I can feel a voltage on the chassis of Lexicon rack unit

[pvision]

A newly-acquired Lexicon MPX110 has a few volts on the case when you plug in the PSU.  I get the usual "furry feeling" when touching the case lightly

It has a genuine Lexicon external transformer with 9 V AC output and a 2-wire cable.  So there is no ground for the unit.  It's not in a rack currently

I was going to recap the internal PSU but it looks well-organised and I suspect some thought went into it.  The PSU has two 3300uF 16 Volt caps, both 25 mm tall but one is 10 mm diameter and the other 15 mm.  The PCB is marked with those exact diameters so someone planned to have two caps of the same value but of different sizes… My conclusion is that they know better than I how the PSU works

Regulator is an LM2940CS

Ideas? Just earth it in a rack??

Thanks

Nick Froome

 

[JohnRoberts]

The external PS should be double insulated.  Confirm 9v and that there is 0V from either lead to ground.
======
Then measure voltage between the chassis and real ground.

Then measure current... (carefully).

ASSuming current is low ground it.

I would look at the PS inside the unit to find how/why current is getting to the chassis.

JR

 

[PRR]

> Just earth it in a rack??

Yes.

Working as designed.

It has RF bypass caps to the line. UN-racked you have 60VAC through 0.005uFd, "furry". But this current is so small that any decent audio ground will absorb it.

 

[JohnRoberts]

> Just earth it in a rack??

Yes.

Working as designed.

It has RF bypass caps to the line. UN-racked you have 60VAC through 0.005uFd, "furry". But this current is so small that any decent audio ground will absorb it.

I don't understand.  Are you saying the external 9V transformer has an RF cap from primary hot to secondary?

Perhaps, but I am not sure why. It seems more logical to put an RF cap across the primary.

Some unbalanced, ungrounded, consumer designs try to ground the chassis through a cap to mains neutral. I prefer a 3 wire line cord and a real wire to the safety ground.  Such a cap between chassis and neutral plugged into a mis-wired outlet, or backwards in an unpolarized outlet could energize the chassis, but with presumably non-dangerous amounts of current.   

That's why I suggested you measure what you have, to see if it is harmless or dangerous.

JR

 

[PRR]

> I prefer a 3 wire line cord and a real wire to the safety ground.

Me too.

If Lexicon has departed from that practice I am disappointed. But unlike Nick, not Shocked.

 

[druu]

While on this topic (and apologies for the hijack) but I have a rack containing an eq (3 wire IEC) and a dac (2 wire). If I have the dac on and switch on the eq, the dac is somehow affected and something odd happens to the sample rate, sync LED flashes momentarily and sound is ugly until I reset power on the dac then all is well.

Any idea what would cause this?

 

[hymentoptera]

I've got a pair of dbx 242 parametrics that do this. Not the chassis though, but the sleeves of the outputs. It hits me sometimes when plugging the outputs into something else, and it doesn't seem to matter what I'm plugging it into. A "furry" little zing is right.

I put the Fluke between the 242's output jack shield and earth, set to measure volts AC, and I see 60vAC no matter what I touch the other lead to. I always forget about it until it happens again!