External PS weirdness

BlueMDPicker

I had an incident last weekend that was really quite unexpected. While routing interconnect cables from my Dodd MLP to a R2R deck (with all components in the system powered off), the ground shell of one of the IC's RCA plugs touched a screw in the bottom of the MLP's external power supply. Arc...POW! Power supply dead.

It's on its way to Gary's for repairs, but I thought I'd share this to save someone else a bit of grief. As I mentioned, all the components were powered off--but the external power supply always has power to it. And (EEs jump right in), there was apparently a large enough potential between PS chassis ground and preamp chassis ground to create the arc. The screw that was touched and the RCA are both pitted from the event.

The only way to avoid this seems to be to unplug the external power supply any time you are making connections. At least that's the methodology I'll employ in the future.

TNRabbit

Bizarre; are you sure there's not a ground fault somewhere??

BlueMDPicker

Yeah, pretty sure there are no electrical issues. The entire rig is powered from a PS Audio Quintet power center. I think it would go into "blinking blue logo" mode if there were a ground fault.

The only other scenario I can envision is some sort of cap bleeder circuit inside the preamp. It was the last thing I powered off, maybe 10 seconds before the welding demonstration. That could have created the potential I suppose??

ben62670

The best thing is to play music while powering down the pre or even amps. This will drain the PS caps.

madmax

BlueMDPicker wrote: »

While routing interconnect cables from my Dodd MLP to a R2R deck (with all components in the system powered off), the ground shell of one of the IC's RCA plugs touched a screw in the bottom of the MLP's external power supply. Arc...POW! Power supply dead.
.

The only reason that should have happened is if one component or the other has hot and neutral switched. If hot and neutral is switched it pulls hot (instead of neutral) to the same potential as ground, if one of the grounds is compromised there is then 120V potential between the two chassis. You found this out by connecting the two chassis together when the ground shell touched the MLP. This must be fixed and DO NOT touch both components before verifying with a volt meter when reconnected!

You may be a very lucky non-charred man.

Edit: This also implies that one of the ground connections in the plug, recepticle or equipment has been compromised and should be repaired.

BlueMDPicker

madmax wrote: »

This must be fixed and DO NOT touch both components before verifying with a volt meter when reconnected!

You may be a very lucky non-charred man.

Edit: This also implies that one of the ground connections in the plug, recepticle or equipment has been compromised and should be repaired.

Thanks Chuck. I'll go over everything with a VOM when I get back down to FL and again when Gary returns the PS.

I've used this same lineup (ST-70, Dodd MLP, EE CDP) for literally years without issue. Only variable was attempting to connect the R2R. But, it never really came into play because I connected the ICs to the preamp first and was routing to the R2R, and never made the first connection, before the arcing. I know your science is sound, I just can't visualize the errant piece.