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Author Topic: Cheap 10 meter amp  (Read 78548 times)
KA5PIU
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Posts: 446




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« on: June 07, 2014, 02:29:54 AM »

Cheap 500 watt HF amplifier.

Hello.

Although they are beginning to get hard to get, Mitsubishi produced some of the largest CRT monitors at 36".
The TV like sweep circuit had to deal with several scan frequencies, but not exceed a safe CRT voltage.
So, there is the ultra linear sweep module and some pretty precise current control.
The Sweep module is of special interest.
Due to the sheer size, the module can do 500 watts, and is good for at least 32 MHz.
The TV sets have an independent power supply for sweep.
By removing the sweep unit, removing the flyback and other parts unneeded, and making correct coupling, it will output 500+ watts.
I did 28.385 Mhz with 500 watts, both into a dummy load, to check purity, and to see how much it would heat.
It does not heat, even after 5 hours of operation, and the purity is well within even that of a type accepted transmitter, but only on 10 meters.
Drive was 4 watts.
The heat sink is massive, the entire back and bottom of the TV, but no fan.
Around 50 volts DC is supply, 10 amps.
There is also 14 volts and 5 volts.
There is no filtering of the RF whatsoever, so at 10 MHz, harmonics are a problem.
I am going to have a wooden box made for it as it is a complete 10 meter base station, less radio.
But, there is no reason a ham could not make a full range HF amp.
In Mexico, 10,11,12 meter radios are popular.
The villages use them to communicate much like one would use a telephone here.

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WB2WIK
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Posts: 21836




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« Reply #1 on: June 07, 2014, 03:36:36 PM »

Sounds like it's an independent "module" from that model TV/monitor.

Does it have a P/N on it somewhere, so folks can look for surpus ones on the market?
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KA5PIU
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Posts: 446




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« Reply #2 on: June 07, 2014, 06:10:40 PM »

Sounds like it's an independent "module" from that model TV/monitor.

Does it have a P/N on it somewhere, so folks can look for surpus ones on the market?

Hello.

Yes, it is indeed an independent module.
What TV manufacturers do is come up with a basic board.
A rear projection set set would be a regular TV, same board, with added boards for the 3 independent guns and special HV section.
Deluxe audio is the same thing, an extra module for that.
But, I find old Mitsubishi large CRT TVs, frequently for free.
The reason for the very high frequency response is that this is the cheap way to do things.
If only a few thousand of something is produced, it is cheaper to buy off the shelf parts if at all possible, and use as much of the parts from the main production as possible.
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WA7PRC
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« Reply #3 on: June 08, 2014, 01:04:23 PM »

By removing the sweep unit, removing the flyback and other parts unneeded, and making correct coupling, it will output 500+ watts.
...
Around 50 volts DC is supply, 10 amps.
So, the efficiency is 100%...
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WB2WIK
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« Reply #4 on: June 08, 2014, 04:20:51 PM »

By removing the sweep unit, removing the flyback and other parts unneeded, and making correct coupling, it will output 500+ watts.
...
Around 50 volts DC is supply, 10 amps.
So, the efficiency is 100%...

Mine goes up to 11.
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KA5PIU
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« Reply #5 on: June 08, 2014, 04:25:29 PM »

Hello.

That is the dissipation, and yes, 100%.
The nominal 50 volt supply is more like 57 volts.
The actual figures are.
58 volts.
10.6 amps.
Almost exactly 500 watts on a "quality" dosy TR-2000 meter!
Why this CB stuff?
The "load bank" is this 25 ohm globar resistor pair in a steel pipe with a UHF connector I made years ago.
The resistor is rated to 1000 watts and has a HV ceramic standoff on each end and filled with transformer oil, so it can handle 10,000 watts for a few seconds
2000 watts all day, real world.
We measure the average power by the temp rise.
It does have a tap for 50% of power.
But, yes, not 100% efficient, but the fact that it is a TV sweep amplifier with NO FILTERING and "10" meters should put you on warning.
What is its nominal impedance?
How pure is the signal?
It needs to be run around 300 watts for anything close to a nice signal, and it is a 100% class C amp, don't even think about anything but CW or FM! even at 300 watts.
But, at 500 watts CW, it is clean!
At 250 watts, external modulation, AM mode, it does nice.
Input AC can be anything from 100 to 270 volts.
And, it is constant duty.
My guess is that around 700 watts is the upper limit.
The power supply can do almost 19 amps without any sag or excess heat on the 50 volt output.
So, 4 watts of drive gives 500 watts nominal out.
Remember that this is loose experimenting.
I will be running at around 27 MHz as an ISM device and loop for induction heating experiments, very little actual radiated power.
I also dismantle microwave ovens for the power supply and magnetron, so this is nothing new.
Remember that safety is paramount.
Be safe, and enjoy!

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WB2WIK
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« Reply #6 on: June 08, 2014, 04:45:16 PM »

Hello.

That is the dissipation, and yes, 100%.
The nominal 50 volt supply is more like 57 volts.
The actual figures are.
58 volts.
10.6 amps.
Almost exactly 500 watts on a "quality" dosy TR-2000 meter!

Yeah.  That would be over 81% efficiency, and that can't be linear.  That's what Bryan was hinting at.
Quote
It needs to be run around 300 watts for anything close to a nice signal, and it is a 100% class C amp, don't even think about anything but CW or FM! even at 300 watts.
But, at 500 watts CW, it is clean!
At 250 watts, external modulation, AM mode, it does nice.

