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CHALLENGE - SS rig for the AM PW (QRP) net.




 
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Author Topic: CHALLENGE - SS rig for the AM PW (QRP) net.  (Read 51004 times)
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N4LTA
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« Reply #75 on: December 16, 2011, 05:12:17 PM »

No current on the final with no signal. Usually a resistor or choke to ground from the base works well with a single transistor.  You can probably adjust the bias until the bias current is just barely cutoff with no drive signal. Should find some examples in the literature for CW transmitters.

It will be a little harder to drive but the efficiency should be quite a bit better.

Pat
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ssbothwell KJ6RSG
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« Reply #76 on: December 16, 2011, 08:34:56 PM »

i finished assembling the modulator.

frits, what would be the proper procedure for testing this beast?

does the 100Kom pot on the power input control the peak output of the modulator?

edit: i am pretty clueless on how to test the modulator. i hooked it up to a 12V power supply and checked the Vout on a DMM. it read 10V regardless of how i adjusted the pots. i also tried inputing a 200Hz audio signal and that didnt seem to change the output.

i'm nervous about hooking it up to an amplifier without first verifying that it is working properly and is set for the right peak output so that it does not damaged the amplifier.

any suggestions?


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« Reply #77 on: December 17, 2011, 09:29:28 AM »

i finished assembling the modulator.

frits, what would be the proper procedure for testing this beast?

does the 100Kom pot on the power input control the peak output of the modulator?

Hi Sol, No that sets the carrier level.

Quote
edit: i am pretty clueless on how to test the modulator. i hooked it up to a 12V power supply and checked the Vout on a DMM. it read 10V regardless of how i adjusted the pots. i also tried inputing a 200Hz audio signal and that didnt seem to change the output.

The basics of AM modulation, at carrier level with no audio applied your voltage should be half of your maximum voltage. If you double the voltage (under the same load) you have 4 times the output power. And that is what you want to achieve. If you apply 12V to the modulator , set the output to 6V (carrier level) and feed it some audio, you'll see that the audio peaks reach almost 12 V (there's some loss in the 2N3055)
Hook up your scope to the emitter of the 2N3055, you don't have to put a load on it , it will show on the scope (if not put the probe on the base). Now with the carrier potmeter all the way down, there should be little DC out, slowly turn the the potmeter to 6 Volt and put some audio into the circuit. Watch the audio being carried on the DC.


Quote
i'm nervous about hooking it up to an amplifier without first verifying that it is working properly and is set for the right peak output so that it does not damaged the amplifier.

any suggestions?

Perhaps it is better to modulate the drive circuit and leave the amplifier as it is?
In that case you have to make sure the amp is really linear.
If you decide to modulate the amp keep in mind that that a higher current needs to be modulated and a single 2N3055 might not be able to handle it.

If you do try it on your amp, monitor the current to it and the temp of the 3055, if something fails, it's likely the 3055. It's a good idea to stock up on a few of them  Grin

Frits
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ssbothwell KJ6RSG
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« Reply #78 on: December 17, 2011, 07:00:21 PM »

frits, thanks for the explanations. i wont have a chance to test anything until tomorrow but i will report back with my results.
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ssbothwell KJ6RSG
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« Reply #79 on: December 19, 2011, 04:17:35 PM »

so i'm having some problems with the modulator circuit. when i first power on the circuit it jumps to 10V regardless of the carrier pot setting. it then drops down to around 2V.

at this point, if turn the carrier pot to full resistance between the wiper and ground then i see a 750vrms dc signal on the 2n3055 emitter.

if i turn the carrier pot to zero resistance between wiper and ground then i see a ~2V DC signal that turns into a transforms into a ~9mhz sine wave every couple seconds.

do you think the circuit layout is causing instability? i didnt use a ground plane on this circuit layout.
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« Reply #80 on: December 19, 2011, 05:21:31 PM »

so i'm having some problems with the modulator circuit. when i first power on the circuit it jumps to 10V regardless of the carrier pot setting. it then drops down to around 2V.

at this point, if turn the carrier pot to full resistance between the wiper and ground then i see a 750vrms dc signal on the 2n3055 emitter.

if i turn the carrier pot to zero resistance between wiper and ground then i see a ~2V DC signal that turns into a transforms into a ~9mhz sine wave every couple seconds.

do you think the circuit layout is causing instability? i didnt use a ground plane on this circuit layout.

Sounds like, you have to put a load on the circuit. a 100 ohm resistor should work nicely or a 12 V light bulb.
http://youtu.be/-vqEPzr8HCY
If you measure at the base of the 2N3055, over the 467 Ohm resistor, you should be able to test it without a load on the 2N3055
as of the voltage jump, I've noticed that to , when the circuit gets powered it jumps the voltage for a split second. That might be the TL071 going in "high" state for a moment.




