Subject: DX440 whine removed

From: [email-masked] (Bill Shymanski)
Date: 15 Mar 92 18:15:10 GMT

It  is  possible  to remove the whine that the  DX440  emits   by 
shielding internal wiring, however, having done it, I don't think 
everyone will find it worth the trouble. 
 
The  problem has to do mainly with lead dress in the area of  the 
volume  control.   The tuning knob apparently is scanned  by  the 
microprocessor  just as the keyboard is scanned; this results  in 
square-wave-like  signals being fed to the knob. As the knob   is 
rotated,  one  of 8 radial spokes inside the knob  makes  contact 
with   three  fingers  in  the  knob  assembly;  I  surmise   the 
microprocessor  watches  these  three,  notices  which  one   was 
contacted  first, and then deduces which way the user is  turning 
the  knob.   The  whine  is audible when one  of  the  spokes  is 
touching one of the fixed contacts. 
 
Immediately adjacent to the wire assembly that connects the  knob 
to  the  CPU board is a gray shielded two  conductor  cable  that 
connects  the volume control on the left side of the  DX440  with 
the  audio  power amp on the right side. This  wire  was  dressed 
right against the CPU/display board on my radio.  As well,  there 
is a green wire (common ?) connecting the audio amp with the  CPU 
board in the area of the volume control.

Due  to the close proximity of these wires and the mix  of  high-
level digital and low-level analog signals, the whine is  coupled 
into  the  audio  signal  at a point  downstream  of  the  volume 
control;  thus,  the whine is most noticable at  low  volume  and 
masked when you turn up the level. 

I  tried  bypassing the contacts in the tuning  knob  with  small 
capacitors;  this was totally ineffective since any  value  large 
enough to reduce noise stopped  the tuning knob from working.
 
I  came to the conclusion that the shielding of the audio  signal 
in the area of the CPU/display board was inadequate, and  decided 
to double-shield this wire.   
 
I used two pieces of braid stripped off co-axial cable.  A piece
from  RG 8, about 200 mm long, was slipped over the  tuning  knob 
wire  assembly ( slightly flatten the braid so that you can  work 
the  connector  through the braid), then insulated  with  tape  ( 
heat-shrink  tubing  would  be much tidier  -  don't  start  this 
project  at 9 PM on a Saturday night).   I ran the gray  shielded 
pair through a piece of braid stripped from RG58 - this  requires 
un-soldering the gray pair from the audio amp.
 
On  each  braid, leave enough length to make a  pigtail  you  can 
solder to a convenient nearby ground.  Tape or use heat-shrink to 
make  sure the braid doesn't short against the underside  of  the 
tuner/IF board.   
 
After  doing  all  the  above,  the  noise  was  almost   totally 
eliminated.   Slight  manipulation  of the green  wire  from  the 
volume  control to the audio amp reduced and then eliminated  the 
noise  -  I taped the green wire down over the  little  piece  of 
copper shielding found in the area. 
 
After doing all this, the radio has absolutely no whine no matter 
what   position  the  tuning knob is in,  both  on  the  internal 
speaker and on stereo headphones.   
 
It  took  a long time to do this; I advise anyone  attempting  to 
duplicate this result to make careful note of the screws used and 
where  they  came  from. I usually  suffer  from  the  Brazil-nut 
syndrome  when  reassembling consumer  electronic  gear,  however 
someone  not all thumbs would have less trouble than I.   Observe 
the usual precautions when attacking a prized possession  with  a 
soldering  iron.  And, of course, you blow the warranty away  the 
moment you open the back of the case.  
 
Someone  was kind enough to post the observation that  installing 
an  extra  diode  on the CPU/display board would  extend  the  FM 
tuning range down to 76 MHZ.  Well, it does...but FM  sensitivity 
goes  into the pit. Two local 360,000 watt EIRP stations gave  me 
no LEDs at all, and lesser FM stations were inaudible.  I had  to 
dismantle  the radio again and cut the diode I'd put in. I  could 
verify the loss of sensitivity on the bench, powering up with the 
diode  in and out and showing the loss.  This seems very  odd  to 
me;  I  don't know why the CPU is telling the radio to  go  deaf, 
maybe  there's so many strong FM stations where they use  76  MHZ 
that  you must decrease sensitivity.   I also verified  that  the 
first  of  these  three ( D404) blocks  tuning  above  26.1  MHZ.   
Still  don't  know what the third diode was for; didn't  give  me 
aircraft  band or lower LW or faster scanning, so I left it  out.    
My DX 440 works fine on the CB and 10 metre ham band ( although I 
can't get 10 metre FM, of course).
                                     
I  tried  the  filter  also posted earlier  here;  and  it  works 
wonderfully  well.  The lower SW bands are actually  useable  now 
with an external antenna when before they were wiped out by local 
top-40.  Doesn't help MW or LW reception of course; but the  ARRL 
handbook lists other filters that look useful for low-pass, too.   
 
As always, I take sole responsibility for my actions and for  the 
opinions expressed above, and I expect users of this  information 
to behave similarly. 
 
Bill VE4STW

[email-masked]
The Inquiring Mind BBS, Winnipeg, Manitoba  204 488-1607