Eddystone 888A Mains Supply and RF earthing - UK Vintage Radio Repair and Restoration Discussion Forum

carnivalpete · 15th Sep 2015, 2:05 pm

I am just setting up my third example of the brilliant if ancient Eddystone 888A in my study so I am once more giving consideration to mains supply earths and RF earths. A previous owner has fitted this 888A with a 3 wire mains lead but on balance, as it is a vintage set, I think I would feel happier if it remained isolated from the domestic mains. To this end I think I would prefer to refit a 2 wire lead and connect the radio via my 230V/230V isolation transformer. This should keep me free of any future mains earth issues and I believe that an isolation transformer can usefully act as a low pass filter to some mains borne noise. We have the usual computers and other electronic stuff in the study and the rest of the house. I am constantly experimenting with my aerial system. I know a long wire is the simplest but I have had very good results in the past with dipoles and I am dead keen on 20m and 40m. I currently have a 20m dipole with a coax down-lead to an ATU and a piece of coax between that and the radio's A1 (coax centre) and A2 (coax braid) aerial terminals. Eddystone sensibly recommend that the A2 terminal should be connected to a good RF earth via a jumper to the E terminal. At present the only accessible earth in my study is the mains supply earth which I assume will be noisy. Also, having that directly connected to the radio or the aerial system would spoil my isolation plan. It occurs to me that it would be quite practical for me to install an earth rod just outside my study and bring a short lead into a dedicated connecting box from which I could run a lead to the radio E terminal. This is as far as my thinking goes at the moment so before I start driving earth rods I would be most interested to know what the more experienced guys on here have found to work for them. All comments gratefully received. pete

G6Tanuki · 15th Sep 2015, 3:33 pm

If your dipole is a 1/2-wave and is properly resonant, you don't need an ATU since the 888 has an input-impedance of 50-75 ohms and will match a half-wave dipole directly.

Connect the inner and outer of the coax to the A1 and A2 terminals, and keep the chassis connected mains earth (which is there for safety not for RF purposes).

If you use a non-balanced antenna for general listening (end-fed wire for example) connect one end of it to A1 and an earth-rod, buried mesh groundplane or other RF-earth to A2.

If your property has old-style "PME" [Protective Multiple Earthing] then be careful with earthing as your RF earth rod etc. may bypass the "Earth Leakage Circuit breaker" and leave the rest of the electrical system unprotected. Fault-conditions - either in your home or other properties fed from the same phase - can then result in some *very* high currents flowing in inappropriate earth connections.

carnivalpete · 16th Sep 2015, 10:26 am

Yes, thanks G6, ok with the resonant thing. The atu does no harm and gives me some possibilities on other bands.

The key points of my ramble where the feeling that I might benefit from mains isolation and an RF earth.

As for my mains supply. Mine is a rural location. I have a two wire incoming supply and an adjacent earth rod. Any additional RF earth rod would be around 25 feet away, the other side of the house.
pete

15th Sep 2015, 2:05 pm	#1
carnivalpete Tetrode Join Date: Dec 2014 Location: Pembrokeshire, Wales, UK. Posts: 68	Eddystone 888A Mains Supply and RF earthing I am just setting up my third example of the brilliant if ancient Eddystone 888A in my study so I am once more giving consideration to mains supply earths and RF earths. A previous owner has fitted this 888A with a 3 wire mains lead but on balance, as it is a vintage set, I think I would feel happier if it remained isolated from the domestic mains. To this end I think I would prefer to refit a 2 wire lead and connect the radio via my 230V/230V isolation transformer. This should keep me free of any future mains earth issues and I believe that an isolation transformer can usefully act as a low pass filter to some mains borne noise. We have the usual computers and other electronic stuff in the study and the rest of the house. I am constantly experimenting with my aerial system. I know a long wire is the simplest but I have had very good results in the past with dipoles and I am dead keen on 20m and 40m. I currently have a 20m dipole with a coax down-lead to an ATU and a piece of coax between that and the radio's A1 (coax centre) and A2 (coax braid) aerial terminals. Eddystone sensibly recommend that the A2 terminal should be connected to a good RF earth via a jumper to the E terminal. At present the only accessible earth in my study is the mains supply earth which I assume will be noisy. Also, having that directly connected to the radio or the aerial system would spoil my isolation plan. It occurs to me that it would be quite practical for me to install an earth rod just outside my study and bring a short lead into a dedicated connecting box from which I could run a lead to the radio E terminal. This is as far as my thinking goes at the moment so before I start driving earth rods I would be most interested to know what the more experienced guys on here have found to work for them. All comments gratefully received. pete

15th Sep 2015, 3:33 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Eddystone 888A Mains Supply and RF earthing If your dipole is a 1/2-wave and is properly resonant, you don't need an ATU since the 888 has an input-impedance of 50-75 ohms and will match a half-wave dipole directly. Connect the inner and outer of the coax to the A1 and A2 terminals, and keep the chassis connected mains earth (which is there for safety not for RF purposes). If you use a non-balanced antenna for general listening (end-fed wire for example) connect one end of it to A1 and an earth-rod, buried mesh groundplane or other RF-earth to A2. If your property has old-style "PME" [Protective Multiple Earthing] then be careful with earthing as your RF earth rod etc. may bypass the "Earth Leakage Circuit breaker" and leave the rest of the electrical system unprotected. Fault-conditions - either in your home or other properties fed from the same phase - can then result in some very high currents flowing in inappropriate earth connections.

16th Sep 2015, 10:26 am	#3
carnivalpete Tetrode Join Date: Dec 2014 Location: Pembrokeshire, Wales, UK. Posts: 68	Re: Eddystone 888A Mains Supply and RF earthing Yes, thanks G6, ok with the resonant thing. The atu does no harm and gives me some possibilities on other bands. The key points of my ramble where the feeling that I might benefit from mains isolation and an RF earth. As for my mains supply. Mine is a rural location. I have a two wire incoming supply and an adjacent earth rod. Any additional RF earth rod would be around 25 feet away, the other side of the house. pete