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Syntor X audio hum on receive

RFI-EMI-GUY

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Okay guys so here is the final conclusion:

The J1 connector on this faulty radios is connected via 37 feedthrough capacitors. One of the feedthrough capacitors is broken, somewhere deep inside. The hum disappears when I push really hard on the plastic cover of J1 connector inside the radio. When I stop pushing it, the hum comes on again. On my other Syntor X, the J1 connector connects directly to the board, without feedthrough capacitors. Since there is no spare board or spare J1 connector in my shack, the radio will be dismantled for parts.
Are you saying the feedthrough capacitor shorts out? I doubt a broken one would affect audio frequencies and cause a hum. I would suggest an open ground pin.
 

Notarola

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If its a cap to ground and its open I can see a hum being able to be generated.
 

RFI-EMI-GUY

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If its a cap to ground and its open I can see a hum being able to be generated.
If that were the case a chip cap or disc could easily be soldered in to replace it. But feedthrough caps on a radio connector are usually fairly low values primarily to keep RF in or out.
 

Notarola

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Agreed. Scoping should narrow down the pin being affected.
 

WB6NVH

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Seems a shame to junk it out now that the problem has been found. Mine was cured just by loosening and tightening screws. Today I would also squirt some DeOxit on them.
 

RFI-EMI-GUY

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Yeah these radios are over 30 years old. Reseating the boards with cleaner and tightening screws might be all that is needed.

I bought some DeOxit. Pricey stuff even at 5 oz. This weekend I found 11 oz CRC QD Electronic Cleaner at Home Depot for $9. You can get even cheaper on Amazon. I am going to reserve the De Oxit for switches and use the CRC for the bigger stuff.
 

Notarola

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I too use cleaners. be sure to select the non residue types. You don not want 'conductive' material to migrate over the board. For switches I flush well with non residue cleaner than use a siicone or very light oil type residue cleaner to 'reinsert' the lost lubricant to the switch.