radio. But it's ok to use the self bias BA board in a fixed bias radio. Never had a issue with doing that. Curious on your thoughts about that. You shouldn't use the self bias BA in a fixed bias radio either, unless you remove the fixed bias circuit. Otherwise you will over bias the output tube and a reduction in power and increased distortion will result.
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Post by mark4 on Jul 17, 2017 5:45:14 GMT -5
Called Barkett about this issue. He makes one BA board for both and told me the added bias is not enough to cause a problem.
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Post by mark4 on Jul 17, 2017 5:47:36 GMT -5
I have a Tram D201A fixed bias version. What I noticed was a added yellow wire that goes to the very bottom right side of the post on the mode switch. This wire goes to the terminal strip to a spare terminal not used on other versions. Then a 100K 1/2 watt resistor attaches at this point to a trace on the audio board behind the added blue pot. Can someone fill me in. Every fixed bias version I have seen has this added wire to the mode switch. And I have never heard anything about it. When returning the unit to the original bias circuit does this remain connected? That wire most likely provided the negative grid bias voltage to keep the tube from "cherry-ing" while operating. They did away with the cathode resistors (located on the BA board) that provided the self bias that was employed in previous models. Not sure why Tram changed over. Based on tube data notes, the grid bias scheme will result in slightly more power output than the self bias method. But the self bias design was a lot more stable. If you are converting back to self bias, make sure to add the cathode resistors back onto the BA board, and never use a BA board for the fixed bias version in an older radio. Think your exactly right about this . I metered it around -22 volts on AM goes to zero on keyup. SSB it stays at -22 volts
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Post by Sandbagger on Jul 17, 2017 11:20:22 GMT -5
Called Barkett about this issue. He makes one BA board for both and told me the added bias is not enough to cause a problem. If by "problem" he means you won't cook the tube, then I would agree. But adding those self bias cathode resistors (and the cathode bypass cap) back in without either removing or adjusting the fixed bias back to zero will drive the tube's bias further toward cutoff, and out of class A operation. The result of which will be lower output and increased distortion.
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Post by Sandbagger on Jul 17, 2017 11:27:06 GMT -5
That wire most likely provided the negative grid bias voltage to keep the tube from "cherry-ing" while operating. They did away with the cathode resistors (located on the BA board) that provided the self bias that was employed in previous models. Not sure why Tram changed over. Based on tube data notes, the grid bias scheme will result in slightly more power output than the self bias method. But the self bias design was a lot more stable. If you are converting back to self bias, make sure to add the cathode resistors back onto the BA board, and never use a BA board for the fixed bias version in an older radio. Think your exactly right about this . I metered it around -22 volts on AM goes to zero on keyup. SSB it stays at -22 volts Interesting. Looks like they were trying to get a little more audio boost on AM transmit by knocking out the bias. But that can cause the plate to glow red on long keyups. On receive, you don't need the extra audio power boost, and SSB doesn't use the modulator on transmit at all, so it would make sense no to switch it there. Someone did a mod similar to that on my Sonar FS23. They added an additional relay which reduced the value of the cathode bias resistance on transmit, giving the audio modulator more power to support the 6 watt carrier it was putting out. The Lafayette HA-410 is setup to do this from the factory. But don't key down for too long....... I never liked how the tube would start to glow dull red on transmit.
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Post by mark4 on Jul 17, 2017 18:55:01 GMT -5
Called Barkett about this issue. He makes one BA board for both and told me the added bias is not enough to cause a problem. If by "problem" he means you won't cook the tube, then I would agree. But adding those self bias cathode resistors (and the cathode bypass cap) back in without either removing or adjusting the fixed bias back to zero will drive the tube's bias further toward cutoff, and out of class A operation. The result of which will be lower output and increased distortion. Nope, he doesn't mean that at all. I called him again today and we spoke extensively about this issue. I have swapped out both types in my fixed bias Tram and there was no less output whatsoever. I have monitored the audio and it was crystal clear. I'll test it on a scope next. When I get around to it. But this is from a longtime expert on these radios. For the record it's not my opinion.