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« Reply #81 on: December 19, 2011, 06:38:23 PM »

oh. i was measuring off of the 2n3055 emitter.

i just put a 100ohm load across emitter->ground and now the carrier is a solid dc signal but i am only able to adjust it between 800mV and 2V when measuring across the emitter.

if measure across the 467ohm resistor on the 2n3055's base then i see a 500mV  10-20MHz waveform that doesnt change when i adjust the carrier pot.


i tried inserting an audio frequency sign wave from a function generator and it is definitely modulating the power supply. this is good news. Smiley i just need a higher voltage carrier.
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« Reply #82 on: December 20, 2011, 12:42:49 PM »

oh. i was measuring off of the 2n3055 emitter.

i just put a 100ohm load across emitter->ground and now the carrier is a solid dc signal but i am only able to adjust it between 800mV and 2V when measuring across the emitter.

if measure across the 467ohm resistor on the 2n3055's base then i see a 500mV  10-20MHz waveform that doesnt change when i adjust the carrier pot.


i tried inserting an audio frequency sign wave from a function generator and it is definitely modulating the power supply. this is good news. Smiley i just need a higher voltage carrier.

Hi Sol, This circuit should be able to (almost) match the DC input on the 2N3055's output, by adjusting the carrier potmeter to full open.
I have three of these circuit up and running, with no problems. If you're sure
about the circuit design, try lowering the 100K resistor a little bit and see if
you can reach the 12 Volt out.
I have to ask, are you doing a realtime test or an LT-Spice simulation?
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« Reply #83 on: December 20, 2011, 06:20:06 PM »

i was using a real circuit board. i think the problems might have been with my board layout. i designed the board to include a large groundplane and thicker traces and the new one seems to be performing better.

i'm attaching side by side comparisons of the two boards.

with the new board i am able to set the carrier voltage to whatever percentage of the total power supply input. tried inserting an audio waveform and was able to modulate the signal successfully.

i set the audio input to a 440Hz sinewave at +4dBu and was able to produce a nice clean 6V modulated carrier. Smiley

i want to try modulating the driver board next but i only have one signal generator. i need one to act as vfo and one for the audio test tone.

maybe i should build a two tone audio test oscillator? is that the standard way of testing radios?


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« Reply #84 on: December 22, 2011, 04:57:47 PM »

i put together a basic Twin T oscillator as an audio source for testing the modulation. i'm attaching images of the unmodulated and modulated signals when powering the driver board off of the modulator board.

how can i measure the modulation index and how can i check make sure there is no distortion in the modulation?


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« Reply #85 on: December 23, 2011, 02:35:15 AM »

hi frits. are you sure that when the carrier is modulated it should reach 12V?

i have the carrier set to 6V and if i turn the audio up the audio then it clips at 7Vrms (8V Pk-Pk).

edit: hmm. i think something weird is going on with the driver. i just noticed that my variable power supply wont go above 8V went hooked up to the driver.  Huh

i tried using a 13.8V/3A fixed supply and the VN-10 and IRF510 started smoking.
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« Reply #86 on: December 23, 2011, 08:54:15 AM »

hi frits. are you sure that when the carrier is modulated it should reach 12V?

i have the carrier set to 6V and if i turn the audio up the audio then it clips at 7Vrms (8V Pk-Pk).

edit: hmm. i think something weird is going on with the driver. i just noticed that my variable power supply wont go above 8V went hooked up to the driver.  Huh

i tried using a 13.8V/3A fixed supply and the VN-10 and IRF510 started smoking.


Hi Sol,
In the design stage, I recommend to monitor the current at all times. I've learned the hard way by blowing up 2N3055's with my 30W RF Deck.
As of the modulator , yeah it should be no problem. The best thing is to set the carrier potmeter full open, take note of the voltage output (and current) and then set it back with carrier potmeter to half voltage, while doing that keep an eye out on the current. Repeat that step a few times and make sure the max output stays the same. If not, see if it is the power supply or the driver circuit.

As of the driver circuit;
The VN-10 and the 2N push pull transistors should be connected to a fixed voltage, you only want to hook up the output of the modulator (through a RFC) to the IRF510.

