[Racal] RA6830JD Microprocessor Noise

[Dallas Lankford]

The first 6830 I bought was a JD model, and it was plagued with =
microprocessor noise throughout its tuning range.  It was especially =
noticeable at higher frequencies where man made noise is low.  Very =
disappointing, and a sad commentary on human nature which is not really =
relevant to the solution described here.  I spent quite a bit of time =
trying to find the cause of the problem, and consulted several otherwise =
quite knowledgeable people, but to no avail.  I even met a fellow how =
had another noisy 6830.  I don't recall if it was a JD or not.  =
Recently, due to success at eliminating microprocessor noise from =
RA6778C's, I had another go at the JD.  To my surprise, I discovered =
that the noise was being picked up only by the 1st IF at 40.455 MHz on =
the 2nd mixer board.  Further examination showed that the short mini =
coax with SMB connectors which connects the 1st mixer board to the 2nd =
mixer board was acting as an antenna.  I fabricated two small 40 MHz =
baluns using 8 bifilar turns of #24 on Amidon FT-50-68 toroids, soldered =
the unbalanced leads directly to SMB (m) connectors, and the balanced =
leads to the center conductors of pieces of RG-316 approximately the =
same length as the original mini coax.  At the center points of the two =
pieces of RG-316 I soldered their braids together with a short length of =
#22 solid, and in turn soldered that to a ground lug that I added to one =
of the mounting screws.  I routed the dual RG-316 through the side =
plates, as the original had been routed.  Actually, the routing took =
place before the second balun was soldered to RG-316.  About 99 % of the =
microprocessor noise is now gone.  A few isolated spots of noise remain, =
but most of these are also heard on my RA6830-516's, although more =
weakly.  I have also had another interesting experience with =
microprocessor noise when I added spin knobs to my -516's, which I will =
describe in a second posting.

--- StripMime Report -- processed MIME parts ---
multipart/alternative
  text/plain (text body -- kept)
  text/html
The reason this message is shown is because the post was in HTML
or had an attachment.  Attachments are not allowed.  To learn how
to post in Plain-Text go to: http://www.expita.com/nomime.html  ---