General Schematic Questions

What does the open circle and filled in square mean on a scematic? See attached

The radio I'm working on (Philco Model 90 Middle) has two capacitors on each leg of the incoming AC power going to ground. Are they necessary? It seems to me to be an accident waiting to happen. See attached

• Schematic (44 KB)
• Schematic 2 (12 KB)

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Dear James,

you should post your questions from the model. It makes it easier to answer and the question is linked to the model for ever.
Normally in "talk" no reditor answers.

1) The condensers at the line-in are dedicated to protect the radio from high frequency disturbance from the line. I myself never found a difference without these condensers connected. So you can disconect them because as you said these are often reason for a failure.
If you like to replace them make sure you use condensers which are dedicated for the AC line voltage (not DC!).
2) The circle and quare obviously symbolize a plug or connection to the electro-dynamic loudspeaker. The output transformer and the driving coil for the loudspeaker (No. 32, 33, 34) are outside the chassis mounted below.

Best regards

Rüdiger Walz

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Thank you for your response and answers. In the future I will post questions from the model. It makes sense now that it's the speaker connections. I think I will leave the capacitors out as I have no AC rated ones available. Good tip BTW.

Thanks again

Jim Hochstetler

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Another reason for having the capacitors between AC line and chassis ground is that the RF decoupling from chassis to mains can substitute a ground connection as counterpoise for the AM antenna. However, if there is a high noise level on the AC line, as it is often today with lots of switch mode power supplies and other power electronics, reception might be better without those capacitors.

When using correct AC rated capacitors, they are not causing any safety issues. Make sure that any new capacitors are approved as safety capacitors and have an Y2 rating. To fulfill today's requirements on leakage currents, those capacitors should be maximum 10 nF for 100-120 V mains, or 4.7 nF for 200-240 V mains.

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