SB-303 - Wierd noises/AGC

[Charles Seitz]

Dale,

My SB-303  whistled and  sometimes  made a
truck airbrakes sound when I moved the AGC
switch through its positions.

I checked the AGC quiesent voltage and found it
to be many volts above the proper value.

First check that all the boards are properly seated
and be sure all ground contacts at the card
guides look OK.  Make sure the shields on the many
miniature coax leads are soldered securely to their
ground points.  Tighten the sheet metal screws and
other nuts and bolts; especially those for ground lugs.
Reseat the RCA plugs.  Get some contact cleaner
for the switches.  Look for anything else that doesn't
appear right.  After all, this is a forty year old piece.

Measure 35V, 15V, 10V and -10V as marked on the
chassis near the corresponding pin off the card cage.

With the back of the radio away from you, look down
into the large card cage.  You will see a trim pot 3/4
of the way from left to the right of the rear board.  It
adjusts the 15V power regulator.  If you cannot set
15V stop.  You must corect the problem.

Next look at the board closer to you.  It has three trim
pots.  Left to Right: Meter Full Scale, Meter Zero, Bias
adjust.

You will need a VTVM with many megohms input
resistance to make the AGC Bias measurement. The
AGC source has a very high output resistance.

Turn the AGC switch  to OFF, the Function swith to OPR
and the RF Gain fully clockwise.

Look at the bank of small boards that the band switch
goes through.  The ACC bias is on the far left top pin
off the second board from the front of the radio.  Set
it with the Bias Adjust pot to 3.5Volts.

Set the S meter to 0.  Then turn the RF Gain  fully counter
clockwise and adjust the Meter Full Scale until the meter just
pegs.

Getting here is one 'Vital Sign'  of life for an SB-303.  If you
cannot get to this point or your problem persists,  you will
need to get the schematic for further trouble shooting.

Asside from a wiring error, one of the dual gate MOS FET
transistors may be leaky (my problem).  I also changed the
trim pots as they became intermittant with age.

By the way, the SB-303 is not a very good receiver.  It
is sensitive enough but it's not very useful when the bands
are crowded.  It's dynamic rang is abyssmal.  But, it looks
pretty on the shelf.


                                          ---  CHAS  WA2DYA

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