Keep in mind adding things inline with the antenna like a TVI filter will change the reactance of the load that the amp sees. This is the most common thing to cause an unstable amplifier to shift its operating conditions. Be it from stable to oscillating or vice versa. Harmonics are almost never the cause of high VSWR because even in the worst case single ended solid state amp, harmonics never reach more than a few percent of total output power. Oscillations on the other hand can produce substantial power on undesired frequencies.
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Texas Star DX500 and high SWR
- Thread starter 2RT307
- Start date
Hi all, I am still planning to work with this issue. Just trying to find the time to get it done.
Perhaps a good project over the winter months.
Stay tuned....
Perhaps a good project over the winter months.
Stay tuned....
Awesome thread! There are some extremely smart guys on here!
The issue I'm seeing with my TS500v is that when I use the variable pot and cut the power back to 3/4 or less my swr's are 1.4:1 and they go down as I turn the power down. Now if the pot is wide open or the variable is off my swr's jump through the roof.
I'm running RG8x for coax and with the Amp off my SWR is 1.1:1 on ch38 using a steel whip.
My radio is a Sears Road Talker 40 AM/SSB. It DK 2 watts and Swings 20ish.
I guess I need to start with the Chokes and Sandbars?
The issue I'm seeing with my TS500v is that when I use the variable pot and cut the power back to 3/4 or less my swr's are 1.4:1 and they go down as I turn the power down. Now if the pot is wide open or the variable is off my swr's jump through the roof.
I'm running RG8x for coax and with the Amp off my SWR is 1.1:1 on ch38 using a steel whip.
My radio is a Sears Road Talker 40 AM/SSB. It DK 2 watts and Swings 20ish.
I guess I need to start with the Chokes and Sandbars?
These symptoms sound more like a problem with the variable power circuit itself. Try pushing the AM or SSB switch in and see if the VSWR is better with those settings. This changes the fixed value resistors that are inline with the variable. if the other settings are noticeably better, you probably have a burnt resistor on the switches.
If the power drops out or reduces when you turn the variable up, the control itself has been damaged by too much drive. If that's the case, leave the center wire connected to the control and move the wire on the end from one side to the other.
That would restore the variable but it would work in reverse. Turning it down will increase power. Don't be alarmed if none of this turns out to be the problem since these power control circuits are poorly designed to begin with.
To do this job right requires a properly designed attenuator, not just some low value resistors inline with the RF. The attenuator circuit will apply more resistive loading in parallel to the driver (radio) as the resistance in series feeding the amp is increased. The overall effect is the radio continues to see a 50 ohm load as the resistance to the amp goes up and output goes down.
This is not how it is done in the Texas Star. The problem is they are using an idea for power control when the correct parts to do the job are not even manufactured, so they use the wrong parts instead. The largest non inductive variable resistors were made by Clarostat and were 2 watts. That part was small for the job and not made in 2 decades.
I use to rip all of those resistors and the variable right out of the circuit and build my own 50 ohm tuned input circuit to feed the amp without the variable. Then control the power at the radio where it's much easier to do at lower levels.
If the power drops out or reduces when you turn the variable up, the control itself has been damaged by too much drive. If that's the case, leave the center wire connected to the control and move the wire on the end from one side to the other.
That would restore the variable but it would work in reverse. Turning it down will increase power. Don't be alarmed if none of this turns out to be the problem since these power control circuits are poorly designed to begin with.
To do this job right requires a properly designed attenuator, not just some low value resistors inline with the RF. The attenuator circuit will apply more resistive loading in parallel to the driver (radio) as the resistance in series feeding the amp is increased. The overall effect is the radio continues to see a 50 ohm load as the resistance to the amp goes up and output goes down.
This is not how it is done in the Texas Star. The problem is they are using an idea for power control when the correct parts to do the job are not even manufactured, so they use the wrong parts instead. The largest non inductive variable resistors were made by Clarostat and were 2 watts. That part was small for the job and not made in 2 decades.
