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Deaf DEM 2400-144RX - Cured!



This morning I swapped out the first MMIC (MAR3) after the 188Mhz OSC, and 
lo, my signal generator is now S9+60 instead of barely audible in the noise 
in the VR500 IF RX.

I am still unsure of the exact failure mode, since I broke the input pin of 
the first MMIC while removing in. A couple of thoughts, though:

1. While building the kit, I was very careful to clean out the MMIC holes 
with an exacto knife when I first mounted the MMIC's. I thought I might have 
seen a small burr when I removed the MAR3, but I am unsure. I cleaned it out 
again and checked the pads with an ohm-meter this time before installing the 
second. The MAR3 location also aligns with the crystal and the crystal 
heater location on the backside of the board, so it could have been an 
artifact of mounting the crystal.

2. Regardless, the input voltage of the MAR3 was suspiciously low, (possibly 
a resistive short) which pointed me there. I was dubious at first, however, 
since two other devices on the board (MAR11, FET) have voltages 
significantly outside the published checks, but was told "they're fine" by 
DEM.

3. There were also appreciable s-meter changes in background noises by 
powering up and adding the UNLA pre-amp I have, which pointed to the LO 
injection versus the incoming RF path.

4. I could have zapped it, or did something to it after it was built during 
the time I was trying to find the incoming generator signal (my crystal is 
at 187.9998, which puts the 2400 marker at 144.028 after the crystal comes 
to temperature (allowing for all the errors in the counter, crystal, and RX 
freq counter).

Anyway, I'm off to check the downlink on the next usable pass of AO-40, and 
then I'll work on my uplink (FT790 in the wings).

I want to publicly thank those of you who sent me suggestions and 
experiences, and to especially thank Rick K9KK, and Terry ZL3QL who via our 
email exchanges patiently acted as techs over my shoulder, helping me to 
check and isolate each part of my receive chain, and each block of the 
downconverter, using just the simple instrumentation I have here in the 
shack from all my HF/QRP building. It's been a great learning experience.

As a "thank-you", I promise to do the same for someone else.

73,

Bill W1PA

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