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Author Topic: SB303 LMO jack resistor  (Read 663 times)
KD0ZGW
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Posts: 1056




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« on: June 20, 2019, 07:57:50 AM »

Just received an SB303 receiver that .  no audio output phones or speaker although it seems to power up ok.  Meter stays at zero. A corner of the FedEx package was damaged but no apparent damage to the rig (i.e.no dents or rattles)

Started troubleshooting and noticed a resistor added across the LMO output jack on the rear.

Looking for suggestions and reason for the resistor.

thx
KD0ZGW
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KD0ZGW
Member

Posts: 1056




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« Reply #1 on: June 20, 2019, 02:41:57 PM »

Got it working.  loose board.

Still wondering what the resistor is for.

KD0ZGW
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K6BSU
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Posts: 66




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« Reply #2 on: June 27, 2019, 04:15:38 PM »

That resistor terminates the internal LMO.  I haven't researched this, but I know it will not work without this resistor.  It's soldered to an RCA plug.
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W5RKL
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Posts: 1113




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« Reply #3 on: June 28, 2019, 05:41:31 AM »

That resistor terminates the internal LMO.  I haven't researched this, but I know it will not work without this resistor.  It's soldered to an RCA plug.

The terminating resistors does not disable the LMO but rather it affects the LMO's stability when not used.
I have an SB-303 that I have tested with and without the LMO terminating 50 ohm resistor at the rear LMO socket.
Without the terminating resistor, the LMO still works but is not as stable as it is with the terminating resistor installed on
the rear panel LMO socket.

An added comment on an option for the SB-303.
I installed a Kenwood TS-520 Noise Blanker in my SB-303. It works great eliminating arcing noise. I rewired the front
panel RF gain control Push/Pull audio switch to enable/disable the Noise Blanker board. The board is powered from the
+15VDC at the Function switch "Orange" wire, no changes to or added resistor voltage divider networks are required.
The Kenwood Noise Blanker board operates from the +14VDC supply in the TS-520 series radios. Any of the TS-520
Noise Blanker boards will work. I installed the board on the left inside panel directly above the SB-303 mixer board.
The link below shows a short video where my SB-303 with the TS-520 noise blanker installed is eliminating power
line arcing noise (power company has since fixed this arcing problem).

https://w5rkl.com/wp-content/uploads/2015/02/Kenwood-TS-520-Noise-Blanker-in-a-Heathkit-SB-303.avi

73
Mike W5RKL
https://www.w5rkl.com



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