Weird earthing problem

[Jem]

Hi all

Hope someone can shed some light on this problem (no pun intended )

Just been finalising all the wiring on the projector and fitting the interlocks, shutters, keyswitches etc.

A pal of mine has been doing most of the electronicy bits such as fitting relays etc., as i'm not particularly good at that sort of thing. We've also done away with the pot that came with the wiring for the DT40Pro's for adjusting the scan angle and gone for the full differential signals. Scanangle is now adjustable within Mamba.

All wiring of the DB25 has been done according to ILDA spec's.

The problem is that the scanamps have some interference on them which results in a double image, it's definately the scanamps and not the lasers. The weird thing is that if I unplug the power lead from my laptop the interference disappears and I get a perfect image.

This probably suggests it's some sort of earthing problem but the question is what could it be? We've tried removing and reconnecting various earths but it doesn't seem to work. The only thing that works for definate so far is removing mains power from the laptop that controls the DAC. I suppose the easy way would just be to run on battery power, but that's not getting to the root of the problem.

I'm running Mamba Black software with the Medialas DAC, DT40Pro scanners, a 200Mw Lasever blue and a 200Mw Lasever green laser, and a Maxyzmodules twin red.

Any ideas???

Cheers

Jem

[stanwax]

The fact that it goes away when the laptop has no mains suggests that all the stuff in the projector head is wired up ok. Im assuming that the laptop has the usual brick in the lead rather than mains straight into the laptop (this went out of favor a few years ago). If so then there may not a real earth to the laptop at all. Does the mains power in to the brick have 2 or 3 prongs? (usually have 3 but some have 2). If only 2 - live and neutral then the laptop will be floating in realtion to earth and so therein may lie your prob. Try measuring between a good earth on the projector to the laptop chassis somewhere using a multimeter on AC Volts range. If you gat a reading - which may surprise you in its magnitude (180 to 200v is not uncommon) then you may need to link the 2 with a jumper wire. If you do use a jumper wire connect it with the power off to everything as it will likely spark if you connect with power on. This isnt too dangerous but enought to give you a fright and you dont wannt jump and kick your projector in the head. It should actually earth through the sheild of the usb lead - which might be worth checking for continuity with everything off ie see if there is a good path from the usb connector to the laptop chassis.
unfortunately this is a problem when you have loads of different items that connect together that all need or have separate power sources. This is why I try to use common supplies as much as I can - to minimise the probs.

keep us informed

Rob

ps have you a wiring diagram or can you or your pal make one ....might help

[dave]

Sounds like an earth loop to me.

[rob]

i'm with Dave. i had the sme problem, plug the projector, computer and dac int the same outlet or power strip. and see if it goes away.

[Wiring Monkey]

The laptop power lead is 3 pin with the earth connected directly through to the -ve side of the laptop power plug, the dac isn't earthed via it's PSU but i'll check the usb earth. I guess that means that pin 25 of the db25 should have laptop earth on it.

Internally all three psu's have their 0V lines bonded together and to mains earth - this wasn't the case before. This has cured the wonkyness that affected the image with the laptop power disconnected (wonky lines as opposed to the jitter problem we have now).

All three units were in the same power strip.

It's the first time I've worked on something like this but I have plenty of experiance with audio earth loops as does jem.

Craig

[DZ]

Jem,

Any chance you can post a picture of both ends of your cable?

[Jem]

Hi again folks

Err... and Hi Craig (Wiring Monkey), welcome to the Forum

Just to clarify... Craig is the Pal who is doing the wiring on my projector... Glad you've joined in as I could easily get out of my depth here !

I'll let Craig take it from here if you don't mind.

Cheers folks

Jem

P.S. 2:05am here, I couldn't sleep either

[Wiring Monkey]

Jem,

Any chance you can post a picture of both ends of your cable?

Yes sure, what do you want to see?

Craig

[dave]

You have a earth loop as you have 2 sources of earth back to the dac - one thru the ilda cable, and one thru the mains earth.

I suggest you earth to the dac thru the ilda cable, and remove the secondary to mains earth link inside the projector..

The laptop power lead is 3 pin with the earth connected directly through to the -ve side of the laptop power plug, the dac isn't earthed via it's PSU but i'll check the usb earth. I guess that means that pin 25 of the db25 should have laptop earth on it.

Internally all three psu's have their 0V lines bonded together and to mains earth - this wasn't the case before. This has cured the wonkyness that affected the image with the laptop power disconnected (wonky lines as opposed to the jitter problem we have now).

All three units were in the same power strip.

It's the first time I've worked on something like this but I have plenty of experiance with audio earth loops as does jem.

Craig

[Wiring Monkey]

You have a earth loop as you have 2 sources of earth back to the dac - one thru the ilda cable, and one thru the mains earth.

I suggest you earth to the dac thru the ilda cable, and remove the secondary to mains earth link inside the projector..

Hi Dave,

You're suggesting seperating the 0V line in the projector from earth? I can remove the wiring strap easily enough but there is also a connection through the servo driver boards. Unbolting those from the chassis is a job for Jem ;-)

Craig