Argon Laser - Dual Switch Mode Powersupply Issue

[bobhaha]

Hey guys,

Hope this is in the right section, if not... please move it...

OK on to business... I have a LaserPhysics Reliant 50S-488 which I've been working on for a while now... But I've hit a wall... I've checked out everything that seems like an obvious place to look... But can't seem to find a resolution. So I thought I would make a video which will hopefully make diagnosis easier.



Where the scorch marks were found...







If you have any ideas or suggestions please let me know here!

Thanks
-Adrian


PS: If you would like to see Dave tear this down and do a repair on it... Please let him know! We live 30 mins from each other!

[Rerouter]

while i do not know while its labelled as C23, that is clearly a metal oxide varistor that in the past has popped scorched the board and been replaced, likely a ptc, type so it would limit current as it heats up, though for a switchmode its normally the other way around :/

[Things]

Burn marks are on the opposite side of the board though, if the MOV had've gone, the burn marks would be on the top. Kinda looks like an arcover to me.

[TerraHertz]

Yeah definitely an arc over. The blackening is surface deposited metalization, from metal vaporized by the arc.
First thing to do is remove it, as it tends to be conductive. Scrape the fibreglass board surface between tracks clean with an exacto knife blade, you won't get it off with solvents.

Next question - how did the arc happen, from where to where, and how widely did the damage spread through the system?
Seeing as it's a laser, I expect there is a very high voltage supply involved, and there could have been an arcover somewhere else, with this as a side effect.


It might have been between two solder pad areas near the center of the blackening, but it's possible it was between one of more of them and the metal chassis under the PCB. Have a look at that metal closely - any pinpoint arc melts?

If you don't have a circuit, good luck. It might be best to try the tedious 'remove and test every component with any kind of proximity, and replace anything that don't do exactly what they should.'

[bobhaha]

Hmm... starting to look that way. Would help if I had a full electronics lab *hint hint*

But its strange, because there is a fiberglass base plate that insulates it from the chassis...



It would have arced from pad to pad I would imagine....

Or something got really hot....

[Things]

How I see it:

Because there is a small amount of power leaking into the output capacitors, enough to fire the ignitor a couple times before diminishing, the SMPS is still running - at least partially.

But, because the power supply can be running without the laser itself running, I'd imagine there would be a way to switch off the tube voltage. Those MOSFETs, being IRFBG30's, rated at 1kV, 3A ea, may possibly be used to disconnect the tube from the power supply.

If they happen to be dead, it's possible maybe they're leaking the small amount of power into the output caps to fire the ignitor a few times. However, it's also possible the SMPS is disabled at a higher level, such as at the switchmode controller itself - I'm not sure.

However, I believe these are either 3, or most likely 4 layer boards, so it's possible an arcover may have occurred in middle layers - but not sure. If you can wipe all the soot off and it looks OK, then it probably was on the top layer, and the soot on the fiberglass kind of points to that also.

But I'd be testing all the semiconductors around that area - don't think it's all that likely for a passive to fail vs semiconductors on something like this.

[TerraHertz]

Hmm... starting to look that way. Would help if I had a full electronics lab *hint hint*

Where in Australia are you? Sydney by any chance?

But its strange, because there is a fiberglass base plate that insulates it from the chassis...

The photo isn't very hi res, but there's a hint of a 'crater' in the center of the sooty area on that base fiberglass sheet.
Have you looked at it under magnification? I suppose it's glued to the aluminium and you can't look underneath.
It's not impossible the arc-over punched a pinhole through the fiberglass.
One way to be sure, is to use any HV supply able to generate low-current sparks of a few mm, and see if electrons can get through the fiberglass at that point. (With all the electronics removed!)
It's a technique used to find leaks in vacuum glassware - using what's literally a hand-held Tesla coil. Which I have one of, for just that purpose.

It would have arced from pad to pad I would imagine....

In which case the soot on the base plate would be more even.

Or something got really hot....

You're really trying to avoid the idea of a serious arc-over, aren't you? Can't say I blame you.
[/quote]

[bobhaha]

Yup I do in fact live in Sydney... out Liverpool way.

Just a little update....

I've done some probing around... the ignitor seems to be recharging after the key switch is turned off, and giving full power strikes down the tube when turned back on.

The cathode leads to the heaters are also a dead short for a moment, then the resistance rises... I have a feeling there must be a cap in parallel with the output.

[Rerouter]

or more likely the heaters are strongly PTC, in such they dramatically increase in resistance once power is applied to them and they heat up, 10A running current means its around 0.3 ohm when hot, so less than that is reasonable when cold,

[TerraHertz]

@ bobhaha  See PM.