I use to rip all of those resistors and the variable right out of the circuit and build my own 50 ohm tuned input circuit to feed the amp without the variable. Then control the power at the radio where it's much easier to do at lower levels.
Well when I first noticed this the other day I would turn the variable up and the power would go up along with the SWR. When I did this I was on AM.
I will test your idea this evening and report back.
Thanks!
Shockwave is correct. the variable control circuit in these amplifiers is useless And changing out the sandbar resistors and replacing the stock chokes with the updated ones with the thicker wire wrap will only have a margional impact, And they may already be using the uopdated chokes in the newer productions of these amplifiers.
you could replace the variable with a fixed resistor and control the input wattage from the radio as Shockwave mentioned. As far as the custom made input tuning coil goes I would imaging by itself will have only a marginal impact on these units that are having these issues like the before mentioned resistor and coil change will knowing the amplifier is self oscillating because of the corner cutting bias circuit design.
The most benifical upgrade for this amplifier would be to build the bias circuit correctly and revamp the feedback circuit to lose the harmonics riding on the bias line and rebuild the feedback circuit to a lesser degree. I have also seen this change that he mentions when adjusting the variable power control and with the current design it uses would effect the swr as you are describing.
I have this project in the works to change the design of it, But I am having trouble locating a non-inductive metal film 100 ohm 4 watt resistor for the feedback circuit. If anyone can provide a link to source these from please feel free to post it here.
you could replace the variable with a fixed resistor and control the input wattage from the radio as Shockwave mentioned. As far as the custom made input tuning coil goes I would imaging by itself will have only a marginal impact on these units that are having these issues like the before mentioned resistor and coil change will knowing the amplifier is self oscillating because of the corner cutting bias circuit design.
The most benifical upgrade for this amplifier would be to build the bias circuit correctly and revamp the feedback circuit to lose the harmonics riding on the bias line and rebuild the feedback circuit to a lesser degree. I have also seen this change that he mentions when adjusting the variable power control and with the current design it uses would effect the swr as you are describing.
I have this project in the works to change the design of it, But I am having trouble locating a non-inductive metal film 100 ohm 4 watt resistor for the feedback circuit. If anyone can provide a link to source these from please feel free to post it here.
Does your meter require resetting when you change power? Mine does, and it would affect the SWR reading.
73,
RT307
This thread is kind of old, but full of a lot of things that a lot of users of these amplifiers see (especially on the newer models, using DEI transistors). I also notice a lot of misinformation, like "harmonics riding on the bias supply" which without offending anyone, is completely inaccurate, and downright not even possible, for which I will explain if no one minds opening up this can of worms. There are simple changes and things that can be done to correct these issues, but want to make sure I won't put a lot of time into this for no reason.
I don't think you will offend anyone, I would just love to hear your take on this age old issue.
I haven't had the time to complete my project so I cannot say that seperating the RF from the bias voltage was the cure or not.
So....Let's have it.
Ok I will do this in parts, one, because I don't want to cover each part in a full essay type approach, and put people to sleep, and also allow people to ask questions or add input. I have been following this thread for some time, and I certainly don't want to offend anyone. I think it's great that everyone has given input to the discussion. BUT, I feel it is nessasary to chime in, and add my input to the discussion.
Let's start with the "sandbar" resistors and choke issue....
First of all, while the components MAY eventually increase in resistance, this will affect the idle curent to the transistors. Each board, or set of to transistors has a switched voltage divider supply going through contact of the relay, that are also used to carry the RF output of the amplifier as well. The purpose of the chokes, is to allow both (the DC source present during TX for the bias supply to both boards, and the meter lamp) and the RF output path for the amplifier as well. If the chokes were not there, this arrangement would not work at all. They allow you to share the same relay contacts, traces, and paths for both, the bias, and the RF out while keeping them electronically isolated. That's all there is to it. It also allows you to use the same contacts on the relay.
