I have to fess up and admit that I am still new in the linear build process.? I bought a DIY Kit 70w SSB linear off of eBay for about $18 US.? With it, I also bought an XP-LPF-HF Filter kit for around $40 US (I cannot find the confirming email to get the exact price) two heatsinks and some spare parts.? It was a challenge to build because of the lack of clear instructions, the small size, the gap-filling that is needed to manufacture and install the two transformers, lack of an accurate schematic, and the fact that the bias circuit as delivered could not be adjusted correctly.? Coupled with my wanting to just get it to a stage to test, I had lots of issues to resolve.
The reason for bringing this up is that in another thread I stated that the finished linear did not meet FCC requirements.? Without going into the gruesome details, the final result is that the linear is delivering 50 watts with 4.5 watts in from my Hermes Light HL2 (I will get around to interface to my uBiTx).? It is compliant as measured with an RSP1 and version 1.04b of RSP Spectrum Analyzer.? This after destroying the original and one spare set of MOSFETs (IRF530), a relay, and a LOT of head-scratching and searching online for information.
The main issues have been intermittent connections, a missing resistor to tame the input SWR, and a very touchy bias adjustment that ultimately required the circuit to be modified.? There are no clear directions on the kit, only documents that others have provided.? It was the bias adjustment that brought the rig into compliance after I corrected all of my bad connections and the input SWR issue.? Originally the bias was not able to reach 2.7 volts as delivered.? I had to add another 1k resistor in parallel with one of the original resistors to get the available bias adjustment above 2.35 volts (the value on the schematic was a range of 500 to 1K with 1K provided).? I then went the route of adjusting the bias the same way as the uBiTx (WRONG).? Could not get the standby current above 335 ma with 176 ma for the fan and relay board.? Left it at the max value to get in the "linear" region.? That was when I did the purity measurements and found it out of compliance.? I put it aside for a day and then went back and started adjusting the bias down from the 3.31 volts to see what would happen? ?The bias was last tweaked to 2.91 volts where the 2nd and 3rd harmonics disappeared into the noise of the RSP spectra.
So if you are good at working things out for yourself from limited info, you have a steady hand at construction, and a junk box (or purchased spare parts) to cover the mistakes, then this is a good investment.
For me, I learned a lot through this process and wound up with a new 50 watt rig for WSJT-X and other modes as a bonus!
Based on the above, I must now change my recommendation and say that the linear and filter kit combination is a good buy.? I just wish there were better directions on how to set the bias and the construction overall.
YMMV, it is not a kit for the light of heart,? and for sure do your homework before proceeding.
73
Evan
AC9TU? ?
