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Re: Deaf DEM 2400-144RX



Hi Bill:

    I'm running the same downconverter with very good results. In fact I
left out U5(MAV-11) because the gain was sufficient and didn't want to
add extra noise. When I first tested my downconverter after assembly
I exibited the same results that you have on yours and it did not work.
What I found was the same problem you had below:

> 2. The IF MMIC should have voltages of 3.6 and 6.5, but is 1.5 and 4. 

I had dead shorts to GATE in the holes where the MMIC's mounts on the
board. I used an ohmmeter to check this. You need to cleanup these holes
before mounting the MMIC's. There were 2 MMICS that I had to remove to
cleanup the holes. I used a rolled up piece of sandpaper through the holes
to remove the foil pieces then rechecked using an ohmmeter. 

Hope this helps. Good Luck

Tom / N7HXP
CN87RG
mjrtom@home.com

----- Original Message ----- 
From: "Bill Acito" <w1pa@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Tuesday, October 16, 2001 8:57 AM
Subject: [amsat-bb] 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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