So, if the components which we have thus far referred to as "sandbar resistors" which are actually called *wirewound resistors" and will be referred to as such for the purpose of this discussion, in this case, change value, which would be really rare, but could happen, the only thing that will happen is that the set of transistors (not both sets, since they each have their own voltage divider supplying bias, respectively) will not have foward bias, it will be lower than normal, or fluctuate. If the small 1.6 or 1.8 ohm (depending on model, week, year, etc) counter part that is the second part of the voltage divider increases in value, or opens (burns out) you would have ay too much current and the transistors would just burn up from too much forward bias. Once again, rare, but I have seen it happen. That is why, with the amplifier keyed, it is important to make sure you have between .48 and .68 volts with the amplifier keyed, with no drive. This will tell you right away if the bias circuit is OK, and all associated components. Depending on temperature of the heatsink (read: transistors) and supply voltage, it will vary, but should be around this range. If it is too low, then the wirewound (sandbar) resistor or resistors have increased in value. If too high, the small 1.6 or 1.8 ohm resistor needs checked, or you have a leaky transistor (2SC2879, 2SC2290, etc) and also, if you have a leaky transistor, the voltage will be there all the time, weather the amp is keyed or not. (remember, in these amplifiers the bias is switched and should only be present during TX mode)
Moving on...
The statement that "harmonics are in the bias supply" is just downright obsurd. I don't want to offend anyone, but it makes no sense, what-so-ever. I will explain. First of all, the 10uf chokes used to keep 27Mhz (28mhz, whatever) off of the bias supply look like a huge amount of resistance at that frequency. So although they allow DC to pass with no issues, at RF it looks like a big resistor. The RF flows to the path of least resistance, so it doesn't get back into the bias supply, at least on paper. To make sure it is "snuffed" out, there are bypass capacitors to ground that take care of what may have gotten past the chokes.
Ok, now that we understand that part... let's dive a bit deeper. While this circuit works well at 27Mhz, let's talk about what a harmonic is. It is a multiple of the fundamental frequency. So, the second harmonic of 27Mhz would be 54Mhz. At this frequency, the chokes and bypass capacitors would be EVEN MORE efficient at removing any RF from the bias path, not to mention what I left out before is that the 25 ohm wirewound resistors (sandbars I cringe saying it that way) also would act as chokes as well at 54Mhz. So that theory is just crazy. Not to mention this this amplifier is a push pull design, and by nature cancels out the second harmonic very well. The third harmonic, same thing, but just better. The ferrite mix, the chokes, bypass caps and resistors all would definetly keep any amount of the third harmonic off the line as well.
The ONLY thing that would effect SWR in, or out, in an arrangement like this, is that if the bias supply changes, it can and will affect the input and output impedance of the transistors a little, which would cause the INPUT SWR to change a bit. It would also detune the output a little as well.
Let's go ahead and stop here, and next time we will go into the input and output circuit and impedance matching curcuits of the amplifier, and I will also go into some simple changes that can be done, especially on the newer DX500 / DX500V designs using the DEI transistors that can be done to bring the input SWR of the amplifier down which will allow your radio to drive the amplifier properly, and not foldback power if it is an HF rig, and if it's not, it will keep your radio cooler, as it will "see" a flat match into the amplifier. This is important too, to help with IMD, or distortion products. No more changing coax length, or other band-aid fixes.
We will also go into some RF bypassing, which will keep RF off of the positive lead, exiting the amplifier, and also the output circuit as well, as how we can better match it to the output impedance of the transistors, and get it to about 52-58 ohms or so over a 3Mhz bandwidth so that we can get more power output, and less heat as well that is wasted in untuned circuits.
But, as I am very busy these days, we will take it one step at a time.
Let's start with the "sandbar" resistors and choke issue....
