Tuesday, February 28, 2017

On the workbench: 70 watts for $20

A recent thread on the Elecraft reflector led me to make an eBay purchase of a 70W amplifier kit.

Back in the NPOTA days - seems so long ago, doesn't it - there were times (particularly on SSB) that I was wishing for a bit more power from my KX2. Its 10 watts are fine when conditions are good and when the QRM/QRN levels accomodate that power level, but the other 99% of the time would have been more enjoyable without all the repeats those 10 watts were often requiring.

My new kit arrived from China only 8 days after ordering. A tracking number was sent to me within an hour of the order.

Only the circuit board and parts are included - no documentation whatsoever and none to download either. I knew this before ordering and chalk it up to the low price. It should be easy enough to build directly from the photos on the eBay order page but I will have to do a bit of research regarding the number of turns on the inductors.

Spectrum analysis images show it to be a very clean amp but that is more a function of the low pass filters in use - none of those are included in the kit. I plan to use filters from another project and will then test the combo on a spectrum analyzer and post the pics at a later date.

A few surface mounts components are used in the kit and, although I don't really enjoy working with them, they are so few in number that I'm almost sure I'll be able to cope without resorting to too much colorful language employed during the build.

With any luck I'll start the build this Saturday.
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China to Texas in 8 days
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Contents of the kit
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From KC2IRV:


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10 comments:

  1. Hello John, I know some very gifted HAMs here in the Netherlands are experimenting with this PA. As well as my most loyal blog reader PE1BVQ. Too technical for me. You can find schematics and more info in the biggest dutch amateurradio forum. See http://www.zendamateur.com/viewtopic.php?f=4&t=17798 It is in dutch but you can probabely translate it with a translator if you see something interesting. Good luck...73, Bas

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    1. Thanks for that info Bas - I'm going to check it out right now.

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  2. I built the baby sister of this amp and found the surface mount stuff challenging. I don't think it will ever be something I am good at, though. I blew the amp up during initial testing. Not sure what I did wrong. I suggest being quite alert when you get to that part. Let us know how it works out.

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    1. I'm the same way with surface mount parts - struggled through some ATS-series rigs years ago and don't want to deal with such a large number of tiny parts again. From what I've read on the link from Bas PE4BAS (above), these amps are highly susceptible to an antenna mis-match and many users report having to replace the finals after an accidental 2:1 SWR presents itself.

      I'll post results here of the smoke test ;-)

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  3. I am making this PA working butiful with mcHF SDR transceiver

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    1. I'm very interested in the mcHF rig. Not in building one myself but in reading the reports of others...it seems to be a unique project.

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    2. Does this amp have an RF sensing PTT, or did you ahve to plug it into the accessory jack on the mcHF? I ask because I have a v. 0.6 mcHF, and am currently building a WA2EBY 40 watt amp, which DOES have RF sensing RX/TX switching.

      If you had to switch via the radio's accessory jack, did you have to do any mods to protect the radio and/or the amplifier? Or use specific settings in the SDR's menu? -de KC8MWG

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    3. Yes, it does have RF sensing and a T/R relay.

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  4. My only concern is the surface mount connectors. I have had bad luck with these. Usually end up pulling the traces right off the PCB. Other than that, looks like a great project for putting boots on my HW-8. Nice video BTW.

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    1. Hi David,

      That's not my video - it was made by KC2IRV, and I agree, it is helpful. Hopefully it will my amp will turn out to be as successful as his...

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