Model 20 mystery wire? - Printable Version
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Model 20 mystery wire? - Mondial - 06-11-2013
I was looking under the chassis of a Model 20 today and noticed an extra black wire coming out of the power transformer. It is connected to the tapped power resistor and is the only wire from the transformer, all the other connections are made by 12 solder lug terminals.
Certainly looks like an afterthought and does not seem to appear on any of the published schematics, as all the transformer terminals are numbered 1 to 12.
Whats going on here? The wire looks original.
RE: Model 20 mystery wire? - GarySP - 06-11-2013
I thought the same thing when I was recapping my model 20. I have the same wire coming out of a small hole in the phenolic board and going to the B-C resistor. It's also the only wire that appears to be rubber insulated. All the rest of the wiring on my chassis is braided cloth insulated. I'd sure like to know as well! Take care. Gary
RE: Model 20 mystery wire? - exray - 06-12-2013
Faraday shield, perhaps? Does it connect to terminal 3 of the tapped power resistor?
Often a Faraday shield, when incorporated, is simply wedged into the laminations or mounting screws somehow and you never see it. This could be an external connection for better grounding.
RE: Model 20 mystery wire? - Mondial - 06-12-2013
No, it goes to terminal 2, which is the tap on the resistor for the grid bias of the 27 and 71A audio stages.
Faraday shield would be a good choice if it went directly to chassis ground terminal 3, but it does not.
Perhaps it is the connection to the center tap of the 2.5V filament winding? The schematic shows the center tap of the HV winding and the 2.5V winding both connected to terminal 6. Maybe they separated them, but I don't know why as they both connect to the same terminal 2 of the resistor.