First of all, while the components MAY eventually increase in resistance, this will affect the idle curent to the transistors. Each board, or set of to transistors has a switched voltage divider supply going through contact of the relay, that are also used to carry the RF output of the amplifier as well. The purpose of the chokes, is to allow both (the DC source present during TX for the bias supply to both boards, and the meter lamp) and the RF output path for the amplifier as well. If the chokes were not there, this arrangement would not work at all. They allow you to share the same relay contacts, traces, and paths for both, the bias, and the RF out while keeping them electronically isolated. That's all there is to it. It also allows you to use the same contacts on the relay.
So, if the components which we have thus far referred to as "sandbar resistors" which are actually called *wirewound resistors" and will be referred to as such for the purpose of this discussion, in this case, change value, which would be really rare, but could happen, the only thing that will happen is that the set of transistors (not both sets, since they each have their own voltage divider supplying bias, respectively) will not have foward bias, it will be lower than normal, or fluctuate. If the small 1.6 or 1.8 ohm (depending on model, week, year, etc) counter part that is the second part of the voltage divider increases in value, or opens (burns out) you would have ay too much current and the transistors would just burn up from too much forward bias. Once again, rare, but I have seen it happen. That is why, with the amplifier keyed, it is important to make sure you have between .48 and .68 volts with the amplifier keyed, with no drive. This will tell you right away if the bias circuit is OK, and all associated components. Depending on temperature of the heatsink (read: transistors) and supply voltage, it will vary, but should be around this range. If it is too low, then the wirewound (sandbar) resistor or resistors have increased in value. If too high, the small 1.6 or 1.8 ohm resistor needs checked, or you have a leaky transistor (2SC2879, 2SC2290, etc) and also, if you have a leaky transistor, the voltage will be there all the time, weather the amp is keyed or not. (remember, in these amplifiers the bias is switched and should only be present during TX mode)
Moving on...
The statement that "harmonics are in the bias supply" is just downright obsurd. I don't want to offend anyone, but it makes no sense, what-so-ever. I will explain. First of all, the 10uf chokes used to keep 27Mhz (28mhz, whatever) off of the bias supply look like a huge amount of resistance at that frequency. So although they allow DC to pass with no issues, at RF it looks like a big resistor. The RF flows to the path of least resistance, so it doesn't get back into the bias supply, at least on paper. To make sure it is "snuffed" out, there are bypass capacitors to ground that take care of what may have gotten past the chokes.
Ok, now that we understand that part... let's dive a bit deeper. While this circuit works well at 27Mhz, let's talk about what a harmonic is. It is a multiple of the fundamental frequency. So, the second harmonic of 27Mhz would be 54Mhz. At this frequency, the chokes and bypass capacitors would be EVEN MORE efficient at removing any RF from the bias path, not to mention what I left out before is that the 25 ohm wirewound resistors (sandbars I cringe saying it that way) also would act as chokes as well at 54Mhz. So that theory is just crazy. Not to mention this this amplifier is a push pull design, and by nature cancels out the second harmonic very well. The third harmonic, same thing, but just better. The ferrite mix, the chokes, bypass caps and resistors all would definetly keep any amount of the third harmonic off the line as well.
The ONLY thing that would effect SWR in, or out, in an arrangement like this, is that if the bias supply changes, it can and will affect the input and output impedance of the transistors a little, which would cause the INPUT SWR to change a bit. It would also detune the output a little as well.
Let's go ahead and stop here, and next time we will go into the input and output circuit and impedance matching curcuits of the amplifier, and I will also go into some simple changes that can be done, especially on the newer DX500 / DX500V designs using the DEI transistors that can be done to bring the input SWR of the amplifier down which will allow your radio to drive the amplifier properly, and not foldback power if it is an HF rig, and if it's not, it will keep your radio cooler, as it will "see" a flat match into the amplifier. This is important too, to help with IMD, or distortion products. No more changing coax length, or other band-aid fixes.
We will also go into some RF bypassing, which will keep RF off of the positive lead, exiting the amplifier, and also the output circuit as well, as how we can better match it to the output impedance of the transistors, and get it to about 52-58 ohms or so over a 3Mhz bandwidth so that we can get more power output, and less heat as well that is wasted in untuned circuits.