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Post by mark4 on Jul 17, 2017 19:19:40 GMT -5
Well, dang! I know I posted a rundown of this issue with the title "Danger Will Robinson, that D201 is gonna EXPLODE!" Meant to warn against using the fixed-bias version of the BA board (the jumper-wire version) in a radio that lacks the fixed bias mod. Had pics of the differences. But it must have been posted on the defunct CB Tricks forum. Can't find my copy on a hard drive here. Dang! Found the pics, though. Here's the fixed-bias version of the BA board. This is the only yellow wire that figures into that discussion. It connects the wiper of the audio tube's bias control to the grid resistor of the audio tube. Best long-term strategy is to remove the yellow wire and solder a wire jumper across the cut in this foil pad. The fixed-bias version of the BA will now work properly in that radio. I consider it better because it's more stable. The fixed-bias setup is just less reliable in the long run. But a yellow wire to the mode selector? Not so sure about that. I'll have a peek inside a D201A and see if I can make sense of that issue. This is our solution to BA board problems. Uses wirewound resistors for the audio tube's bias. They won't drift with age like the factory-original resistors. 73 Great board. Will purchase another soon.
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Post by Sandbagger on Jul 18, 2017 8:58:39 GMT -5
If by "problem" he means you won't cook the tube, then I would agree. But adding those self bias cathode resistors (and the cathode bypass cap) back in without either removing or adjusting the fixed bias back to zero will drive the tube's bias further toward cutoff, and out of class A operation. The result of which will be lower output and increased distortion. Nope, he doesn't mean that at all. I called him again today and we spoke extensively about this issue. I have swapped out both types in my fixed bias Tram and there was no less output whatsoever. I have monitored the audio and it was crystal clear. I'll test it on a scope next. When I get around to it. But this is from a longtime expert on these radios. For the record it's not my opinion. Just as a point of clarification, I'm not talking about RF power output, I'm talking about audio power from the modulator tube. The reduction of which, and the subsequent flat topping of the modulation waveform on negative peaks when the tube cuts off, may not be readily noticeable to the ear, especially if the carrier power has not been "juiced up". But it should be easy to spot on the scope. When I rebuilt my 2nd D201, I had to replace the self bias resistors, and I didn't have the exact values and I ended up being 100 Ohms higher in value than what was called for and THAT made a difference in audio power and waveform symmetry. Obtaining the correct value parts, took care of it. Think about it for a minute. The tube is happy with somewhere around -22 to -25 volts bias on the grid. In the self bias version, this is accomplished by raising the cathode voltage above ground (in this case it rises to about 31V) using the resistors on the BA board. The grid runs at about 7 or 8 volts giving a difference of ~ -23 V. In the fixed bias version, they bring the cathode back to zero volts, by jumping out the BA resistors, and add in a negative 22-23V bias voltage to the grid, accomplishing the same goal. So what happens when you add the self bias resistors back in? You now are raising the cathode voltage above ground, effectively adding in potentially up to an additional 20 volts of negative grid bias. Now how can that have absolutely no affect on the operation of the tube? I can't believe Greg would tell you that.
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Post by mark4 on Jul 18, 2017 21:44:51 GMT -5
I know full well what was clearly discussed 100%. I have had multiple conversations about this issue. I had to edit my post because I don't think this is the place for this type of discussion when it gets into challenging what someone said.
I'm going to leave it at that.
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Post by Sandbagger on Jul 18, 2017 22:37:30 GMT -5
I know full well what was clearly discussed 100%. I have had multiple conversations about this issue. I had to edit my post because I don't think this is the place for this type of discussion when it gets into challenging what someone said. I'm going to leave it at that. I disagree. This is precisely the place to discuss the things that are done to radios, and the potential effects that can occur when non-factory or non-standard mods are done. As for not challenging what others may say, well, applying that standard to everyday life situations, could result in some really bad advice being taken without question. I ALWAYS question what others say, especially when it flies in the face of established theory. In the 47 years that I've been in CB radio, I've been the recipient of hundreds of "CB Science" and other technically dubious claims, that needed to be given the hairy eyeball. Not saying that Greg falls into that category, as he has certainly earned a great reputation. But no one in this hobby is infallible, and if you are asking me to choose between someone's word, and established tube theory, the engineer in me sides with the theory each and every time. I did not mean to offend you, but it just didn't seem right, and I'm more concerned that people do the correct things to their radios. But I do agree that we've gone as far as we need to here. No sense in belaboring the issue any further.
