"Vibrating" faceplate (actually has current) on vintage amplifier - diyAudio
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Old 2nd January 2012, 02:39 PM   #1
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Question "Vibrating" faceplate (actually has current) on vintage amplifier

Hello

I have an old amplifier which, when plugged in, has a "vibrating" faceplate. It's not actually vibrating though, but rather, has current running through it. It really feels like it's vibrating. It doesn't matter if the power switch is on or off. It goes away the moment I pull the plug. It doesn't matter which way the plug is connected into the socket.

The amp works normally, otherwise.

What could be causing this? There is a "boing" sound every time I turn it on. It is not the relay click, which comes later. It might be coming from the transformer.

Also of note:

It used to have an arcing power switch, so I replaced it. I do not know if it was like this before I replaced it.

The new switch is wired exactly like the old one, and is of the same type. I measured resistance and voltages across every pin on the old before taking it out, and then on the new one after putting it in, and everything is the same.

The power switch got broken during installation, some of its plastic casing broke off when I bent oneof the pins to make it fit better. The switch still worked normally, but I could see the internal workings of the switch through the hole. I glued the plastic piece back in place.

The switch is of the type dpst. It is wired as follows:

1 2
| |
3 4
.B.

1 = to transformer (via fuse)
2 = to transformer
3 = to live
4 = to neutral
B = button
| = X-capacitor (0,022uF)

There are X-capacitors across 1-3 and 2-4. Both of these have numerous cracks in them, but I kept them in because I do not have replacements.

Last edited by Coconuts 500; 2nd January 2012 at 02:43 PM.
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Old 2nd January 2012, 02:45 PM   #2
jaycee is offline jaycee  United Kingdom
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Something is causing earth leakage. The unit is dangerous - do not use it until the fault has been found.

Does the unit have a mains ground wire from the outlet, or is it 2 pin?
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Old 2nd January 2012, 02:46 PM   #3
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I am not using it - I am too afraid to. Just tried it again today to confirm it was not a one-time thing. Not going to plug it in again!

It is a regular two pin device.

To clarify on the power switch issue, the X-capacitor was bent too much outward for the switch to fit, so I bent on it, and that caused one of the switch pins to bend outward, cracking open the plastic casing.

Last edited by Coconuts 500; 2nd January 2012 at 02:49 PM.
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Old 2nd January 2012, 03:06 PM   #4
jaycee is offline jaycee  United Kingdom
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Check to see if either of the mains lines has been connected to the chassis via an X-rated capacitor. Some old equipment does this - a practice long since banned.

Check to see if you get any continuity between the front plate and either pin on the mains plug, on high ohms setting. There is a proper tester to do this, but a regular multimeter will do for starters.

Some pictures would be helpful for suggestions

edit: oh, definitely replace those x-caps across the switch. Remove them for now - the purpose of the caps is to suppress switch arcing. The caps being dead is probably why the old switch died.

Last edited by jaycee; 2nd January 2012 at 03:09 PM.
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Old 2nd January 2012, 04:05 PM   #5
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The mains line goes directly to the transformer via the power switch and a fuse. There is no connection to the chassis. The multimeter does not detect any continuity between the chassis and the mains plug. I guess I would need a megger to properly measure this?

I tried to take some pictures but of course, the batteries in the camera died right away. Here's what I managed to get. I can take more tomorrow when they are charged.

Click the image to open in full size. Click the image to open in full size.
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Old 3rd January 2012, 06:52 AM   #6
jaycee is offline jaycee  United Kingdom
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Yeah, a Megger would be the proper tool to do the job.

Try eliminating the internal mains wiring to rule it out, temporarily wire a line cord straight into the transformer.

What kind of transformer is it? When you can get more pictures please do
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Old 3rd January 2012, 07:05 AM   #7
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Coconuts 500 View Post
The mains line goes directly to the transformer via the power switch and a fuse. There is no connection to the chassis. The multimeter does not detect any continuity between the chassis and the mains plug. I guess I would need a megger to properly measure this?

I tried to take some pictures but of course, the batteries in the camera died right away. Here's what I managed to get. I can take more tomorrow when they are charged.

Click the image to open in full size. Click the image to open in full size.
You can measure the earth leakage using an AC milliammeter: just connect it between the chassis of your amplifier and the earth of your house, first via a 1 to 5K power resistor, and then directly is the reading is reasonably low.

I have noticed that some kinds of paint or anodizations can give this curious "vibration" sensation at very low current levels, in the tens of µA range, which is perfectly safe.
The inter-windings capacitance of the transformer is sufficient to generate that kind of currents.
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Old 3rd January 2012, 11:32 AM   #8
djk is offline djk
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Flip the 2 pin power cord over and see which orientation has less leakage, one way will have virtually none. If that is the case, then that is normal.
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Old 3rd January 2012, 12:00 PM   #9
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I removed the power switch and wired a brand new power cable directly to the transformer wires (although the fuse assembly is still in series with one of the wires). No difference, same problem still. I guess it wasn't the power switch, and this makes me believe that it has been like this all along, I just didn't notice it until I put in the new switch. It's not that easy to detect - I can only feel it with my knuckles/finger joints. Not if I push my fingertips against it.

The faceplate is anodized aluminum.

I've read something about transformer capacitance. Could that be it?

I have tried flipping the power cord. No change. I am in Europe, by the way.
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Old 3rd January 2012, 12:31 PM   #10
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Some more pictures of the power supply:

The loose cord is not connected to anything, it is the original power cord that I temporarily disconnected in favour of a new one. That's the thick cord that can be seen in the first picture.

Click the image to open in full size. Click the image to open in full size.
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