Mind you.. I've never been really satisfied with the AM produced with
this driver circuit. It most be something in the linearity and classes
of operation. But it is a great circuit to boost VFO power up a to a
few Watts. (I turned you on to this circuit because my original
understanding was that you wanted to modulate your amp)

If you still like to modulate the driver,
I recommend using a dedicated driver IC , the IDXF604PI, so much more easier, just drive the IRF510 with that. This chip is good up to and a little beyond the 80 Meter band. take a look at the schematic of my 80M RF deck, you can do the same with the 510 and probably directly drive the 604 chip with a VFO

Frits


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« Reply #87 on: December 23, 2011, 08:01:24 PM »

i think the modulator might be fine and the problem is with the rf driver.

i suspect the voltage limiting has to do with it triggering the crowbar protection on my power supply.

if i connect the driver to a 13.8V/3A fixed power supply then the VN-10 and the IRF510 immediately start smoking.

i also noticed that when the VN-10 gets burnt out, the power supply voltage (when using my variable power supply) is no longer limited.
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« Reply #88 on: January 15, 2012, 02:45:44 AM »

hey Frits, its been a while since i posted in here but i wanted to give you a little update. i havent had as much time to work on electronics projects lately but this week i did get a chance to sit down at the work bench.

i tried using your modulator circuit with a 100mW AB amp design i found in "experimental methods in rf design."

here is the AB schematic: http://i.imgur.com/0Ucf4.png
and here is my 'ugly style' board: http://i.imgur.com/hasZW.jpg

i'm using a homemade hartley VFO (dont have a photo or schematic on hand currently) to drive the amplifier.

here is my modulator board (i think i posted a photo of this board in this thread earlier): http://i.imgur.com/dLT5Y.jpg

here is the output from the AB amp WITHOUT using the modulator circuit: http://i.imgur.com/lVa3K.jpg

when i set the modulator board to 6V carrier and power both the 2n3904 the 2n3966 from AB amp with the modulator then i get this really distorted waveform: http://i.imgur.com/YKD30.jpg

if i input a 500hz sinewave into the modulator and turn up the audio level then i end up with this waveform: http://i.imgur.com/I5txm.jpg

any idea why the waveform is getting so clipped when i try to use your modulator circuit?
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« Reply #89 on: January 15, 2012, 12:17:38 PM »

hey Frits, its been a while since i posted in here but i wanted to give you a little update. i havent had as much time to work on electronics projects lately but this week i did get a chance to sit down at the work bench.

i tried using your modulator circuit with a 100mW AB amp design i found in "experimental methods in rf design."

here is the AB schematic: http://i.imgur.com/0Ucf4.png
and here is my 'ugly style' board: http://i.imgur.com/hasZW.jpg

i'm using a homemade hartley VFO (dont have a photo or schematic on hand currently) to drive the amplifier.

here is my modulator board (i think i posted a photo of this board in this thread earlier): http://i.imgur.com/dLT5Y.jpg

here is the output from the AB amp WITHOUT using the modulator circuit: http://i.imgur.com/lVa3K.jpg

when i set the modulator board to 6V carrier and power both the 2n3904 the 2n3966 from AB amp with the modulator then i get this really distorted waveform: http://i.imgur.com/YKD30.jpg

if i input a 500hz sinewave into the modulator and turn up the audio level then i end up with this waveform: http://i.imgur.com/I5txm.jpg

any idea why the waveform is getting so clipped when i try to use your modulator circuit?


Hi Sol,
I think, lowering towards 6 Volt (carrier) changes the class of operation. Looking at the schematic of the 100 mW amplifier. Try modulating Q2 only (V3), keep Q1 at 12 Volt (V2).
Now with Q2 at 12V, you want to see 100 mW out (if that is the max) and at 6 Volt (carrier voltage) about 25 mW.
You may have to experiment with the feedback on Q2 (R5 and C6) or eliminated it completely. (this might however lower the maximum output of the circuit)

Frits W1FVB
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« Reply #90 on: January 15, 2012, 04:31:05 PM »

ok, i've been kinda following this thread, and i got an idea of something i want to do with some of these designs. i have a grounded grid amp that uses a quad of 572Bs and it needs about 15 to 20 watts to drive it. however, it doesn't have a tuned input, so my icom 718 will just barely put out enough power to drive it, and my yaesu ft-901 has to be retuned everytime i adjust the amp tuning or the antenna tuner. what i want to know is can i make a broadbanded output network on a low power solid state transmitter that instead of having an impedence of 50 ohms over its range, the impedence matches the input impedence of the grounded grid amp.
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« Reply #91 on: January 15, 2012, 07:04:50 PM »

Hi Sol,
I think, lowering towards 6 Volt (carrier) changes the class of operation. Looking at the schematic of the 100 mW amplifier. Try modulating Q2 only (V3), keep Q1 at 12 Volt (V2).
Now with Q2 at 12V, you want to see 100 mW out (if that is the max) and at 6 Volt (carrier voltage) about 25 mW.
You may have to experiment with the feedback on Q2 (R5 and C6) or eliminated it completely. (this might however lower the maximum output of the circuit)

