Electronics > Beginners

Mains voltage found on an old pre-amp chassis - was I in danger?



I have an old transistor preamp (PS Audio 4.6) that isn't working so I decided to open it up and see if I could figure out what is wrong.

The pre-amp is in a small metal chassis. Its power comes from a separately packaged mains transformer that has a two-prong plug for 120V input and has a 56VAC output secondary with a center tap. That transformer is in a separate box and a three-conductor umbilical cord connects the transformer box with the pre-amp itself. The center tap wire is attached to the chassis of the pre-amp.

Here's the crazy thing. I found a 120VAC potential between the pre-amp chassis' ground and the earth ground (from another instrument that is properly grounded through a three-prong plug). Fortunately, I didn't get shocked.

I am very puzzled how this could happen. After disconnecting everything, I could find no short between the mains plug and the secondary output. Shouldn't I be able to expect that the transformer would provide isolation between the secondary voltage and mains ground? (Or am I misunderstanding something fundamental?)

- Ken

Is there a very low resistance between chassis and the earth pin? Is there a connection between the center tap and earth? Is the socket wired correctly? The chassis can sometimes be at 120 VAC (is this your mains voltage or half the mains voltage?) due to leakage from the mains input filter caps (they from a capacitive voltage divider). In this case the current would be very low, so it would not be very harmful, but a proper earth connection should prevent this from happening. If the secondary is not connected to earth (common for double isolated equipment), then the 120 VAC may either be capacitive coupling inside the power supply, or from another piece of equipment (eg. source, amplifier).

Thanks for your help!

There is no earth pin on the pre-amp. It's a two-prong plug, 120V. The center tap from the secondary in the power transformer connects to the chassis ground. There is no provision for grounding the chassis to the wall plug. There are no other pieces of equipment, so the 120VAC must be coming from the power supply some how, but since it's isolated by the transformer I wouldn't have expected there to be such a potential.

Where should I look for sources of capacitive coupling? Is this a fault that can develop with age?

Would it be an Autotransformer?

if it is then maybe your mains wires need swapping, to give a voltage on the chassis near earth potential.

If it is an Autrotransformer then I would ditch it.

Most "Audio" equipment no matter what "Audio" always connect center tap to ground but most of the time that ground is not chassis ground and if you notice they bolt the transformer to the panel , the panel went through being inducted by the transformer's small and weak field and therefore you can find full voltage but nothing will happen . Shouldn't happen with a non-inductive panel ( aka non-magnetic panel )
but yeah , if you have a big MOT you know what i mean .


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