Wednesday, December 23, 2015

Low cost 45W HF linear amplifier kit

I can't resist the low cost electronics kits out of China these days. On the bench at the moment is a 45 W HF linear for US$20.

My kit was missing the large capacitor and had extras of many of the surface mount components. The strips were marked with the values.

It's a familiar design with a pair of IRF530s in push pull on the output. The surface mount components are tiny and I have run in to a few problems with bad connections due to my giant sized soldering iron but it has come together.

The instructions are machine translated from Chinese and are quite amusing.

Here's some excerpts:

* I will Stress that the fourth point is Pandora's box, and you will feel good, if you do well. But if you do it not very well, you will reinstall after buying a new one!
* The welding way should be correct, no mistake, and no
* Postscript: Pay more attention please, because of the discreteness of component, the initial conditions may be not the optimization. In order to make discharge waveform best, experienced worker can make adjustments. In practical use, you must insert low pass filter after outputting, and filter the higher harmonic.

At this point I'm debugging the input stage which isn't biased correctly for some reason.


OK, I had destroyed Q3, a 2SC3357. I've roughly replaced it with a random NPN transistor and now for 2V in I see over 50V peak to peak out. The waveform looks pretty bad but that's mostly coming from the input stage having a totally wrong device in there. They are easy to source so I've ordered a bunch more.

So, it looks like the kit is good. I had one bad surface mount joint but otherwise it seems to have come together ok.


Razvan M0HZH said...

I think that's cheaper than the cost of the parts alone. Any idea how much input level does it need for the full output ?

Razvan (YO9IRF)

Peter Marks said...

They claim 5mW, which on reflection might be the explanation for my non-working input stage - I put much more power in to it... I'll update when I get it working.

Razvan M0HZH said...

That's never good, you should probably check the first transistor. If you don't have a replacement 2SC3357 at hand, you can try a 2N5109 instead just to test the circuit, it has similar characteristics. I also noticed there is no wire soldered to VCC_TX point in the picture, have you fixed that afterwards ?

Razvan (YO9IRF)

Peter Marks said...

Thanks Razvan. There's a link under the board for power and there's voltage there so that's not the problem. I fed it with 5W so will test the transistor. 73

David said...

I built one of these kits but I get negligible output at the output terminals (with 50 ohm load). Most of my output appears at the bottom of L1. With one mosfet biased on I get the correct frequency output at L1 but when I start increasing the bias on the other mosfet the output becomes twice the input frequency. From my understanding this is what I would expect as L1 impedance is much higher than T3 impedance most of the output would appear across L1 and it would be twice the input frequency as the output would peak for both positive and negative peaks of the input signal. However if I connect a 0.1uF from the bottom of L1 to ground (it gets warm) then most of the output appears across T3 and I do get an output at the proper output terminals (distorted). What am I doing wrong ? Has anyone got one of these working properly ?