Frits W1FVB


i was wondering if it was a problem that i was modulating both transistors. i isolated Q2 powered it off the modulator. When i turn up the carrier i get a nice clean sinewave: http://i.imgur.com/DCoDD.jpg

i am still having trouble with modulation. the signal gets really distorted as i turn up the audio input.

here is a photo of the modulated carrier: http://i.imgur.com/MuxAr.jpg
and here is a photo of it in XY mode: http://i.imgur.com/xzzvZ.jpg

i noticed that if i reduce the carrier to the point where it looks distorted (such as in my previous post) the trapezoid pattern looks more like a trapezoid (but everything still looks pretty distorted).
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« Reply #92 on: January 17, 2012, 08:07:38 AM »

Hi Sol,
I think, lowering towards 6 Volt (carrier) changes the class of operation. Looking at the schematic of the 100 mW amplifier. Try modulating Q2 only (V3), keep Q1 at 12 Volt (V2).
Now with Q2 at 12V, you want to see 100 mW out (if that is the max) and at 6 Volt (carrier voltage) about 25 mW.
You may have to experiment with the feedback on Q2 (R5 and C6) or eliminated it completely. (this might however lower the maximum output of the circuit)

Frits W1FVB


i was wondering if it was a problem that i was modulating both transistors. i isolated Q2 powered it off the modulator. When i turn up the carrier i get a nice clean sinewave: http://i.imgur.com/DCoDD.jpg

i am still having trouble with modulation. the signal gets really distorted as i turn up the audio input.

here is a photo of the modulated carrier: http://i.imgur.com/MuxAr.jpg
and here is a photo of it in XY mode: http://i.imgur.com/xzzvZ.jpg

i noticed that if i reduce the carrier to the point where it looks distorted (such as in my previous post) the trapezoid pattern looks more like a trapezoid (but everything still looks pretty distorted).

I'm curious to see, what the circuit does without R5 and C6 installed. (Keep the modulator on Q2 only) Output might be way down, but hopefully no distortion.

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« Reply #93 on: January 17, 2012, 08:21:56 AM »

ok, i've been kinda following this thread, and i got an idea of something i want to do with some of these designs. i have a grounded grid amp that uses a quad of 572Bs and it needs about 15 to 20 watts to drive it. however, it doesn't have a tuned input, so my icom 718 will just barely put out enough power to drive it, and my yaesu ft-901 has to be retuned everytime i adjust the amp tuning or the antenna tuner. what i want to know is can i make a broadbanded output network on a low power solid state transmitter that instead of having an impedence of 50 ohms over its range, the impedence matches the input impedence of the grounded grid amp.

You can certainly design an L-Network impedance matcher, if you know the amps input impedance , but it wouldn't be very broad banded. Using a transformer probably gives you a better range. I'm certainly no expert on this, maybe you should post this question as a new technical Q&A post. I'm sure there are other people on this board with more experience then me  Wink
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« Reply #94 on: January 22, 2012, 04:44:00 AM »

hi frits. my previous tests had been done without much attention to details. i decided that before i try rebuilding or changing the modulator i should go through one more round of tests where i record everything i do and i use an amplifier that is not entirely home made. i used an hfprojects PA-100 which you can see here: http://www.hfprojectsyahoo.com/sla5wamp.html

here is the unmodulated output from the PA-100: http://i.imgur.com/XT78K.jpg

i'll just walk you through my test process of testing the modulator. maybe i am making some critical mistakes that i dont realize.

1. connect the modulator output to my scope input with a dummy load (50ohm). adjust the carrier level until it is half the power supply voltage (6V). http://i.imgur.com/9MCj8.jpg

2. adjust audio input level to carrier for largest possible Pk-Pk signal without it clipping at the highest desired audio frequency. http://i.imgur.com/6ykiO.jpg

3. connect modulate output to power input of PA-100 amplifier and observe signal with varies audio frequencies inserted into modulator.

5kHz: http://i.imgur.com/bEt2i.jpg
500Hz: http://i.imgur.com/ktkZS.jpg
50Hz: http://i.imgur.com/Zc0DL.jpg

-does this behavior seem normal? why is there way less modulation at higher frequencies?

-i can physically hear the audio signal coming from the modulator circuit. i have no idea what is doing this but is it anything to be concerned with?

