[Heath] SB-303 Noise Blanker

[Guy Giacopuzzi]

First of all, 303's never had noise blankers.  So the "blanker control" 
at the lower left control position is /not/ original...Originally, 303's 
had an "rf attenuator" which, when you look at the schematic, is merely 
a 600 ohm linear pot across the antenna.  Now, a bit of history;....when 
the SB-/104/ came out, true to Heath tradition, a noise blanker was 
offered that was optional....that meant it was sold as an "optional" 
kit....and that meant that someone could fit one to a 303'....which was 
featured as a QST article "modification"....if you have a NB board,  it 
will be a 104' board, and  you should find it mounted to the left inside 
of the chassis, above and to the left of the FSK board.   I haven't been 
in my 303' in a while, but as I recall, the board is rather clearly 
marked, if you have one, and roughly about 2 x 3".  Note this was an 
"after market" modification, and was never authorized, much less 
encouraged by Heathkit.  As I recall, the 104' NB mod simply had an 
off-on switch, not a control.  So I'm not sure exactly what or how your 
radio was modified, but if you don't find the NB board inside your 303', 
you should be aware that as collectors started buying up 104's, a number 
of them didn't have the treasured NB'er, and, because the 
303'-with-a-104'-NB-board was a somewhat popular modification, it's 
possible someone subsequently stole the NB board from your 303' to put 
it into a 104'....
I personally find the excitement over the NB board to be a bit over 
done...remember, NB's were designed chiefly for automobile generated 
"pulse" type noise.  I don't know if they had any effect on general 
backround noise; compared to a modern rig's DSP system, they're like 
swatting mosquitoes one at a time as compared to a helicopter doing an 
insecticide  spraying.  This may explain why Heathkit offered NB's 
"optionally", as relatively few 104's ended up on the transmission humps 
of  cars of the 70's and 80's....most were bought as rigs for home.

Guy
WA6OQQ,
One of the "hams at Heathkit..."
.
On 3/13/2014 10:06 AM, pbrickey at verizon.net wrote:
> Hi,
>   My SB-303 has a Noise Blanker control in place of the RF attenuator control, but the noise blanker board is missing. Does anybody have an idea of why Heathkit eliminated the noise blanker and substituted the RF attenuator? Anyone have a SB-303 noise blanker board?
>   73's,
>   Peter
>
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