Tektronix TM502A Transformer noise problem

[Gulftown]

Hello,

i have a problem with my Tektronix TM502A with an AM503 plugin.
I get a weired noise signal on the output of the amp as well as when i touch the case with a probe.
It somehow seems like bad grounding, but i measured the gound and it is connected and has low resistance.
The signal seems like a 3rd harmonic of the transformer mains.
I unplugged everything from the transformer except the primary side and still get that signal on the case and outputs.
Funny is that i see the signal even if i clip the gound clip of the probe to the probe it self and touch the case, indicating there is some current flow into the gound of the oscilloscope.
Anyone ever had a similar problem?

Help would be very appeciated.

[David Hess]

Is the transformer running hot?  What happens if you disconnect the earth ground?

I would suspect an insulation failure in the transformer.

[Gulftown]

Is the transformer running hot?  What happens if you disconnect the earth ground?

I would suspect an insulation failure in the transformer.

Thanks for the Tipps.

I disconnected earth ground from the grid input and the noise went away.
I instantly suspected a ground loop problem, but in a instrument like that, this should not happen in my opinion.

I used a 12V TVS diode between the chassis and the grid earth, so in case of a fault the GFCI will pop.
This solves the problem for me.

[David Hess]

I disconnected earth ground from the grid input and the noise went away.
I instantly suspected a ground loop problem, but in a instrument like that, this should not happen in my opinion.

I used a 12V TVS diode between the chassis and the grid earth, so in case of a fault the GFCI will pop.
This solves the problem for me.

The transformer has an electrostatic shield which is brought out on connector pin B10, and also connected to ground through a low value resistor.  Disconnecting it will allow more coupling to the outputs, but maybe it will matter for your application.

[Gulftown]

I tried to disconnect the shield from ground, but that did not change the waveform present on the scope.
We have the same Amp with a serial number close to mine in my workplace and, as far as i know, it does not have the problem, thats why i suspected a fault in my case
I wonder if someone had modified it in the past and just disconnected the gound wire (i will check tomorrow).
I would assume, that any of those TM502A will suffer from a ground loop problem if both the scope and the amplifier are connected to ground.

[Gulftown]

I measured the amp in my workplace yesterday and it does not seem to have the problem.
And no diode between line ground and chassis. There might be something wrong with my transformer, but the voltages are fine and it does not get too hot,
I used a variac to reduce the voltage to test if its a saturation problem, but i needed to go down to 170V to completely get rid of the noise.
The Supply is set to 240V so that should not be saturation.