ComboStar Build

I’ve started work on my ComboStar HF DSP based radio. Overall with a few hard to find parts, it’s a great project to build. Assembly has been easy with a large selection of alternate parts on on the board. The BOM and schematic kept confusion thus far to a minimum. Big thanks to W4ZCB and SP5TAA for sending critical parts (Crystals and EEPROM). Mouser does sell a 16.67mhz but not in the package provided on the board. Thus far I have the DSP booting and awaiting more parts to complete the build. I completing my build with 805 size parts and hot air aides in getting everything straight. There are a few 603 exceptions such as LED’s and in a few places I used 603’s with parts I had laying around.

I’ve done about 98% of the board using mouser for parts sourcing. RFParts has the filter. Several members of homebrewradio provided critical crystals and EEPROM. Kitsandparts has magnetics and wire. The only items that were hard to come by seems to be the mini-circuits parts. I found a close match that pulse manufactures that I will try in place of the MCL 4:1 part (This likely won’t work and W4ZCB was kind enough to send me an alternate). I was able to locate the amp on Ebay in small quantities. I’ve been assembling as parts arrive and testing voltage prior to setting any major components. So far so good and only a few mistakes that were easy fixes with hot air assistance.

DSP Testing the ComboStar.  This is the first step in testing and be sure to check voltages prior to placing anything expensive.  Also on the DSP be very careful as the pins are fragile.  The technique I used was flux first, then tack a pad placing the part where you want it. You have to use very light pressure with iron to prevent bending pins.  Ensure you have no burrs on the tip prior.  If you evenly apply solder as you go down each row of pins you won’t have to wick any excess.  I’ve found that a continuous motion leading a bead of solder over the work does well with no or little cleanup.

More progress and DSP section is loading with Blinking LED.  Just as a note for troubleshooting – No LED check LED orientation (You should have check voltages prior to DSP soldering).  Solid LED – Check pF on crystal. Likely not enough capacitance = lazy clock.  I used 22pF.  Also for clarification to power DSP attach to DSP section and install jumper.

The OCXO section was tested and a nice clean output is seen. Any OCXO could be used in theory. The DDS input reference is 400mhz max and the IC has a 4x-20x multiplier. The schematics are for a 3.3v OCXO but this could easily be modified to work with several units. Assembly was straightforward but a heat-sink had to be made to fit in the board space. I drilled and tapped a hole to allow the heat-sink to sit off the board to stay clear of the components for the regulator underneath. A heat-sink is required as this part gets very hot. With the heat-sink shown the temps get warm to the touch but dissipation is well within the 317’s spec.


Various Photos during the build.


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