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Clark Pace
12-12-2014, 3:50 PM
Hello,

I had a Shengui 350 machine. Worked great. Sold it to someone last week. I did not really need it anymore. I setup it up for them. Spent a day training, on the do's and don't. When ran and bunch of stuff not problems when I was there. Cut and engraved. Did some 1/4" cutting.

So I'm sure they have been using it since. They called and said they ran the laser yesterday for 2 hours. Today they turned it on, and after about 45 sec they heard a large popping sound? Or kind of like a whip?

The laser does not fire any more, but the machine mechanics works. I asked there was any leaking? They could not see any. The said the water was running still.

I'm guessing the tube or the power supply. I asked about bubbles in the tube, they said tiny ones. I guess that could be the problem. I'm going to stop by and take a look, but wanted to get some laser heads together.


Thanks All.

Neil Pabia
12-12-2014, 4:06 PM
A popping sound could be a capacitor on the tube. That would be easily fixed.

Clark Pace
12-12-2014, 5:11 PM
A popping sound could be a capacitor on the tube. That would be easily fixed.

I didn't even know the tubes had a capacitor? Guess I'll look at mine again.

Chris DeGerolamo
12-12-2014, 5:41 PM
As a CYA thing, be sure to see if they have it on a properly rated UPS. If it were me, I would not want to pay for someone else's mistake(s).

Clark Pace
12-12-2014, 6:03 PM
Hi. They guy has not ask for a refund or anything yet. He just want to know how much a new tube would be. I don't know if it's the tube. But I am willing the try to help out. He is in a new building. Could be bad electricity? I did check the polarity and ground of the wall socket before I turned it on originally. I had one building that had been wired wrong. I sold the laser with no guarantee. Things break? I also don't know if he had a setting wrong. Forgot to turn the water on this time, a bunch of things could be the problem.

Bill George
12-13-2014, 11:33 AM
I'm wondering if either the HV lead burnt off or fell off the tube connection and arced to ground. Could be the moving around loosened things up. Or he forgot to turn on the water and the tube went.

Dave Sheldrake
12-13-2014, 6:12 PM
Almost certainly arc to chassis, the wire doesn't have to crack off or be broken , at 25kV an arc will defeat what would normally be very good insulation. Have a look at the joint between the PSU out line and where it meets the main laser tube supply line (usually a screw fitting) that's usually the place they leak from.


A popping sound could be a capacitor on the tube. That would be easily fixed.

DC excited lasers don't have capacitors in the tubes, they are basically a big glass tube full of gas with two mirrors (one a partial reflector, one a total reflector) and two tungsten electrodes. A through glass line to carry water for cooling and that's pretty much it :)

cheers

Dave

Clark Pace
12-13-2014, 6:40 PM
So the tube seems intact. I opened up the laser power supply after turning off the machine and disconnecting the wires from the tube to the power supply. I found that the F6AMP 250 Volts had blow. I can order a replacement online, but locally I can only find 5FAMP 250 Volts fuse. Do you think that will be ok?

Machine is shenhui 350 50wat

Bill George
12-13-2014, 8:52 PM
Before replacing that fuse look for the HV wire shorted someplace. If you can only get a 5 amp fuse try it and then see what happens.

Dave Sheldrake
12-14-2014, 8:28 AM
The Shen 50's don't have a current limiter in the supplies, that particular fuse is to prevent current building and cooking the PSU windings in the event of a dead short in the HT line.

As Bill said, DON'T replace the fuse until you isolate the short or you risk killing the supply, the tube or both.

cheers

Dave

Clark Pace
12-14-2014, 10:58 AM
The Shen 50's don't have a current limiter in the supplies, that particular fuse is to prevent current building and cooking the PSU windings in the event of a dead short in the HT line.

As Bill said, DON'T replace the fuse until you isolate the short or you risk killing the supply, the tube or both.

cheers

Dave

Thank. I will look again. I did look and visibly I can see anything. No obvious short or burn marks.

I never run my laser at 100%. I normally run it at between 11% to 80% at max. After I left the customer wanted to cut some things and ran the unit at 100%. That is when the problem occurred. The machine popped and the laser lost power. I was not their, but that was according to him.

Dave Sheldrake
12-14-2014, 11:30 AM
100% on a Chinese PSU is never a good thing, if the supply is badly adjusted at the factory it will over - current the tube and insulation. Tell the guy to stay below 90 or 95% at max.

It's an HV arc almost certainly, that whip sound is a perfect example of the HV arc to ground on Chinese machines. Given the poor quality of the small Chinese PSU's the arc *may* have been inside the PSU itself so probably a good idea to check there as well.

cheers

Dave

Bill George
12-14-2014, 2:38 PM
On my GWeike the HV line is/was running right along the metal sides of the laser. I took some spiral wire wrap loom and did the entire line, and then moved it away from direct contact with any metal. I would check the connection to the tube at both the Hot end and the grounded wire end. If and when you replace that fuse have the covers open if you can and be sure not to touch anything.... and watch for arcs.

On my machine there does not appear to be any limits in the PS, its all in the software. So 100% could be 2x the rated current of the tube.

Dave Sheldrake
12-14-2014, 5:03 PM
Should be either an internal pot or external pot Bill, they tend to fire them up in machines and adjust by power on an LPM (so you get 50 watts etc on occasion from a 40 watt tube) by measuring emitted power they can get round the "This is a 50 watt laser" bit pretty easily while neglecting to mention neither the tube or PS are rated for it :( What they *should* be doing is adjusting to the Max-Q of the tube current and then measuring emitted power followed by selling at the peak emitted power.

eg: @ 18mA this is a 38 watt laser instead of "This is a 50 watt laser on the LPM" while neglecting to mention it's running at 24mA.

Easy way..

press laser button (it will fire at 100%) measure current, reduce current to the maximum rating for the tube on the PSU pot.

Given that at the tube length of the SH350D it isn't possible to get 50 watts within the current rating it kinda makes a bit of a mockery of their figures :(

Clark,

Handle with care brother, the HT arc on an unlimited 40 watt PSU will jump 4 inches in dry air

302110

This was fully screwed together with a minimum of 6mm of insulation at the thinnest point until the HT went off on a jolly

cheers

Dave