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Post by mark4 on Jul 19, 2017 14:46:19 GMT -5
I know full well what was clearly discussed 100%. I have had multiple conversations about this issue. I had to edit my post because I don't think this is the place for this type of discussion when it gets into challenging what someone said. I'm going to leave it at that. I disagree. This is precisely the place to discuss the things that are done to radios, and the potential effects that can occur when non-factory or non-standard mods are done. As for not challenging what others may say, well, applying that standard to everyday life situations, could result in some really bad advice being taken without question. I ALWAYS question what others say, especially when it flies in the face of established theory. In the 47 years that I've been in CB radio, I've been the recipient of hundreds of "CB Science" and other technically dubious claims, that needed to be given the hairy eyeball. Not saying that Greg falls into that category, as he has certainly earned a great reputation. But no one in this hobby is infallible, and if you are asking me to choose between someone's word, and established tube theory, the engineer in me sides with the theory each and every time. I did not mean to offend you, but it just didn't seem right, and I'm more concerned that people do the correct things to their radios. But I do agree that we've gone as far as we need to here. No sense in belaboring the issue any further.
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Post by mark4 on Jul 19, 2017 14:57:49 GMT -5
Think you may have missed my point. And I was trying to tone this down a bit. It is the place to correct misinformation. I'm not disputing that. What I have a problem with is the remark "I find it hard to believe Greg would tell you that" Believe it or not that's what I was told your remark leans on the side of calling me a liar. That's where I have the problem. I also have many years of experience. What I don't want to do is get into a pissing match whether right or wrong. I simply stated what I was told. And I stand behind it.
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Post by Sandbagger on Jul 19, 2017 22:02:12 GMT -5
Think you may have missed my point. And I was trying to tone this down a bit. It is the place to correct misinformation. I'm not disputing that. What I have a problem with is the remark "I find it hard to believe Greg would tell you that" Believe it or not that's what I was told your remark leans on the side of calling me a liar. That's where I have the problem. I also have many years of experience. What I don't want to do is get into a pissing match whether right or wrong. I simply stated what I was told. And I stand behind it. This is one of those cases where the absence of verbal tonal inflections can completely change the implication of a comment. When I said that "I can't believe Greg would tell you that", I did not mean to imply that you were lying about it. I completely believe that you are telling it as you heard it. What I meant was that I can't believe that someone of Greg's reputation, and supposed skill would tell you something that flies in the face of well established tube theory. My problem is with not with you repeating it, I'm just amazed that he would claim that a potential change of up to 20V in the bias would have no discernible effect on the tube's output. If you have the time, check it out on a scope between the proper BA board for the fixed bias, and then a BA for the old self bias, without changing the fixed bias and see if you see no difference in the amplitude and symmetry of the output. If you don't see a difference, I would really be surprised. That's never been my experience with audio amp tube bias Please accept my apology for implying that you weren't being honest. That was never my intent.
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Post by mark4 on Jul 19, 2017 22:27:55 GMT -5
Thanks for clarifying your comments. It is appreciated.
I found another difference between my self bias and fixed biasD291A. There is a row of wires terminating in front of the relay socket. The wh/or & wh/ blue have a jumper across them. On the fixed bias version. This appears to be done by the factory, but I maybe wrong about this. I should take a pic and post it.
I'm in the middle of a complete overhaul of a D201A. A local heard me talking on one and 20 minutes later I was buying his for the cost of eBay shipping on one.
I now have receive and transmit power! This has been a basket case fixing multiple golden screwdriver attempts that are evident on the board. Only a couple lifted traces I can repair. The chassis is in very good condition. I have a excellent condition face plate and cover That will be changed out.
It takes a lot of time catching the small things that people do to these old rigs.
The black channel selector still works. It will be completely disassembled , cleaned lubricated with Shield to prolong the life. Mikes radio repair has a informative video on YouTube.
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Post by 2600 on Jul 19, 2017 22:48:35 GMT -5
Wow!
Very cool!
Congrats to you both and 73!