That doesn't make much sense either.  How do you modulate it externally?  250W carrier power is 1000W PEP, and would require a 250W modulator.  How are you doing that?  And it really handles 1000W PEP output?  I'd love to see that.
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WA7PRC
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« Reply #7 on: June 08, 2014, 05:05:42 PM »

By removing the sweep unit, removing the flyback and other parts unneeded, and making correct coupling, it will output 500+ watts.
...
Around 50 volts DC is supply, 10 amps.
So, the efficiency is 100%...

Mine goes up to 11.
Like this?  Wink
http://www.youtube.com/watch?v=N3L4EZwmRrA
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KA5PIU
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Posts: 446




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« Reply #8 on: June 08, 2014, 06:22:56 PM »

Hello.

correct, 25 or 50% of the carrier is needed for external (High Level) modulation.
http://en.wikipedia.org/wiki/Amplitude_modulation
TV sweep modules are super easy to do with the built in DEW circuit.
http://www.google.com/patents/US4082986
In the case of the really large sets, the high voltage must be monitored on a dynamic basis as they are right on the edge.
But, the CRT will at some point no longer emit enough electrons, so the set will blink 7 times and turn off.
This is just fine for me, I do not want the CRT.
What I want is that driver module and power supply and the audio amplifier and its power supply.
I have also ripped apart old rear projection TV sets and although good for solid state Tesla coils, the transistors will max out around 2 MHz.
That is why I see this as so much of a treat, 32 MHz!
I normally only go 160 and 10 meters in HF.
Again, the large Mitsubishi sets will yield all the parts for an AM ten meter amp or transmitter.
This is the really large Mitsubishi Diamond-Vision with the SVGA connector.
I had no concern over the TV proper, just the audio section at first.
I took the HV as it was connected to this nifty power supply.
It was only after I took it apart that I saw the module.
This set yielded 2 tuners, 3 power supplies, 6 speakers, and a host of parts such as the input panel and its PC board.
Remember that this was all free.
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W1BR
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Posts: 4189




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« Reply #9 on: June 08, 2014, 08:30:16 PM »

Hello.

At 250 watts, external modulation, AM mode, it does nice.
 
My guess is that around 700 watts is the upper limit.


It would to do 1000 watts peak for 100% AM modulation for 250 watt carrier.

pete
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KA5PIU
Member

Posts: 446




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« Reply #10 on: June 08, 2014, 08:53:26 PM »

Hello.

OK, so we are on the same page.
In CW, I am measuring carrier.
On AM, I am measuring peak.
The audio amplifier in question does not seem to have enough power for 100% modulation.
Remember that this is a CB wattmeter and that super high quality dummy load I made with plumbing pipe, so the precision? what precision?
I used a Motorola service monitor to look at purity of signal, and that is excellent, but at about 80% modulation.
But, the rest of the figures, kind of sort of like a gas gauge, in the ballpark.
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KD8MJR
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Posts: 5557




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« Reply #11 on: June 08, 2014, 10:39:21 PM »

Any pictures of this Amp?  I would love to see it.
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“A lie can travel half way around the world while the truth is putting on its shoes.”  (Mark Twain)
KA5PIU
Member

Posts: 446




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« Reply #12 on: June 08, 2014, 11:52:50 PM »

Hello.

In hindsight I should have taken pictures.
But, this was just an old direct view TV, one of hundreds getting scrapped.
Think of this as just a scrap TV.
Bars are where you will find them, just sitting there due to the DTV transition.
Even if yours does not have the nicer HV, it will have nice speakers and an audio section.
This has already been mounted in a box and sold.
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K6AER
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Posts: 5726




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« Reply #13 on: June 09, 2014, 09:00:25 AM »

Several issues are at play with what is being proposed.

Is the power supply intrinsically isolated from the AC mains?

What is the input and output impedance as viewed on a Smith chart for 160-10 meters?

You mentioned it is linear but how linear. The FCC requires the even and odd harmonics to be down 43 dB of the power. A bank of low pass filters will be required.

Now we get to everyone’s favorite subject…IMD. Linear in the Audio Visual world only requires 20-30 dB of IMD reduction. With the efficiencies you have reported, these devices look more like Class “C” or “E” in operation. Is there a schematic available anywhere?

Also the gain will have to be tamed. 10-15 dB maximum. Transceivers have terrible IMD at low power outputs.
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KA5PIU
Member

Posts: 446




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« Reply #14 on: June 09, 2014, 04:06:06 PM »

Hello.

First off, only the schematic for the TV is available.
The power supply is a fully isolated switching unit, there is no connection to the AC supply, it fully floats.
It is not configured for anything but 24 to 32 MHz, no low pass or high pass filtering is required.
The second harmonic of the lowest frequency would be 48 MHz, well outside the range of the transistors to amplify.
The second harmonic is down 100+ Db.
The third harmonic? there is NO third harmonic!
IMD? 50+ dB down.
Again, I am using the idea that this is a 10 meter amp.
True, class C biased, it can NOT deal with an AM or SSB signal!
Linear? not hardly!
In a TV set, yes, it is linear, but in this  application? no!
What they mean by linear is the ability to control power output over a broad range of frequencies, not an RF application.
Again, this is a TV sweep unit, NOT an HF amplifier.
The way I get away with it is the fact that it is run on one limited area.
Think of it as a super clean 11 meter amplifier.
The power control circuit can accept an AM signal.
That was your first hint.
The fact that it can do AM with external modulation?
Or FM?
"CW is possible"?
No, you are expecting WAY to much, it is a cheap 10 (Or 11) meter amp.
And, to make this clear, in Mexico, running 1000 watts on 11 meters is perfectly legal.
That is the market for something like this.
Thank you.
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