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It could be the preamp transistor or a relay You need to figure that out. If you looking at the amp from the front, the preamp relay is on the left and you'll see a small transistor and a metal can that is your preamp transistor.
I had a look at the base assembly. There is a pin (rivet?) that goes through the insulator. Perhaps this is the connection? I am going by online visuals, so I can't be certain. The obvious telltale, would be to ohm it out.
I also looked at the specs on the Sirio store, that also state DC...
I would power the receiver from a battery and turn off the main breaker, if it is suspected to be a source, within the house. It would be more important, to first identify the type of noise, before trying to locate the source. It may not be in the house at all.
This stuff, is out of my analog kid realm.
I do like what the DDS VFO people are doing with the 980 SSB though. Seems simple enough, except for the Brazil guy/Troy radio collaboration 980 SSB vaporware, that will never be seen.
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