[elfiself]

Working on a 42-PT-7. I have some basic questions. Is the chassis connected to power supply common and if so where? I believe it may have been at the riveted connection at one end of the filament resistor. In general, once the common is connected to chassis it becomes the only ground with no other isolated grounds so why do some schematics indicate two different grounds? Is B- always line neutral? how is chassis ground achieved if not connected to line neutral? Is it a matter of being at a lower potential ? And finally, is B+ power supply line in or rectifier DC out or both?
TIA elfiself

[morzh]

No, the chassis is not connected to the PS common which is the right side of the Power Switch and the electrolytics' minusses.

They are connected through the 1A capacitor as AC-coupled.

[elfiself]

Is the 1A cap grounded to PS common? Is the #5 cap grounded to PS common? I'm trying to identify the separate ground locations. I'm doing a total rebuild with only the schematic to go by.

[morzh]

Sorry that was not 1A, that's #7. 1A designator is just close to it.

Both caps are grounded (7 through a small choke, likely wrapped around the cap itself) to chassis.
The ground symbol is the chassis.


PS. Do you read schematics?

[elfiself]

I'm learning

[morzh]

Learn symbols first. Including power nodes and grounds.

[Arran]

There is common negative, which is connected to the neutral side of the power line in an AC/DC set like this one, and then there is chassis ground which is connected to common negative through that funky capacitor with the coil wrapped around it. Common negative, or B-, and chassis ground are two different things, the resistor you speak of may have it's case riveted to the chassis, but the resistor itself is insulated from the chassis. and is actually connected in series with the hot side of the power line along with the tube filaments. On some schematics they will use two different symbols to differentiate between chassis ground and B-, one looks like an arrow made up of lines, the other looks like a garden rake, they will sometimes do this to simplify the schematic drawing, an arrow means it goes to B-, rake goes to the chassis, this set actually has the B- or common negative drawn out as a line so the arrow means chassis ground in this case.
Regards
Arran

[elfiself]

Thank You Arran, I have been trying to find a basic explanation without much success. The terms B+, B- and where to establish the desired ground points arrow and rake are the source of my confusion. As far as the riveted resistor it was in fact isolated from chassis, there was however a chassis ground solder connection attached by one of the rivets securing the combo resister.
elfiself

[Arran]

That blob of solder is probably there in part to aid the chassis in acting as a heat sink for the resistor, it probably also helps in preventing the AC from inducing hum into the audio circuits, but I think drawing the heat away was the main idea.
Regards
Arran

[elfiself]

Still not clear on establishing a chassis ground. If I understand your explanation all chassis grounds would be tied to a point ahead of the coil wrapped capacitor and the coil wrapped cap is grounded to line neutral. I'm trying to understand the actual physical location to make chassis ground connections to.

[morzh]

The physical location usually- any point on chassis itself. I could be a solder spot, several of them, or a riveted solder terminal.

[elfiself]

Thanks Morzh, in all the research I've read and youtube vids I've watched no-one states the obvious and so it has remained a question. Can I further assume that it is only DC voltage that is chassis grounded?

[morzh]

No. Anything can be chassis grounded.

Chassis is, in its nature and function, a low-inductance low impedance common reference point.
In this sch there are more than one common references, typical for AA5, the chassis being one of two, so they have to be tied up but since they are at different DC potential, they are AC-tied, through the aforementioned capacitor with coil.

[elfiself]

Hi Arron, I'm back on the Philco 42 PT-7 rebuild which you responded about last year.  Could you explain what determines when the "common ground" is used and when "chassis ground" is used.  Is chassis ground  always established by AC couple through a capacitor to common ground?  Do I understand correctly that a "floating chassis ground" consists of wired chassis ground system separate from actual chassis and if so how is it terminated?  I'm still circling around understanding this concept and appreciate your time.
Larry

[Arran]

In this set, the 42-PT-7, the common negative (electrical ground) or B- wiring is routed back to the power switch where it says 15A on the schematic, you can trace out where it goes from that point. Philco was rather kind in the way this schematic was drawn in that they show the actual electrical connections of the B- wiring, other manufacturers would leave this out and denote what is the chassis ground verses the B- wiring by using a rake styled symbol for the B- connection, and an arrow looking symbol for the chassis connections. However B- or electrical ground, and chassis ground are not completely isolated in most cases, in this set part #7 couples the B- and the chassis together, it's a coil and .2 mf capacitor connected in series, other sets have a capacitor and a high value resistor connected in parallel between the B- and the chassis. I don't know if this explanation helps but if in doubt just print off the schematic and traces out the lines going back to the power switch with a high lighter pen, at any point where two lines meet and there is a black dot that's a connection point, if there is no dot there is no connection. 
Regards
Arran