[Premium-Rx] RA6830 processor noise

[Carcia, Frank A. HS]

    Hi All,
    Saturday I was trapped inside due to heavy rain. I found it a good day
to chase
    processor noise in one of my receivers. One was really bad with two s
meter
    bars lit when the 16 KHz filter was selected and the frequency was above
20 MHz.  I removed P1 from A9 IF strip and the radio went quiet so it was
further up stream. Then I did P1 on the A10 filter module. A bit of noise. 
I tried bypass caps all over the mother board with no change.
I removed A11 P1 and the noise didn't change so I was close.
I then grounded the shell of A11-J1. The radio took off into oscillation.
I found if I put a 330 resistor across the output of A10 the radio remained
stable. I placed the resistor between the ground side of A10 C20 and the 47
ohm series output resistor to A10 J3.
I have done this A10 mod on a number of modules to reduce noise
when wide filters are selected. It does not effect the performance of the
radio but reduces back ground noise.
A11  is still making lots of digital noise. A long story short. The U310
first IF amplifier must have a noise figure around 3 dB so it doesn't take
much signal to upset it. I  found the noise was reduced to a point
where no S meter bars were lit (with 16 KHz filter) with a simple
modification. I installed two fingers of a strip of EMI finger stock
on the ground plane of A11 next to L1 along the left edge of the module.
This grounds the section of plane to the module shield 
plate.  I chased my tail a lot with this one because every time I
removed a shield screw things changed. I added caps on the 
mother board at A13 digital signals which didn't really change anything. 
I may revisit A11 ground system to see if I can make it better but this was
a big step. The A2 processor has 2 test points that reset the processor.
A quick test was stick a screw driver across the pins and stall the system.
There is still some performance hiding on A11 that might improve the MDS.
It is very important to install all the 4-40 screws along the side rails for
best
performance. I will eventually install the finger stock on all my units . A
number
of people have reported this problem to me. Most of my units are very quiet
but 
two of them would drive you crazy.   fc
 
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