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Post by Sandbagger on Jul 19, 2017 23:06:58 GMT -5
Thanks for clarifying your comments. It is appreciated. I found another difference between my self bias and fixed biasD291A. There is a row of wires terminating in front of the relay socket. The wh/or & wh/ blue have a jumper across them. On the fixed bias version. This appears to be done by the factory, but I maybe wrong about this. I should take a pic and post it. I'm in the middle of a complete overhaul of a D201A. A local heard me talking on one and 20 minutes later I was buying his for the cost of eBay shipping on one. I now have receive and transmit power! This has been a basket case fixing multiple golden screwdriver attempts that are evident on the board. Only a couple lifted traces I can repair. The chassis is in very good condition. I have a excellent condition face plate and cover That will be changed out. It takes a lot of time catching the small things that people do to these old rigs. The black channel selector still works. It will be completely disassembled , cleaned lubricated with Shield to prolong the life. Mikes radio repair has a informative video on YouTube. I was just going to suggest that Mike's Youtube video. Might be the best chance at preserving that black selector, considering the grey replacement is practically unobtanium these days. There have been a lot of mods out there, that can be done to these rigs. Some are better than others. The problem is finding up to date schematics for each different revision in order to determine what was done at the factory, and what was done by someone else. CB Tricks carries only the hard wired version (with VOX) and the early version of the 201A before the change to fixed bias. But there are more versions of the 201/201A that have differences from what I've been able to find. That makes it tough to tell what belongs there and what doesn't. When I rebuild a D201, my first order of business is to undo any obvious mods, and put it back as close to factory stock as possible (one exception to that is the VFO transmit mod), and then start replacing aged parts and repairing lifted traces, and there are usually at least a few. Other than the electrolytic caps (some of those .22uF parts can short), the biggest issue I find is 2 watt resistors that have drastically changed value. Very weak receive, and poor audio can sometimes be caused by this. It's to the point where a proper 100,000 mile overhaul needs to include all of the 2 Watt carbon resistors as well as the electrolytics.
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Post by mark4 on Jul 20, 2017 22:14:22 GMT -5
Here is a progress pic of the D201A All electrolytic caps changed and power resistors. I was really disappointed in the sound at first. I need to also clean the tube sockets. The National 6GH8 Chinese tubes suck. The pins are undersized. Changed them out to RCA and it sounds awesome. Attachments:
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Post by mark4 on Jul 20, 2017 22:24:58 GMT -5
Ok, here is a pic of a jumper that's installed on a fixed bias D201A. I just tacked on the jumper to show where its installed in the unit in question. This is not that unit. I'm currentlly restoring another ad201A. What is this jumper suppose to be for. Is this necessary with the fixed bias version. ( the pic is just the example of what I'm referring to) Attachments:
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Post by mark4 on Aug 5, 2017 12:25:43 GMT -5
I know full well what was clearly discussed 100%. I have had multiple conversations about this issue. I had to edit my post because I don't think this is the place for this type of discussion when it gets into challenging what someone said. I'm going to leave it at that. Ok, I stand corrected here. After rebuilding a BA board to fixed bias I could hear louder audio and more power output. So I called up my friend 2600 on how to convert his custom BA board to fixed bias, which is a big improvement on the original. I performed the mod on both my fixed bias D201A. Here's a pic of the modified board. 10K resistor replaced with a 3watt 27K & 250ohm 5watt resistors removed and replaced with jumpers.
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Post by Sandbagger on Aug 6, 2017 13:34:13 GMT -5
I know full well what was clearly discussed 100%. I have had multiple conversations about this issue. I had to edit my post because I don't think this is the place for this type of discussion when it gets into challenging what someone said. I'm going to leave it at that. Ok, I stand corrected here. After rebuilding a BA board to fixed bias I could hear louder audio and more power output. So I called up my friend 2600 on how to convert his custom BA board to fixed bias, which is a big improvement on the original. I performed the mod on both my fixed bias D201A. Here's a pic of the modified board. 10K resistor replaced with a 3watt 27K & 250ohm 5watt resistors removed and replaced with jumpers. I am glad you were able to try that out and see for yourself. Just be careful to set the fixed bias carefully so that you don't cherry the tube, or rob yourself of power. From what I've heard, the fixed bias can drift, although it shouldn't. But that's the reason given why many people converted back to the older self bias.
