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DEM 13ULNAK Lessons Learned

Continuing my current hobby of rebuilding everything I own, whether I wanted to or not, here is an installment on the popular DEM S-band preamp.

When I built mine, I ordered a male N-connector for direct connection to the antenna (no coax jumper needed).  It now appears the male connectors supplied by DEM are not waterproof.  I found this out the hard way.  Now, in all fairness, I believe the first fault was mine in NOT sealing the conenction after it was made to the dish feed.  But since I change the hardware as often as my underwear, I opted not to take the time.  Bad decision.  Ingress of water into the fitting made it's way into the aluminum box.  The box, as it comes from DEM, has no drain hole.  Mine does now!

Well, when I could not hear AO-40, but could still hear my "duckling" signal source, I knew something was wrong with the preamp since the d/c was still working.  When I brought it down to the shack (easy to do since the fitings were not sealed!), I was in utter disbelief:  the inside was literally full of water almost to the top.  The insides were coated with white paste and barnacles (it had only been up a couple of months).

After removing the board from the box and cleaning everything, a smoke test produced mixed readings.  Later analysis shows the SMD negative voltage regulator IC, 7660, was bad--arguably the most difficult component on this board to remove and replace.  The RF amp still draws some current so I am hopeful it is still OK and the 78L05 voltage regulator is fine.  Not wanting to make a lifetime project out of reverse engineering this design, I called up DEM on Monday and ordered a complete board and components kit ($45).  It arrived on Friday and was built on Saturday.  The "bad" board will get rebuilt when I have a spare day (2005?) and then I will have a "backup" preamp.

When I built the unit the first time I followed the instructions on tuning--basically you do nothing but set the drain current for the PHEMT and forget it.  This time I decided to test it with a signal source and see what the effect really was.  My test setup was the preamp feeding a TSI 3733 and my signal source was an ICM self-contained oscillator (see Nov/Dec 2001 AMSAT Journal) in a coffee can.  I found the "sweet spot" was quite broad and for my system was at 18.25 mA drain current (1.825 Vdc across R6).  This is within the 15-20 mA recommended by DEM.  The signal increased from 12 mA, but took a dip below that.  I could get additional gain with current up to 22 mA, but the noise increased proportionately.  I also measured the power supply current at this point at 21.5 mA--so the "2-3 mA higher" suggestion from DEM is a little off as I measured 3.25 mA.  

What was very, very interesting is the other end of the scale.  I found at less than 1 mA (0.50 to 0.75 mA) the PHEMT still operated, and although the gain was down (2 S-units on my TS-2000--for whatever that is worth), the noise figure must have been outstanding.  There was nothing in the receiver except the pure note from the oscillator.  No backgound noise... nothing.  I really liked this setting, but was unsure what it would do in the real world on AO-40.  If you have a high-gain antenna and too much noise, you may want to experiment with this option.  It was really sweet sounding!
Jerry, K5OE

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