Switching PSU problem

[richard]

My multimeter says 2KV DC. I'm not sure I can trust it.

I don't doubt the possibility that the voltage is real, as a tv serviceman I have experienced those sorts of voltages between supposedly earthed TV's and properly grounded antennas many times.

maybe a photo of the psu indicating where the measurement is being made could shed some light

ps its not a meter auto_scaling measurement error

[The Master]

I've attached some pictures which I hope will help. The earth pin on the IEC socket has 2 green wires coming from it. (I've attached an old picture so you can see what's going on behind the PCB. There was only 1 wire back then.)
One wire goes to the PSU and then the front panel. The other goes to the screws holding the IEC socket then splits to the 10 metal sockets on the back and the ethernet module (little PCB top-left).
All 3 screws on the ethernet PCB and the 2 screws on the front panel are connected to PCB ground.

The 12V supply goes into the PCB at the back. The negative side connects to this PCBs ground plane. That ground plane is then connected to the ground plane on the ethernet module through the short wires between them (Red: 7V out, Black: ground, Green/Blue: data).

The ribbon cable to the front panel links the ground planes on the ethernet module and the front panel. It also has 5V/3.3V supplies and some data lines.

So all 3 PCBs have their ground planes connected but the ethernet module and front panel also have their ground planes connected to mains earth (and the PSU case). The PCB at the back is the only one without a separate earth connection as it's only contact with the outside world are the 4 pins on each mini-XLR socket. It doesn't even make contact with the metal cases of these sockets.

While the PCB was resetting I connected the multimeter to the 12V screw terminals on the PSU. This is what stops the problem until I take the probs off.
The high voltage was measured with 1 probe touching the top of the PSU case and the other touching the 3 screws on the ethernet module. The screw where the earth wire connects has the lowest voltage. I also put the probe on the back PCB where the ground wire connects to it from the ethernet module and this had the highest voltage.

There were 4 of these that were all turned on several months ago. The other 3 are still running fine now. This one was sat on top of 1 of the others so both would have experienced the same ambient temperatures and electrical noise etc.
There are another 4 of these which are still running after a year.

I opened up every PSU before powering up for the first time to check for solder balls, bad joints, lose components etc. Can never be too careful with cheap Chinese electronics

I'm trying to source another multimeter to double check the reading.

[pedja089]

PCB in top right corner with metal port isn't grounded. Look at ground plane. And solder mask...
Also secondary of PSU isn't grounded. Remove blue capacitor looking thing, and place short. Then measure static build up.
PIC probably resets when spark jumps from screw to GND.
Multiple path to for ground can act as antenna. Move green wire from all PCB's to - terminal of PSU. Then try to test it.

[richard]

I agree with pedja
relying on the psu cover to be screwed on for earthing continuity is not a good look.
ps has meter reading been verified yet ?

[The Master]

I know it might not be clear from the images but every single screw hole in both PCBs (and the metal connector) are all definitely connected to PCB ground. The ground plane is on the bottom of the PCB and top is 3.3V which is why the screws holes aren't connected to anything on the top of the PCB. They are plated through to the bottom though.

The PSU secondary isn't grounded internally (though I will try that as you suggested). It is still grounded though because it connects to the PCB ground which has the green wire connected to it. It's a longer path but it's still connected to earth.

Is removing the green wire from the PCBs just to see what happens or are you suggesting this as a permanent fix? If I replace that capacitor looking thing with a short then I guess everything would still be grounded but I'd feel better knowing I have an actual earth wire attached in case one of the connectors comes loose.

relying on the psu cover to be screwed on for earthing continuity is not a good look.

Is this referencing the blue capacitor like thing that connects to the case rather than directly to the earth terminal?
I don't think that would be so bad as the case is always supposed to be on and I have separate grounding wires for the PCB grounds.
I could add a wire between the earth terminal and the negative secondary and also an earth wire directly to the case just to be sure.

I'll test everything you've suggested as soon as possible and report back. I know someone who has a better multimeter so I will see if I can try with that.

[pedja089]

If - is bottom, then metal connector should go at bottom.
Screw doesn't make good contact. Also on vertical board with LCD, I don't see exposed copper or tin around metal connector.

[The Master]

I can move the connector to the bottom. It shouldn't be causing this problem though. The PCB has about 6mm of tin around the hole on both sides of the board and the hole is plated through.
The LCD board is exactly the same (6mm tinned both sides and plated through). On that board the ground plane is on the same side as the green wire.