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FDP Forum / Amp Mods, Repairs, and Projects / Grounding old gear - opinions wanted

wrnchbndr
Contributing Member
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New Jersey

I'm back with the otters again
Oct 28th, 2016 12:59 PM   Edit   Profile   Print Topic   Search Topic

I'm getting a bit more into repairing "stuff" instead of just guitars and often I run into things with old two-prong damaged and frayed power cords. An example would be something like a vintage vibraphone or vibraharp with an electric speed control. This particular one belongs to a local college music department so I can anticipate it taking a lot of abuse and neglect. My question is, should I replace the power cord with a grounded three-prong plug? The motivation is to make the thing safer. In this particular situation, I did not because I wasn't sure I'd be doing the right thing.

The speed control circuit had AC going into a transformer which dropped the voltage down to 15 VAC on either side of a center-tapped secondary with the center tap grounded. The two 15VAC wires were then rectified via two diodes to about 12VDC with no filtering to drive a DC motor.

Is adding a three prong plug always a good idea? Certainly getting the hot, neutral, and ground, correct is essential but what do you folks think about doing this by default when replacing damaged cords or, should this be done in every instance even if the cord isn't damaged? Are there circuit types where this is a bad idea? Thoughts?

catnineblue

LA , Calif

I try my best
Oct 28th, 2016 01:12 PM   Edit   Profile   Print Topic   Search Topic

I'd say if it's old and has a metal chassis then a three prong grounded power cord is a good idea. Course if the outlets are properly wired is another issue. Not much help.

amphead4
Contributing Member
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Cincinnati, USA

Oct 28th, 2016 01:20 PM   Edit   Profile   Print Topic   Search Topic

I don't know all the codes, UL or otherwise, but it seems everything bigger than a lamp has either a grounded or polarized cord. Wouldn't anything made with exposed conductive material that's handled be safer with a grounded cord?

Outside of safety though you can set up ground loops that cause 60 Hz hum when multiple pieces of gear have grounded cords. An example is the '63 Reissue Reverb. It's based on the old 6G15 unit but updated with a three conductor cord to meet codes. This required a new grounding scheme that would allow safe use without creating hum from a ground loop. I can't remember exactly how it works but you can search FDP and find a prior discussion of it. I think Fender also described it in the owner's manual of the device.

So my advice is converted to grounded but realize that in some cases it may take more thought than just screwing the green wire to the chassis or frame.

Hammond101
Contributing Member
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So. Cal. USA

Oct 28th, 2016 01:25 PM   Edit   Profile   Print Topic   Search Topic

With motor circuits I don't see much of an issue just leaving the circuit as is although an update to a correctly wired grounded circuit certainly couldn't hurt.

With electronic circuits and equipment with ground switches I would say it's imperative to install a grounded power cord and bypass the ground switch.

Peegoo
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Onward

Christian Slater
Oct 30th, 2016 10:10 AM   Edit   Profile   Print Topic   Search Topic

Mark, YES it makes perfect sense (safety-wise) to replace any two-wire mains cord with a grounded three-wire. Why?

Because virtually everything except consumer-grade power tools and most kitchen appliances are not double insulated. This means the chassis/case can become energized if the power polarity is reversed.

Here's what to do.

Look at the old two-wire circuit and see which leg of the mains supply has continuity (or closest to a direct short) with ground.

That leg gets connected to the neutral (wide blade) and ground pin (round blade) of the new three-wire cable.

The remaining leg gets connected to the hot (narrow blade) of the three-wire cable.

IMPORTANT: the hot supply from the mains cable needs to be routed in this order: power comes from the hot side of the receptacle, travels through the hot leg of the mains cable, goes through the fuse, and then goes to the main power switch. From the switch it goes to the power transformer. THIS ORDER IS CRITICAL because if the fuse is placed after the switch, and the fuse blows, there's still power at the switch (and potentially power on the chassis). The fuse serves as The Black Knight at The Bridge. "NONE shall pass!"

But you're not done yet! There are three things to always do when making a two- to three-wire conversion:

1. If there's a ground switch anywhere in the circuit, *REMOVE IT*. Take it completely out and plug the hole. This switch was commonly used in two-wire chasses to reduce noise and sometimes to prevent shocks. Left in, it's an electrocution hazard if flipped the wrong way.

2. Remove any "death cap" (if included) from the AC input stage to ground. Left in, if the cap goes dead short, it's an electrocution hazard.

3. If there are any "convenience outlets" (always a two-prong receptacle) on the chassis, disconnect them from the circuit and fill the receptacle's slots with hot snot (hot glue) to prevent anyone thinking they're still available. Left connected, they are an electrocution hazard.

wrnchbndr
Contributing Member
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New Jersey

I'm back with the otters again
Nov 1st, 2016 06:25 AM   Edit   Profile   Print Topic   Search Topic

Thanks. Everything said here makes sense.

guitarcapo

U.S.A.

Nov 19th, 2016 08:08 AM   Edit   Profile   Print Topic   Search Topic

I agree with all of this but I might sometimes replace a two prong convenience outlet with a grounded outlet.

Peegoo
Contributing Member
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Notice the music

not the guitar brand
Nov 20th, 2016 06:51 PM   Edit   Profile   Print Topic   Search Topic

If the cutout in the chassis allows it. If not, it requires punching the sheet metal.

Here's a pretty clear step-by-step 2- to 3-wire conversion.

Starts about halfway down.

ECS-3
Contributing Member
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USA / Virginia

Nov 22nd, 2016 04:57 AM   Edit   Profile   Print Topic   Search Topic

"Is adding a three prong plug always a good idea? "
Yes, because getting electrocuted is a bad idea.

Peegoo
Contributing Member
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Notice the music

not the guitar brand
Nov 22nd, 2016 03:22 PM   Edit   Profile   Print Topic   Search Topic

Shocking!

SoK66

USA

We had the hit but Van got the money
Jan 8th, 2017 04:24 PM   Edit   Profile   Print Topic   Search Topic

I recently sold off my old EP2 Maestro Echoplex. About ten years ago I shipped it off for a recap and tuneup to a guy in Phoenix who's a renowned Echoplex shaman. He advised me to replace the original two-lead cord & plug as they were disintegrating. He replaced it with a mored three pronger, good to go.

willie
Contributing Member
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Too Near Atlanta GA

Amp Tech Emeritus
Jan 8th, 2017 04:57 PM   Edit   Profile   Print Topic   Search Topic

Never install a Three wire mains cable to an old AC/DC amp...bad medicine. :)

w

stl80
Contributing Member
*****

USA

stl80
Jan 13th, 2017 08:47 AM   Edit   Profile   Print Topic   Search Topic

The ground is the most important conductor in a circuit. If one of the other conductors fails, nothing will work and you will know to fix it. If a hot conductor goes to chassis and only 2 conductors in the circuit, you'll know when you get shocked.
Jim

FDP Forum / Amp Mods, Repairs, and Projects / Grounding old gear - opinions wanted




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