But, as I am very busy these days, we will take it one step at a time.
Hmmm, Very interesting read.
I will however, Hold my reply until the completion of your cause, effect, and cure strategy.
I will however, Hold my reply until the completion of your cause, effect, and cure strategy.
There are other things that can cause a much higher input and output VSWR with an amplifier. Poor isolation that results in coupling the output back to the input is a key factor that can easily cause self oscillations to occur inside the amp. These self oscillations are not on the fundamental frequency and when the feedback is strong enough, it will continue with the exciter off. The RF sniffer may or may not stay keyed to see it at this level. Using the SSB delay may uncover RF without excitation when you unkey.
Oscillations can easily be seen on a spectrum analyzer and can even be spotted on a decent scope. They reveal themselves best when operated into a narrow band resonant load. On the spectrum analyzer you'll clearly see two carrier signals mixed together at different frequencies. The fundamental and the oscillation frequency. On the scope you won't spot the frequency difference but you will see the two competing carriers mixing together with separate envelopes.
The VSWR is poor except when operated into a dummy load. Increasing the drive from the exciter usually causes the power to shift from the oscillation frequency to the fundamental and can sometimes stop the oscillation but is not how to fix it. The chokes and caps used to filter the output from the bias might removed as much as 99.9 % of the RF from the DC bias. That's good enough in low level RF circuits such as preamps or drivers under a few watts.
At 500 watts PEP a 99.9 % reduction in RF would still leave 500 milliwatts of output fed back into the bias circuit that is tied right to the input. You really need to remove 100% of this RF because even at 99.99% you still have 50 milliwatts of feedback excitation. If the transistors are really biased class AB, 50 mw is plenty to start an oscillation since the finals are already on.
This is why changing the bias level or adding a regulator can also reach a point where the oscillation stops. Reducing the DC bias voltage below the RF voltage of the oscillation signal will make the amp not respond to RF below that level. Adding a regulator will add attenuation to the RF at the input of the regulator.
How hard is it to add a small relay? Wire the new relay coil right across the 12 volt meter lamp or existing relay coil. Use the dedicated contacts on the new relay to close the bias circuit in TX rather than the RF output contact on the original relay. If you use a transistor to switch the bias, you cannot control the transistor with the original RF contaminated bias voltage since it will not provide isolation and can amplify the existing RF.
I agree the problem is not harmonic related but it's worth mentioning that push pull amps only cancel even order harmonics with no attenuation of odd order harmonics. In fact if you drive them into the non linear region (common practice with these amps) the odd order harmonics spike up rapidly.
Getting to all of that in the next post... like I said, we're handling one thing at a time. But remember, even after the chokes, you have RF bypass caps, and also the wirewound resistors are inductive, and act as chokes themselves.
Also, I want to make it very clear by what I meant, I think something I said was misunderstood..... if you read my previous post closely, I mentioned:
"The ONLY thing that would effect SWR in, or out, in an arrangement like this, is that if the bias supply changes, it can and will affect the input and output impedance of the transistors a little, which would cause the INPUT SWR to change a bit. It would also detune the output a little as well."
When I said "in an arrangement like this" I was referring to the bias circuit, and what I meant is, that everyone blaming the bias circuit for the input and output SWR issues is a long shot, and while it CAN affect the input and output impedances, it is, as I previously mentioned, a little change.
Or worded yet another way would be: In a longshot, if the bias supply failed, it is not the main cause, the end all, be all of the issues, so please quit randomly changing wirewound resistors and chokes trying to solve the issue.
We also established that these devices have very little gain, if any at 54Mhz, and even less on odd order harmonics.
Oh one more thing... you said that the bias is tied right to the input. Once again, not really. You have to look at the path. It has to go through the choke, then the resistor, 25 wirewound in this case, the leads that have ferrite over them, the input transformer which the way it is set up acts like a choke itself, also a lot of it is shunted via a bypass cap, and what is left over still has a 1.6 ohm resistor, which is part of the divider, and two 10 ohm resistors at the base of each transistor.
