1940 Radio engine static

[JayTee]

1940 Fleetwood 2-passenger convertible (7567) has a stock radio but no hole in the cowl for an antenna. So I assumed it came with an antenna under the running board. I had two fabricated (one for each side), and they work fine when the engine is not running. But when I start the engine the distributor or spark plug noise drowns it out. I have put suppressors and grounds on everything in sight! But no luck.

Does anyone know how to fix this?

Thanks

JT

[Doug Houston]

Interference suppression is an art, and not a science, so answers to your question are not easy to make work!

The first thing to do is to turn the radio on, disconnect the antenna from the radio, and run the engine. If the interference is being picked up by the antenna, the set will be silent. If the interference is being conducted through the electrical system, youll still hear the niose.

Im sure that you are right in that the original antenna was an under runningboard type. While the vacuum operated  vertical rod antenna first appeared in 1939 (I have one on my 39-9057B), mant owners opted for the running board antenna.

If the set has no noise with no antenna connected, the noise is obviously being picked up by the antenna. There are many possibilities to the source of the problem. The lead-in to the set from the antenna needs to be shielded and grounded to the body at he connection to the antenna strips. If the car has undergone restoration, the body may not be properly bonded to the frame, and that could be a source of trouble. In addition, the connecting line from side to side of your antenna strips needs to be shielded and  grounded to the body and / or frame.

Go back to the first step. If there is noise with no antenna, check inside the ignition coil cap. There should be a capacitor connected from the terminal in it to the case of the coil. That could kill a lot of interference. The value of the capacitor would be .25  Microfarad @ 200 Volts. You might need to put a woven wire strap from the hood to the cowl. Ungrounded hoods can cause a lot of grief. on many 41 Cadillacs, a capacitor from the ignition switch terminals under the dash to the dash panel can also snuff out ignition noise. This can app;ly to other models too. Try a ground strap from the engine block to the firewall. That has been the cure for a lot of noise on some cars.

The other thing to have is a capacitor from the generator ARMATURE terminal to the frame of the generator.

The throttle cable may need to be grounded as it enters the engine conpartment, though this isnt usually a problem on Cadillacs.

So,the above for starters, and see if it gets you a feeling for the source of the interference. The prime axiom for interference suppression is to kill it at the source.  

[MARK HEWITT 20209]

DOUG. A GREAT AND IMFORMED ANSWER. MANDATORY KNOWLEGE FOR ANY ONE
DOING A RADIO FROM THIS ERA. HOPEFULLY THE S.S. WILL PICK IT UP FOR PUBLISHING.

[JayTee]

Doug

Thanks very much for your apparently well-informed reply!
I will try all you suggested.

JT