-where should i check the audio if i want to  look at the trapezoid pattern in XY mode? should i check it at the modulator output or the output from my audio signal generator?
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« Reply #95 on: January 22, 2012, 11:33:29 AM »

hi frits. my previous tests had been done without much attention to details. i decided that before i try rebuilding or changing the modulator i should go through one more round of tests where i record everything i do and i use an amplifier that is not entirely home made. i used an hfprojects PA-100 which you can see here: http://www.hfprojectsyahoo.com/sla5wamp.html

here is the unmodulated output from the PA-100: http://i.imgur.com/XT78K.jpg

i'll just walk you through my test process of testing the modulator. maybe i am making some critical mistakes that i dont realize.

1. connect the modulator output to my scope input with a dummy load (50ohm). adjust the carrier level until it is half the power supply voltage (6V). http://i.imgur.com/9MCj8.jpg

2. adjust audio input level to carrier for largest possible Pk-Pk signal without it clipping at the highest desired audio frequency. http://i.imgur.com/6ykiO.jpg

3. connect modulate output to power input of PA-100 amplifier and observe signal with varies audio frequencies inserted into modulator.

5kHz: http://i.imgur.com/bEt2i.jpg
500Hz: http://i.imgur.com/ktkZS.jpg
50Hz: http://i.imgur.com/Zc0DL.jpg

-does this behavior seem normal? why is there way less modulation at higher frequencies?

-i can physically hear the audio signal coming from the modulator circuit. i have no idea what is doing this but is it anything to be concerned with?

-where should i check the audio if i want to  look at the trapezoid pattern in XY mode? should i check it at the modulator output or the output from my audio signal generator?

Hello Sol,
It's easier to tell what the modulation is doing if you slow down your scope to 1 ms, then you can actually see the modulated audio frequency.

If you have a dual scope you can put one channel on the audio signal and the other on the RF output and compare the signals. I've attached an example image, the brighter trace is the audio input (speech in this case). If you compare the peaks you'll notice that they are not perfectly the same.

It's OK to hear some audio coming out of the circuit, most be the 3055 buzzing  Grin

Anyway, I'm curious to see what it looks like if you slow your scope down. (The last image you've posted of the 50 Hz signal looks very strange)
Personally I've never played around much with a XY setup. And I know there's some good information to be gathered out of that. Maybe somebody else can jump in on that subject.




* 100_9017.JPG (321.83 KB, 1280x960 - viewed 465 times.)
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« Reply #96 on: January 22, 2012, 01:44:18 PM »

oh cool. i never realized that was how people got those cool views of the modulation.  i also just found the peak detect display mode on my scope. it should help with the crappy cell phone photos.


heres images of the modulation at 50hz, 500hz, and 5khz. i used the same calibration method i used in the previous post (set signal generator to 5khz and set audio source voltage to maximum undistorted signal).

50hz: http://i.imgur.com/p8g7B.jpg
500hz: http://i.imgur.com/FDEz9.jpg
5khz: http://i.imgur.com/xzJcl.jpg

looking at the signal at 1ms makes it clear that i am achieving full modulation at 50hz but i am not getting anywhere close to it at higher frequencies.
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« Reply #97 on: January 23, 2012, 10:22:13 AM »

oh cool. i never realized that was how people got those cool views of the modulation.  i also just found the peak detect display mode on my scope. it should help with the crappy cell phone photos.


heres images of the modulation at 50hz, 500hz, and 5khz. i used the same calibration method i used in the previous post (set signal generator to 5khz and set audio source voltage to maximum undistorted signal).

50hz: http://i.imgur.com/p8g7B.jpg
500hz: http://i.imgur.com/FDEz9.jpg
5khz: http://i.imgur.com/xzJcl.jpg

looking at the signal at 1ms makes it clear that i am achieving full modulation at 50hz but i am not getting anywhere close to it at higher frequencies.

Do you keep the audio input level the same across all three tests? Thinking, maybe the circuits low frequency response is better then
at higher frequencies?
If so, increase volume on the 500 and 5K test and see if it modulates better.

 
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« Reply #98 on: January 23, 2012, 04:19:00 PM »

i tried that but if i increase the input level at all past this point then it starts to distort really badly at higher frequencies.

if it is not to inconvenient for you, would you be able to post photos of the modulation when using various frequency sinewaves? it would be nice to compare results and see if there is a problem with my board.
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« Reply #99 on: January 24, 2012, 08:03:59 AM »

The best way to test the modulator board is to test at DC level. Forget about any RF parts and hookup up a simple load. Get a 12V 5W or 10W automotive light bulb and measure across the bulb at DC level.
This way you know for sure the board works OK.
http://youtu.be/-vqEPzr8HCY

Try that, and if it still shows up, not being capable of reaching 100% , I'll be happy to compare some results.

Frits

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