Matt -- I know you say you're near a commercial router, but it would generally take a commercial transmitting station of some power (think 50 kW AM station) within 5 miles or so to cause true RF interference -- and then, you'd hear the station as well at that. The RIAA playback response curve inherently has a tremendous amount of HF attenuation built into it, making it very hard for RFI to upset these stages. As well, the MA2275 uses active EQ to achieve the playback curve, which makes the phono stages even less prone to RFI issues and finally, the MA2275 has RFI protection built in to the phono input circuit of these preamps, so again, I would suggest that true RFI is not at play here. I would more tend to think that the noise is digital in nature, and more likely from within the unit itself.
A couple of thoughts that might help diagnose the issue:
1. IF the noise is ONLY through the Phono inputs, and IF the noise is in both channels (basically) equally, then that does tends to narrow things down pretty quick. Problems like this tend to invariably indicate power supply issues, since it is the only thing common to both channels; an issues there affects both channels.
2. However, the MA2275 uses an op-amp unity gain buffer stage between the output of phono preamp tubes, and the input to the high level section. The buffer for both channels is on the same chip (U9). To see if it is a potential issue, remove the two phono preamp tubes (V4 & V5) before powering the unit up. This hurts nothing, and is the same effective failure mode as if the heater of one tube fails since the heaters of these two tubes are connected in series. With the tubes removed, is the noise still there? If so, the problem is in the buffer stage some where. If it is gone, then it's in the vacuum tube preamp stages.
3. Note that each phono preamp tube serves both channels: V4 is the input stage for both channels, while V5 is the output stage for both channels. Therefore, anything affecting either of these tubes (or stages) will inherently affect both channels. When trying different tubes then, realize that one tube is not dedicated to a given channel, but serves both channels -- always helpful to know when troubleshooting or changing out tubes.
4. Further trouble shooting would require the use of an 82Ω 2 watt resistor, and I don't know if you want to go that far in these efforts. But the idea is to use the resistor (leads inserted into pins 4&5 of the V4 tube socket) to simulate the heater of V4 -- which would allow V5 to then operate. Again, if the noise is still there, then now the V5 stage is suspect, and if not, V4 is the new target.
5. If the V4 stage is the issue, my suspicions would direct me to C75 -- the power supply bypass cap specifically for V4. A failure of this cap would allow all manner of power supply noise (remember there are a number of digital circuits in this unit, so digital noise is certainly available) to enter both channels at this stage, and cause the symptoms you note -- including the overly sensitive tubes (although they will inherently be sensitive to a degree simply due to being preamp tubes).
Since you've ruled out the tubes, the noise is in both channels, and only in the phono preamp circuits, then the problem will invariably be found within the efforts noted above, as there's really nothing else it can be -- internally at least.
Good luck with your search -- I hope this helps!
Dave
