To add to the PA madness that is going on here at the moment, I decided today to build one of the 2 QRP Labs 10w PA’s kits i have here. At the risk of inflaming all the QRP Labs fan boys out there, i both love and hate their kits.
I love that they are well thought out, designed and tested and bring a level of professionalism to the whole kit process, but, and this is the clincher for me, they are mostly a total pain in the arse to build and fix when something goes wrong because they are so densely packed.
Now, some of this has a lot to do with my own personal situation, I have arthritis in my fingers and only have about 75% use of some of my fingers and these boards are backed dense. Might be ok if you are perfect dexterity in your hands, but if not, well the build is not impossible, just frustrating. It would be nice to have say 30% more board space on all their kits. It would really make things much easier to work on, for me at least.
Anyway, it works, with 12v, 250ma bias and 15dbm drive I am seeing about 5W out, not 10. To get 10W out i need to drive it into hard clip with 1W input and turn the output into a square wave. Obviously not good. I figure it could probably do with some more bias current. Gain is pretty flat which is a nice thing. I will have to come back to this and play further at some point.
What i might end up doing is copying the design and laying out my own board and having some boards made up using larger ferrites, even if I lose some gain at 28mhz, it would be worth it to me as I dont use 10m anyway. If i do this, i will not be releasing my gerbers, because you know that someone will take them and start selling boards and that would not be good.
Let me tell you a story. It was a dark and stormy night and not a creature was stirring, not even a mouse. And yet, in a little lab in the middle of nowhere, somewhere out beyond the back of Burke, a little Aussie VK Ham was trying to get an order in with a PCB manufacturer before China closes down for Chinese New Year.
So in my infinite wisdom I needed to rush a 2nd board onto my order to make it more cost efficient on shipping. I only had one board ready with updates and fixes, and I have been meaning to do a PA board for quite sometime, so i fired up Diptrace and opened up the schematics I was going to use, and got to laying out 2 separate designs published by Ruben VK5FE which can be found here
So anyway, the two designs were slightly different in components and the two fets had different footprints and in my cut and paste zeal while doing the schematic capture, i forgot to change the components on the IRF510 board from RD16HHF to the IRF510 component and footprint.
Whats funny in this whole story is that after I posted the above picture to a friend of mine, I joked that the mechanical design was such that if i had made a bomb the PCB would shield me from the FET shrapnel and i should not lose an eye.
So lucky for me when it came time to smoke test this thing I use current limiting power supplies and had set the current limit to 2amps, because when the drain and source pins are swapped on the IRF510 you have made a heater as it will draw infinite current until the wires coming from the PSU overheat, catch fire and burn your house down.
So my board looks pretty but does not work. The other board designed for using RD16HHF1 fets should work a treat assuming I have not screwed anything up there 🙂 But before i go and build one of those, i will be checking it over 3 times as I do not want to waste expensive fets for nothing. When IRF510’s costs pennies and the RD16 costs pounds, you dont want to be exploding things to often LOL.
Anyway, the moral of the story is don’t rush. Check things many times and cont use cut and paste when putting 2 similar designs on the one board. Now i just have to wait a couple of weeks for China to re-open for business and send off the board files I have sitting here now just waiting.