Posts Tagged ground differential

Be careful with those vintage radios kiddies! While completing the finishing touches on my station grounding system yesterday, a got good little buzz coming off of the antenna feed line off the YL’s Heathkit transmitter. The power switch was off, which I always make sure to do whenever I’m making any connection changes to it. It felt like a shock from an electric outlet just not as intense. After I had her station grounded, the voltage differential went away, which is what I expected.

Here’s what I discovered:
Upon disconnecting the RG8u feed from it, I had no differential between the ground bus nor the cable shielding. This indicates a proper ground from the bus to the radio, which is where the cable shield is connected to.

After lifting the ground strap from the radio so the radio appeared to “float” electrically, I still had no difference in ground. I soon remembered that I previously hooked an audio line between her receiver and her computer so she could decode and monitor RTTY. When I pulled the audio cable, a 60 VAC differential appeared between the ground bus and the radio’s chassis.

Here’s why:
The Heathkit radio is powered with a 2 wire AC cord unlike the newer radios with external power supplies feeding 12 VDC to the radio. The DC power supplies bond the negative side of the circuit to their chassis ground, which in turn is then usually fed via a 3 prong power cord with ground. Since the Heathkit is essentially floating via AC, they probably attached chassis ground to the neutral side of the power feed. So when you compare the difference between a station ground and chassis ground, 60VAC appears, one half of a 120VAC house feed. Hopefully your not comparing the difference with your hand. It’s definately an attention getter.

One of the things to keep in mind when you start introducing voltage differentials and computers, you run the risk of damaging computer components. Think about it for a moment. The radio is floating electrically and your computer is grounded. Now you attach a 1/4″ to 1/8″ wire from your radio to your computer. While your pushing that connector into the computer’s Line-in jack, it’s possibly contacting other non-ground pins on the audio card, unleashing the voltage difference between the radio and the audio driver circuits in the computer. POOF! Your sound card or motherboard could be woofed. Mine didn’t, that I’m aware of.

Lesson learned: Ground your components before interconnecting them.