[Winky Dink]

I spent several days making silicone tuner mounts to replace the dry and cracked original rubber mounts…

[Image: https://c1.staticflickr.com/5/4214/35347...3a5f_z.jpg]

…and it occurred to me that I don’t know the purpose of the rubber mounts.  They insulate the rotor from the chassis, but then you need a chassis ground for the rotor.  So, what for are the rubber mounts?

And speaking of chassis grounding, this radio has a puzzling way to ground the tuner.  The wire with yellow dots goes from the ground post of a compensating condenser to the ground of a Bakelite block condenser, and then it’s soldered to a tuner-mount screw head.  

[Image: https://c1.staticflickr.com/5/4266/35437...7b23_z.jpg]

It appears to be an original wire, but I’m don’t think the factory soldered it to a screw head.  Isn’t the connection from (1) to (2) superfluous?  How was the tuner originally chassis-grounded?

Finally, was the connection to the speaker hard wired, or was there a plug?  

Thanks for your help.
- Henry

[morzh]

The grommets do not insulate: they are to prevent the microphonics.
As you could see there is a metal bushing inside; it will get squeezed between the washer and the chassis when the screw is fully tightened. But the grommet will prevent the cap from rocking and vibrating. It is not an insulator.

[Steve Davis]

The ground wire soldered to the tuning condenser mounting screw is factory. Philco did that on several models.

Steve

[Winky Dink]

Got it. Thanks.
My self-learned concept of microphonics relates to actual movement of vacuum tube components when the tube vibrates. Is that wrong, or how does vibration make a tuner go microphonic?

[morzh]

Anything that is capacitive or inductive and where vibration can change the characteristics enough for it to become noticeable due to the sound propagation whether through air or through chassis can create microphonic effect. Tubes have large electrodes on flexible enough support that they could experience oscillation; so do the tuning caps.

[Winky Dink]

Okay. I'm a little smarter now. Thanks again.