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Post by mark4 on Aug 7, 2017 14:33:55 GMT -5
Ok, I stand corrected here. After rebuilding a BA board to fixed bias I could hear louder audio and more power output. So I called up my friend 2600 on how to convert his custom BA board to fixed bias, which is a big improvement on the original. I performed the mod on both my fixed bias D201A. Here's a pic of the modified board. 10K resistor replaced with a 3watt 27K & 250ohm 5watt resistors removed and replaced with jumpers. I am glad you were able to try that out and see for yourself. Just be careful to set the fixed bias carefully so that you don't cherry the tube, or rob yourself of power. From what I've heard, the fixed bias can drift, although it shouldn't. But that's the reason given why many people converted back to the older self bias. Will do. Actually more forward power. And no cherried tube. This was probably more of a issue I suspect on aging Trams. But I know the pot can drift with age. I changed one of my units to fixed bias because I wanted to see the difference. From the reports I get operators have all preferred the sound of my fixed bias radios. Does the little more forward get me out further, No it does not.
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Post by Sandbagger on Aug 7, 2017 19:39:23 GMT -5
I am glad you were able to try that out and see for yourself. Just be careful to set the fixed bias carefully so that you don't cherry the tube, or rob yourself of power. From what I've heard, the fixed bias can drift, although it shouldn't. But that's the reason given why many people converted back to the older self bias. Will do. Actually more forward power. And no cherried tube. This was probably more of a issue I suspect on aging Trams. But I know the pot can drift with age. I changed one of my units to fixed bias because I wanted to see the difference. From the reports I get operators have all preferred the sound of my fixed bias radios. Does the little more forward get me out further, No it does not. Based on the tube specifications and data sheets, fixed bias tends to put out more than self bias, because the the cathode resistor isn't there which allows the tube to fully utilize its gain. But self bias is far easier to implement as it does not need a negative bias supply. I just wish there were more factory schematics of all of the various major and minor revisions of the D201(A), so we can determine which mods came from the factory, and which did....... NOT. I'd like to experiment with fixed bias on one of my D201's just for kicks and grins. But I really wouldn't need the extra audio power, since I have the radio set for 3 watts of carrier power, which is real easy to 100% modulate as it is.
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Post by mark4 on Aug 8, 2017 5:26:39 GMT -5
I don't believe in jumping out those resistors for more power output.
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Post by Sandbagger on Aug 8, 2017 9:18:20 GMT -5
I don't believe in jumping out those resistors for more power output. No, I don't either. I subscribe to the school of thought that says leave the radio alone and if you need more power, buy an amplifier.
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Post by bill on Nov 23, 2017 8:05:09 GMT -5
Here's Nomad's original Post. as Copy and Pasted, to CBTricks : DANGER WILL ROBINSON DANGER WILL ROBINSON! Here's a peculiar kind of POOF! problem that is unique to the 40-channel Tram D201A radios. The very last production version of this radio made a major change to the small plug-in circuit board that's alongside the large audio tube. The manual calls this the "BA" or "Auxiliary Audio" board. Because it simply unplugs from the radio, I hear more and more often about radios that just don't have one of these boards at all. Sitting a fraction of an inch away from a really hot, large power tube causes this board to run hot. The resistors mounted on it also produce heat, and the board will literally burn up and turn from green to black after a few thousand hours of running time. That sounds like a long time, But a (non-leap) calendar year has 8,760 hours in it. A radio that is left turned on and running 24/7 for a year will have a lot of hours on it way before it gets to be 30 or 35 years old. The resistors routinely overheat and have to be replaced. The foil traces will eventually lift from the board, making it tricky to repair. As a result, "spare" radios that are too expensive to fix up will become organ donors. A guy with two or three "junk" Trams will use up his entire supply of this board in a few years if he runs the radio a lot. A few years later when someone decides to rebuild that D201, the search is on for a BA board to make it work. Never fear, you can get one from Barkett Electronics at www.goldeneagleradios.comBut here's the "GOTCHA". the very last production runs of the 40-channel D201A made a radical change to this board, and to the bias circuit on the big, fat audio tube. For you tech geeks out there, what they did was to change tha audio tube from cathode bias (sometimes called "self" bias) to a fixed-bias arrangement that added a half-inch thumbwheel trimpot hanging beneath the solder side of the main audio pc board. This "late"-version BA board has JUMPER WIRES where the older version has two resistors, a fuse and a capacitor. Plug this BA board into an older revision of the radio, and you will get a really nice, smelly smoky POOF! from the audio power section. Might even burn up some main B+ power-supply parts to boot! Those resistors serve to control the current in that audio tube. The earlier version of the radio doesn't have the fixed-bias voltage setup, so the audio tube will simply overheat, or "cherry" just as soon as it warms up. It gets hotter, and hotter until the glass melts, or the smoke gets so thick that someone pulls the plug. But by then it's too late. The damage has been done. Here's a pic of the "late" BA board. "Late", as in your "late" radio after you plug this thing into it. Most 40-channel D201A radios require the older version of the BA board, and ALL 23-channel D201 models do, as well. How do you tell WHICH verson of the 40-channel D201A you have if this board is gone. For that matter, if someone has plugged this type BA into an older 40--channel D201A, you will get the same meltdown result that plugging it into a 23-channel model. Here's an underside view of the main audio board in the "fixed-bias" version of the 40-channel D201A. The bias-set trimpot for the audio tube is near the top of the pic. Has a blue plastic thembwheel about a half inch across. If that's not present, along with the yellow capacitor near the center of the pic, you have the older version of this radio. AND IT'S NOT COMPATIBLE WITH THE 'jumper-wire" version of the BA shown above. Plugging a "jumper-type" BA board into this radio will be a really, really bad idea. Here's a closeup of how they feed the fixed bias voltage into the audio tube's grid circuit. The cure is simple. remove that yellow wire leading to the trimpot, and solder a wire jumper across the cut foil. And install the missing fuse, resistors and cap onto the BA. The yellow capacitor wired across the cut can be removed, once you have jumpered across the cut. That's what we do. The 'fixed-bias' setup is not stable. Tends to cause the audio tube to overheat and damage other parts. The setting of the trimpot is not stable, and always drifts in the direction of MORE tube heat, not less. The original cathode-bias circuit is just more dependable, and the fuse prevents meltdwn problems if that tube breaks down. It happens. I don't have a good pic of the 'fixed' BA board. The owner's manual has a nice, large drawing showing where those other parts should go. I recommend that you NOT use 2-Watt resistors for R638 and R639. We use a 250-ohm 5-Watt wirewound part for each of these. Works fine, and they run cooler. A single 500-ohm 10-Watt wirewound resistor works fine, if you string it across the correct two holes in the BA board. You should also use a 4uf or 5uf 450-Volt cap for C621. The factory-original C621 is rated at 160 Volts. Not a good choice. A close look at the schematic and layout will reveal that C621 is on the WRONG side of the fuse. If the audio tube breaks down, it can shoot way more than 160 Volts into C621. Blows it up. And if C621 becomes a dead short, then you might just as well NOT have that fuse where it is. A cap rated for 450 Volts won't be damaged by a failed audio tube. Cheap insurance. But if you 'borrow' a BA board from a Tram D201 to put into another radio, check to see that it's the right one. Helps to keep the smoke in the radio where it belongs. The radio won't work if you let too much of the smoke out of it. And putting the smoke back in just isn't cheap. 73 ReplyQuoteNotify
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Post by 2600 on Nov 23, 2017 21:42:52 GMT -5
Way cool, Bill. Thanks for digging that up. Couldn't find a saved copy here. Here is the same text with the pictures. Turns out ImageShack hasn't lost these yet. DANGER WILL ROBINSON! Here's a peculiar kind of POOF! problem that is unique to the 40-channel Tram D201A radios. The very last production version of this radio made a major change to the small plug-in circuit board that's alongside the large audio tube. The manual calls this the "BA" or "Auxiliary Audio" board. Because it simply unplugs from the radio, I hear more and more often about radios that just don't have one of these boards at all. Sitting a fraction of an inch away from a really hot, large power tube causes this board to run hot. The resistors mounted on it also produce heat, and the board will literally burn