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  1. #1
    Active User New Member YU1OXB's Avatar
    Join Date
    Jul 2016
    City of Cacak, West Serbia

    Universal 120-160W output PA unit with BJT-s

    This is a kind of universal classic solid state PA in 120-160W range. Primarily it has been designed as an alternative PA stage for old ATLAS 350XL transceiver but is very usable as output for almost any radio operating on 12-14V DC. I happened to have a pair of SD 1487 bipolars which are among the best and most robust BJT-s for RF. There is nothing special or unique in this design, just plain straightforward reliable two stage unit to do the job and last.

    PCB is double sided FR4 laminate, and apart from 5 mounting holes (that fit exactly on ATLAS heatsink holes) and cutouts for transistors , there are no holes to drill on a board. I designed this board so that all components are to be soldered on a top layer tracks. That way it is much easier to change or replace components. Bottom layer is whole copper and without tracks. There are numerous plated through-holes between top ground surfaces and a bottom layer which will enhance grounding and overall stability. If you are doing your own PCB-s at home you probably won't be able to make true hole plating but to achieve the same result put 5mm narrow and 0,1-0,25mm thick copper strips along all 4 sides of the board, bent them in U shape so that one wing touch upper side ground copper and another wing touch bottom copper layer and solder continuously all around on both sides.

    Board is designed for RF transistors in a .500 4L Flange case (outputs) and .380 4L Flange case for driver. Many RF transistors are in those cases so board is not just for one or two types. My Atlas now have SD1487 as outputs and SD1285 as driver. Please note that if you are going to use different transistors, some components might need slight change of values.

    In my unit I actualy skip whole driver stage with that wierd "unobtanium" transistor CD3495 (even thou my one is in a fully working condition) and I took signal from secondary of T531 transformer. With SD1285 as a driver I didn't need more than about 1W of drive and that is provided from 2N3866. Someone else might try to keep orig. driver stage in a chain and omit SD1285 stage and connect existing driving signal to the primary of output stage driving transformer... in any case, some tweaking and adjustments will be necessary for sure.

    Outputs needs about 200mA EACH as BIAS current so set it to 400-500mA total for both transistors. To measure that, unsolder one side of RFC going from incoming +14V supply to + pole of big electrolytic cap and put amp-meter in series.

    I forgot to highlight one more thing - I have a small 12V/2A PCB type relay which I added later for switching on supply for driver stage and BIAS block only on transmit. Collectors of output devices are permanently on +14V.On the orig. PC-530 board, one need to make mods according to attached schematic.

    PCB file is in PDF format but I don't know how to attach it here?!
    I can always send it on mail if anyone needs it!
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    Last edited by YU1OXB; 07-25-2016 at 07:12 AM. Reason: correction and addition
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  2. #2
    Active User Contributing Member KD8MJR's Avatar
    Join Date
    Aug 2014
    Blog Entries
    Excellent post, thanks.
    You can use the FILES section at the top and upload the corresponding PDF file in Zip or RAR format and registered users can the download it.

    BTW It wont show up in the file section until it's moderator approved.


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