SB303 Receiver noise.

[Robert Myers]

Hi,

Unless the SB-303 has its LMO output ('LMO out' RCA phono socket on the
back) loaded properly by another cable-connected SB-XXX transmitter by
the connection to its 'LMO input' (and the associated circuitry inside
the SB-XXX rig) then you won't have the proper 50-Ohm load that ALWAYS
has to be attached at the 'LMO out' of the SB-303.

So, if you have no LMO cable between SB-303 and the SB-XXX rig (and
probably no HET OSC, or BFO OSC cables either) then you must stick an
RCA male plug in that SB-303 'LMO out' socket that houses a 50-Ohm
resistor to ground from the centre plug.  It cannot be left with just
nothing attached and function properly.

If you have the SB-303 connected all up to, say an SB-401, and its
reception is still extremely noisy, check that RG-174 cable connecting
those 'LMO out' and 'LMO in' sockets.

Anyway - you must provide that proper 50-Ohm load on the Sb-303's 'LMO
out' socket at all times.

Cheers,
-- Rob Myers,
 VE3JQL. 


-----Original Message-----
From: Heathkit Owners and Collectors List
[mailto:HEATH at LISTSERV.TEMPE.GOV] On Behalf Of Michael Waldrop
Sent: Monday, March 05, 2007 5:05 PM
To: HEATH at LISTSERV.TEMPE.GOV
Subject: Re: [HEATH-TEMPE] SB303 Receiver noise.

I had a very similiar problem with my SB-303. I do not have any
converters
plugged into the rear of the 303 so placing the Converter switch in any
position had no affect on the noise level; it remained at S8. Removing
the antenna also had no affect on the noise level; again still S8.
I found the voltage adjustments needed attention quite often as well.

I checked other receivers, SB-300, Yaesu FT-757GX, FT-857D and a Kenwood
TS-820S, under the same environment, same power strip and antenna,
no noise. This told me the noise was being generated inside the SB-303.

A thorough cleaning did nothing to reduce the noise. The fix was to
replace the electrolytic caps on the PS/BFO and IF/AUDIO boards. The
noise went from S8 to S0 after replacing the caps. The receiver is
really
quite now.

Whether this will fix your noise problem or not, I can't say. I can say
the SB-303 is over 30 years old and more than likely the electrolytic
caps
are becoming quite leaky. Another thing. The voltages, once adjusted
as outlined in the manual, remained constant once the caps were
replaced.
So replacing the caps fixed 2 problems, noise and voltage instability.
By the way, with the noise gone the receiver really pulls in those
weak stations; stations I couldn't hear before because of the noise.

73's
Mike
W5RKL

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