[Premium-Rx] Collins 651-S1 A6 RF Module Problem

[Carcia, Frank A. HS]

    Sometimes you can break a screw loose by heating it with your iron. I
have seen a number of these boards on ebay/ Rockwell Collins
U310s or SD215s for the mixers SD215s are protected gates.         fc

-----Original Message-----
From: Gary Geissinger [mailto:ggeissinger at digitalglobe.com]
Sent: Monday, March 17, 2003 1:57 PM
To: 'premium-rx at ml.skirrow.org'
Subject: [Premium-Rx] Collins 651-S1 A6 RF Module Problem



Gentlemen, 

I have two Collins 651S-1 receivers.  One is fine.  The other is somewhat
less sensitive; instead of 1 uV sensitivity, it is about 15-20 dB less
sensitive across the entire RF tuning range.

I swapped the A6 RF Modules between the receivers.  The lack of sensitivity
followed the module. 

I took the "less sensitive" A6 RF Module apart.  The screws on the "good" A6
RF Module were thread locked are won't budge.  I have a mechanical engineer
evaluating options to extract the fasteners.  If I get it apart, then
comparision measurements may yield the solution directly.  

On the "less sensitive" A6 RF Modle, the variable injection amplifier and
the 99 MHz injection amplifier each have about 10 dB of gain and appear to
be fine from a DC point of view.  The second mixer FETs look good (DC Check)
as does the preselector and the LPF (measured for loss).  Bench testing the
A6 RF Module with lab supplies and RF signal generators was unnerving (and
somewhat unscientific) to say the least.

When I checked the first mixer, I found that it was anything but symmetric
in terms of conversion loss between the two halves.  I suspect that it may
have a problem.  An exact measurement was difficult with out the requisite
fixturing and extender cables.

So here are my questions.  I would really appreciate some help here: 

1.  This is the A6 U1, part number 350-3072-020 mixer.  Is there a
replacement part source that us mere mortals can afford?

2.  Casual analysis shows that this may simply be a pair of matched
n-channel jFETS.  I wonder if it is possible simply match a pair of jFETs
and swap them in?

3.  Is there a source of replacement A6 RF Modules? 

4.  Does someone out there have the necessary extender cables and fixtures
I could borrow/buy? 

5.  Is there an "affordable", qualified repair point for the A6 RF Modules?


Thanks for the help, 

Gary WA0SPM 

Gary A. Geissinger 
Chief Electrical Engineer 
DigitalGlobe 
1900 Pike Road 
Longmont, CO 80501 
(303) 682-3826 
(303) 682-3848 (fax) 
ggeissinger at digitalglobe.com 
"An imaging and information company" 
see: www.digitalglobe.com 


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