This is a updated collection of easy to build projects for radio amateurs and swl's. See all my projects in the OLDER POSTS down this blog.
Have fun.
73' John

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Saturday, November 12, 2016

DIY kits 70W SSB linear HF Power Amplifier For YAESU FT-817 KX3

It's time for a new project.   eBay link to the seller                   

My first impression : High quality PCB ( 100 x 50mm ) , all parts in the kit including 2 power FETs type IRF530N. Capacitors are 10nF instead of 100nF as shown in the schematic but that's not a problem. The 0.5 turns windings are on the PCB by means of the tubes inside the toroids. Construction is easy but a lowpass filter for each band is mandatory. I used the low pass data from W3NON for a 7 element Chebyshev with standard value silver mica capacitors and T68-6 toroids. The provided wire for the power chocke ( 2 turns ) has a weak coating . It's better to replace it to avoid shortcuts .Start soldering the small parts and end with the transformers. Turn the potentiometer anti clockwise and slowly adjust the bias to aprox. 3.7V. I replaced the FETs by a pair of IOR IRF530N resulting in more stabillety en less input SWR however the input swr is highly effected by the drive power . At 2 Watt drive the swr reaches 1:2.7 at 5 Watt this mounts to over 1:3... . Best results with this PA are obtained on CW and FM.
The test was in the original configuration at 13.8V and 5Watt FM input from my FT-817. Max. power was 80 Watt . At  2.5 Watt input the output was about 50 Watt at 20m.but the modulation was as expected distorded in ssb mode at the recommended 2.7V  bias voltage.
At 3.7V ( gate ) bias the modulation is acceptable but far from perfect.  The 150 Ohm resistors over the FETs heat up strongly at full power FM. It's better to reduce the drive power to 2 Watt when working 12V FM/AM . Don't reduce the drive by reducing the Bias voltage for this results in a distorted modulation in SSB mode. Better use a power attenuator : Power att. calculator PA1B
Unfortionally the seller often forgets to add the documentation. I made a copy of the pdf file . Click on the  schematic to enlarge.
A  You Tube impression by KC2IRV : Overall impression of the amplifier
                                                               Testing the PA

Conclusion :  This is a nice kit with acceptable results  for a low price. 

Disclaimer:I'm not responsable for any damage due to improper reproduction of my projects.