How to perform the Hammond safety test?

[M_ichel]

Hi all.

Glad to have found you!

I am NOT a technician! (Heck, I’m not even a musician!)

I’ve been lurking on the forum for a few days. I’ll introduce myself properly in a future post.

The equipment in question is a Hammond A-122.

For various reasons, I’d like to perform the safety test detailed on the first page of the Hammond service manual; but I have several newbie-type questions.

1. I have a voltmeter (Micronta #22-201A, bought at Radio Shack ages ago). On the face, one of the inscriptions is “10,000 ohm/VAC”. Does this mean that it satisfies the manual’s requirement (“Use an AC VOM of 5,000 ohms per volt or higher sensitivity…”)?
2. The organ is in the living room where I do not have a cold water pipe. What would be a practical method of connecting one of the test clips to “a known earth ground”?
3. What are volts “RMS” (as in “any reading of 4 volts RMS or more…”)?
4. A voltage of even 5 volts would be hard to see on the 250 ACV scale of the voltmeter. Should I make this test using progressively lower scales? On the voltmeter, I have 1000 VAC, 250 VAC, 50 VAC and 10 VAC scales.

Before someone asks, I have taken the following measurements at the outlet that the organ is plugged in:
Earth/Hot: approx. 115 V
Earth/Neutral: 0 V

This is not very urgent: I have yet to buy the resistor and capacitor needed for the test.

Please excuse the questions’ detail level: I am not very familiar with these manipulations and I’m scared of electricity.

Thanks for your help!

[indianajo]

If your house was built after 1970, and is in a metropolitan area where the city inspector inspects the wiring is per code after construction, it is likely the safety ground of your 3 prong outlet is connected to earth (a water or gas pipe) in the main panel.
If your house was built before, or was outside a city when built, it is likely a 3 prong outlet has been incorrectly installed with a jumper between safety ground and neutral inside the wall outlet box. check to see if the cable behind the panel in your main box from that outlet has a green wire (romex 2w/g) or not.
In the case of a correctly wired house, you take a three prong NEMA 15 repair plug, attach a (green) wire to the safety prong, and use that as the "earth" in the test illustrated in the manual.
If your three prong wall outlet is incorrectly wired, run a new temporary wire to the cold water pipe in your basement to do the test.
10000 ohm/volt is better than 5000 ohm/volt for meter sensitivity.
You test at the highest voltage first then click down until you are seeing less than the specified voltage on the 10 VAC scale. any full scale readings or readings higher than specified, you stop at that point and have someone replace your transformer(s). there are 3 power transformers in the A100: preamp, power amp, and reverb amp.
The capacitor specified is a ceramic disk capacitor. Do hard wire these on a terminal strip or something so they don't flop around during the test.
Enjoy good fortune doing this. Transformers are expensive.

[M_ichel]

Is this a good ground?

Hi Indianajo. Thanks for the clear response.

I believe the house was built in the 60's.

Looking at the cold water pipe, there's a big copper wire attached to it. That goes off into a wall, in the direction of the electrical panel, which is at the other end of the house.

At the panel, there's a similar wire behind the panel; it enters the panel and is connected there.

Meaning I should be able to use the ground at the outlet. Right?

I'll go and buy the components I need later this afternoon and I'll keep you posted.

Thanks.

[indianajo]

The picture is small enough I can't see the green wires from the romex cables to the outlets attaching to the screw bar at the bottom of the panel. But it looks likely the electricians did the right thing. A third of the expense is that fat ground wire from the main panel ground bar to the pipe. The other expense is buying romex 2w/g instead of romex 2 and running it to each outlet. If they did it right, you can use the 3rd prong of the electrical outlet in the room for the ground of the test.

[M_ichel]

Hi Indianajo.

I don't know why the picture size was shrunk. Here's the same in a different file format: (Hope they're not just thumbnails... Maybe you have to click on them?)


There is no green wire that I can see; just white, black and red! But that may be just a question of different building code standards?

I looked at the electrical outlet where the organ is plugged in and the wire coming into the outlet box has insulated white and black wires plus a bare copper wire.

I managed to get the resistor and capacitor for the safety test. From the reaction of the clerks at the store (Magnavox), they don't sell these things much anymore. Heck, they gave them to me!!!

Best regards and a Happy and Prosperous New Year to all!

[indianajo]

Bare copper wires from the round pin in the wall outlet is fine. The heavy pipe ground wire terminates in a screw on the ground bar in the panel fine. I don't see the bare copper wires going to the grounding bar, but you might be able to. That is where they are supposed to go. White current "neutral" wires, and bare safety ground wires, all going to the grounding bar is fine. The white ones carry return current, the bare safety ground wires do not. That is why you use the round pin of the outlet for the test, there is supposed to be no current flowing through the bare wires so there is no voltage drop on them. The round pin of the outlet should be at the same voltage (potential) as the water pipe from the earth.