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Deaf DEM 2400-144RX



Thanks to all of you who have been helping me diagnose my AO-40 receive
chain.

Based on checks with a microwave oven, using a harmonic of my 2M rig, and a
50MHz crystal oscillator, it appears I have a deaf downconverter. I have to
hold the wire antenna of my oscillator right next to the feed helix to hear
anything on my VR-500 IF receiver. The dish aimed through my basement
ceiling at the microwave in the kitchen yields some broadband noise just
above the background. The pre-amp appears fully functional, and does not
appear to be oscillating. The results are the same, if not worse, with the
pre-amp removed.

One suggestion has been made that my oscillator may not be on the right
harmonic (should be 2256MHz).

What I have, for those of you who have these units:

The DEM page has a PDF. but it seems corrupted here.

Basically, the top half of the DEM board has an 188.000MHz Osc feeding to a
couple of MMIC's,  then a strip line filter, then a MMIC, then a second
filter, then a filter, and then into the mixer (ADE-3G) -- a 2256MHz LO.

The bottom half has the incoming RF feeding a NE3210 FET, then a strip line
filter, then a MMIC, then a stripline filter, and then into the other input
of the mixer.

The resultant 2M output then goes through a MAV11 (optional, I installed it)
adjustable IF gain stage before going to the N connector.

There is a small negative voltage supply in the center of the board to bias
the FET, as well as the main 9V (7809) that supplies the bias for all the
MMIC's.

The Osc is peaked by a small coil (4 turns, 1/8" diameter) and a piston cap.
My crystal is a touch low (187.997). The FET bias is adjusted with a small
pot in the neg supply. Both the tested voltages here are nominal.

My MMIC voltages are all nominal except for two:

1. The FET current. The FET supply (one resister away) should be 4.5 +/- 0.5
but is running at 2.5V. The drain voltage at the FET is ok at 2V, so it
seems to be drawing high current. DEM says this is ok.

2. The IF MMIC should have voltages of 3.6 and 6.5, but is 1.5 and 4. DEM
says this is ok.

I have ordered a replacement FET and MAV11 just in case and will swap them
out.

Any checks you can suggest on the harmonics --- I have a small 2.8GHz freq
counter. Other ideas?

Bill
w1pa

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