up and turn from green to black after a few thousand hours of running time. That sounds like a long time, But a (non-leap) calendar year has 8,760 hours in it. A radio that is left turned on and running 24/7 for a year will have a lot of hours on it way before it gets to be 30 or 35 years old. The resistors routinely overheat and have to be replaced. The foil traces will eventually lift from the board, making it tricky to repair. As a result, "spare" radios that are too expensive to fix up will become organ donors. A guy with two or three "junk" Trams will use up his entire supply of this board in a few years if he runs the radio a lot. A few years later when someone decides to rebuild that D201, the search is on for a BA board to make it work. Never fear, you can get one from Barkett Electronics at www.goldeneagleradios.comBut here's the "GOTCHA". the very last production runs of the 40-channel D201A made a radical change to this board, and to the bias circuit on the big, fat audio tube. For you tech geeks out there, what they did was to change tha audio tube from cathode bias (sometimes called "self" bias) to a fixed-bias arrangement that added a half-inch thumbwheel trimpot hanging beneath the solder side of the main audio pc board. This "late"-version BA board has JUMPER WIRES where the older version has two resistors, a fuse and a capacitor. Plug this BA board into an older revision of the radio, and you will get a really nice, smelly smoky POOF! from the audio power section. Might even burn up some main B+ power-supply parts to boot! Those resistors serve to control the current in that audio tube. The earlier version of the radio doesn't have the fixed-bias voltage setup, so the audio tube will simply overheat, or "cherry" just as soon as it warms up. It gets hotter, and hotter until the glass melts, or the smoke gets so thick that someone pulls the plug. But by then it's too late. The damage has been done. Here's a pic of the "late" BA board. "Late", as in your "late" radio after you plug this thing into it. Most 40-channel D201A radios require the older version of the BA board, and ALL 23-channel D201 models do, as well. How do you tell WHICH verson of the 40-channel D201A you have if this board is gone. For that matter, if someone has plugged this type BA into an older 40--channel D201A, you will get the same meltdown result that plugging it into a 23-channel model. Here's an underside view of the main audio board in the "fixed-bias" version of the 40-channel D201A. The bias-set trimpot for the audio tube is near the top of the pic. Has a blue plastic thembwheel about a half inch across. If that's not present, along with the yellow capacitor near the center of the pic, you have the older version of this radio. AND IT'S NOT COMPATIBLE WITH THE 'jumper-wire" version of the BA shown above. Plugging a "jumper-type" BA board into this radio will be a really, really bad idea. Here's a closeup of how they feed the fixed bias voltage into the audio tube's grid circuit. The cure is simple. remove that yellow wire leading to the trimpot, and solder a wire jumper across the cut foil. And install the missing fuse, resistors and cap onto the BA. The yellow capacitor wired across the cut can be removed, once you have jumpered across the cut. That's what we do. The 'fixed-bias' setup is not stable. Tends to cause the audio tube to overheat and damage other parts. The setting of the trimpot is not stable, and always drifts in the direction of MORE tube heat, not less. The original cathode-bias circuit is just more dependable, and the fuse prevents meltdwn problems if that tube breaks down. It happens. I don't have a good pic of the 'fixed' BA board. The owner's manual has a nice, large drawing showing where those other parts should go. I recommend that you NOT use 2-Watt resistors for R638 and R639. We use a 250-ohm 5-Watt wirewound part for each of these. Works fine, and they run cooler. A single 500-ohm 10-Watt wirewound resistor works fine, if you string it across the correct two holes in the BA board. You should also use a 4uf or 5uf 450-Volt cap for C621. The factory-original C621 is rated at 160 Volts. Not a good choice. A close look at the schematic and layout will reveal that C621 is on the WRONG side of the fuse. If the audio tube breaks down, it can shoot way more than 160 Volts into C621. Blows it up. And if C621 becomes a dead short, then you might just as well NOT have that fuse where it is. A cap rated for 450 Volts won't be damaged by a failed audio tube. Cheap insurance. But if you 'borrow' a BA board from a Tram D201 to put into another radio, check to see that it's the right one. Helps to keep the smoke in the radio where it belongs. The radio won't work if you let too much of the smoke out of it. And putting the smoke back in just isn't cheap. 73
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