When you said that poor isolation is the cause of self oscillation, I would say...yes and no... meaning that you WANT a small amount of feedback, but negative feedback. Old solid state amplifiers would actually purposely take a small lead, insulated and wrap it around the input lead. This was tied to part of the ouput circuit, and sometimes 180 degrees out of phase. Crazy right? Not really. It was to stop self oscillation.
But on this design, it does have negative feedback. Also, weather on purpose or on mistake, RF is routed through the power switch, so once again, purposely, or accidentally because they are routed closely, you get some feedback there. Also the 330pf capacitors help with any out of band oscillations as well.
Even if you push the amplifier near class A, where small signal gain will increase substantially, oscillation isn't an issue, unless something else is wrong.
But all of this can be verified with a spectrum analysis and sweep for in band spurs, oscillations, and while we are at it, check IMD levels (also in band) then check 3rd order harmonic levels.
Ok, going back to the bias. I agree 100% that we need to change this. And we will...we are getting to that. We will use the "contaminated" bias supply to power the relay's coil, and use RF bypassing for good measure, since wiring the second relay in parallel with the other relay's coil will cause too much current across the keying transistor.
But first, we are going to correct the heart of the issue, and that is improper impedance matching.
More tomorrow.
Also, I want to make it very clear by what I meant, I think something I said was misunderstood..... if you read my previous post closely, I mentioned:
"The ONLY thing that would effect SWR in, or out, in an arrangement like this, is that if the bias supply changes, it can and will affect the input and output impedance of the transistors a little, which would cause the INPUT SWR to change a bit. It would also detune the output a little as well."
When I said "in an arrangement like this" I was referring to the bias circuit, and what I meant is, that everyone blaming the bias circuit for the input and output SWR issues is a long shot, and while it CAN affect the input and output impedances, it is, as I previously mentioned, a little change.
Or worded yet another way would be: In a longshot, if the bias supply failed, it is not the main cause, the end all, be all of the issues, so please quit randomly changing wirewound resistors and chokes trying to solve the issue.
We also established that these devices have very little gain, if any at 54Mhz, and even less on odd order harmonics.
Oh one more thing... you said that the bias is tied right to the input. Once again, not really. You have to look at the path. It has to go through the choke, then the resistor, 25 wirewound in this case, the leads that have ferrite over them, the input transformer which the way it is set up acts like a choke itself, also a lot of it is shunted via a bypass cap, and what is left over still has a 1.6 ohm resistor, which is part of the divider, and two 10 ohm resistors at the base of each transistor.
When you said that poor isolation is the cause of self oscillation, I would say...yes and no... meaning that you WANT a small amount of feedback, but negative feedback. Old solid state amplifiers would actually purposely take a small lead, insulated and wrap it around the input lead. This was tied to part of the ouput circuit, and sometimes 180 degrees out of phase. Crazy right? Not really. It was to stop self oscillation.
But on this design, it does have negative feedback. Also, weather on purpose or on mistake, RF is routed through the power switch, so once again, purposely, or accidentally because they are routed closely, you get some feedback there. Also the 330pf capacitors help with any out of band oscillations as well.
Even if you push the amplifier near class A, where small signal gain will increase substantially, oscillation isn't an issue, unless something else is wrong.
But all of this can be verified with a spectrum analysis and sweep for in band spurs, oscillations, and while we are at it, check IMD levels (also in band) then check 3rd order harmonic levels.
Ok, going back to the bias. I agree 100% that we need to change this. And we will...we are getting to that. We will use the "contaminated" bias supply to power the relay's coil, and use RF bypassing for good measure, since wiring the second relay in parallel with the other relay's coil will cause too much current across the keying transistor.
But first, we are going to correct the heart of the issue, and that is improper impedance matching.
More tomorrow.