Home made BLF188 QRO

During big cw contests a good QRO will improve overall score.

Many hams use tubes, others use transistors, home made or factory made. The bad, the good and the ugly. One should balance the price, resilience, size, noise and the heat that QRO produces.I have elected to build a home made  QRO using BLF188 LDMOS transistor about 200 Euros give or take for 1 piece.Oh yes, I’ve burned some.My wallet went thin after 3 pieces and I wanted to drop the project.

I burned last LDMOS when antenna  has fallen down to the ground while transmitting and no protection  implemented at that time.

But a good friend send me another BLF188 and since then, I decided to rebuild it from the ground zero, protection first then testing LDMOS afterwards.

  • Aluminium 1.5 mm was used. From mouser dot com I got SMD components ,  Laird toroids and 1Kv mica silver capacitors
  • An old Eltek Flat Pack 1800 was used as power source and another cheap 12V SMPS  + lm7805 for relays and controller.
  • Aluminium heatsink 300x190x50 mm  without copper heat spreader. 2 PWM high speed fans also from junk box.
  • EasyEDA online software to design PCB’s and JLCPCB manufactured it.Also for SWR bridges I used DJ0ABR design and approach with AD8307 fast log detector
  • W6PQL was the source for tech info but I changed a lot in his design, but still having problems in 21 and 28 Mhz, so for the moment those bands are disabled temporary.

I built the case using plain aluminium 8×8 mm rectangular bars and 3mm screws + nuts .Front panel has a large official 7 inches touch screen for Raspberry Pi and the back plate houses the UHF connectors, Ptt jack and power jack + fuses. Aluminium shield plates were used between power amp and swr bridges, also Raspberry Pi has separate shielding.

Installed  linux Kivy OS on Rpi and start writing pure python ontop of Kivy framework which is a real piece of art, congratulations to Kivy team !

Built a separate protection board using LM339 comparators and CD4013 flipflops based on YO4HFU design but also altered for my needs.

This protection once properly calibrated, it keeps the bad out of BLF188 which is not so resilient as producer advertise it, simply because on CW (not pulsed) if transmitting on 28 mhz with 3.5 mhz filter set, then bye bye , another 200 Euros burned away.

This protection board measures 4 protection stages : wrong filter selected , more than 2:1 SWR , drain over current and input overdrive power.

Why not using Raspberry Pi for protection ? Coz it aint realtime, no matter what bad or good code I write. Raspberry pi in this case is just for design, filters selection, and visual alerts / indicators.

Protection board in realtime will cut off 50 V rail by flipping BTS50085 enable pin and disabling  PTT  T/R relays, then sending to RPI informations about fault occured. I do like building things, but if one wanna replicate it without good knowledges, antenna analizer, good oscilloscope and other lab tools then I advice to stop here and purchase a factory one.Why ? The quantity / price of  LDMOS burned will overcome the price for a factory made one.You have been warned !!!!!

Total cost for this project about 500 – 600 Euro give or take, without burned LDMOS’es, and not counting emotional drama seeing a very nice flame / bang coming from precious BLF188 burning away into oblivion when mistakes are made.

Next step is to matte black paint it and some nice text on the front panel;

Here is a photo gallery, in the future will post schematics and  the python code is here to download. 73’s Gabriel




Leave a Reply

Your email address will not be published. Required fields are marked *