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Philco 70 and similar ground leakage - Printable Version

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RE: Philco 70 and similar ground leakage - cpollock - 02-24-2018

Thanks!  I always learn a lot from this phorum.  I got the 38-7 running today, and I noticed for the local AM stations (all 3 of them in this area) the antenna did not make a lot of difference. I suspect the house wiring is acting as some form of antenna and those caps are helping.  But who knows...in my experience RF and HV (20kV and above) both seem to operate with rules that are not obvious to most us, so exactly where the signal is coming from is anyone's guess.  However, the antenna definitely helps on the shortwave band. 
I'm leaning toward putting the chassis back in its cabinet with the caps as shown in the schematic (if it was good enough for Philco, it should be good enough for me), and let this issue rest.  I'm glad to have it working again.  
Now... on to my 41-300.  

Again, thank you for all the information, I really am enjoying learning from everyone about these radios.

RE: Philco 70 and similar ground leakage - EdHolland - 02-24-2018

Grea
One way to look at the weird behaviour of a radio is that it is a voltmeter - it measures or "hears" the difference between its antenna and chassis terminals.

Now if there is a good antenna and low impedance ground connection, reception is good and noise free. Simple! Most of us deal with something different. Another more common situation is a fair antenna, but no chassis connection except the capacitors linking to the line and neutral. In this case a few things can happen. First, it can be fine - clean mains that has a low rf impedance to ground = good reception. Alternatively, if there is rf noise on the mains line, this adds to the aerial voltage, and a vareity of fuzz and buzz interferes with our stations. The radio is acting honestly to what is across the chassis and antenna terminals, but it isn't what we want to hear - a good ground should clean this up. Another effect is where there is good signal from the antenna, and 50/60Hz on the chassis from the way the ac power line is connected. Sometimes these can mix, resulting in background hum on strong stations because the mains frequency modulates the received station inside the radio. I have seen this several times, and again it can often be remedied by connecting a good earth/ground to the chassis.

It is interesting to experiment - also to see what the original instructions recommended. These sets can receive on a short length of wire, but the balance of signal could then be coming down the mains line - the radio doesn't care. You could even connect the antenna terminal to a ground, leave the chassis disconected and still get plenty of stations from what comes down the ac line, noise and all!

And so, this is why it is recommended to have a good antenna, and a separate earth/ground connection to the chassis. Done right, it defines the paths for RF signals, sending them carefully to the radio circuits, and separates them from the ac line and any RF and interference on it.

Ed

RE: Philco 70 and similar ground leakage - cpollock - 02-24-2018

Any suggestions on finding a good earth ground?  All of the pipes in my house are  bonded to the ground of the power panel, which is also bonded to the neutral, so I'm concerned that attaching to a water pipe won't be  any better than using the power ground..  Do you need to go to a ground rod in the earth?

Electrical code does not allow separate grounds in a building, but I don't know if that counts for an antenna system.

RE: Philco 70 and similar ground leakage - Kenneth F. Besso Jr. - 02-25-2018

Well the circuit breaker box is also grounded to a grounding rod ,,,and the cold water pipe ,,,on the other side of the water meter,, ( street side),,,,,,I have a ground on the street side of the Cold water pipe coming in the house,,,and all seems to work fine,, and I use the dead telephone wires which run down the street for the antenna,,,which I disconnect when done with the radio,,,and everything works,,Really Really Well,,,,just my 2 cents ,,,,CHEERS

RE: Philco 70 and similar ground leakage - EdHolland - 02-25-2018

First off, I would try the water pipe anyway - it is there, and it is easy. If it is connected with the water supply, grounding can be very good.

There is no problem making and using a separate RF ground such as a ground rod, as long as it is not used as part of the electrical supply safety ground.

RE: Philco 70 and similar ground leakage - morzh - 02-25-2018

The fact that water pipe s bonded to neutral is ok, it's still a good ground.