Repairing HeNe power bricks

Impossible right?

I've been working lately on repairing and reverse engineering some of these potted bricks. Thought some here might find this interesting, more to follow as I go. Thanks to Sam for adding my efforts to the FAQ, and for donating some more dead bricks.

Sam's Laser FAQ - HeNe Laser Power Supplies

Sam's Laser FAQ - Complete HeNe Laser Power Supply Schematics

Circuit description of the Liteon brick to follow when time permits, most of it is pretty obvious but there's a few clever tricks that took some thinking to figure out what was going on.

Are those the Epoxy encapsulated Bricks...:thinking:

Those are a PITA to open... :gjn the Sam's FAQ

Jerry

lasersbee said:

Are those the Epoxy encapsulated Bricks...:thinking:

Those are a PITA to open... :gjn the Sam's FAQ

Jerry

Click to expand...

Yep, they're a bit of work to get into, but not impossible. The reverse engineering and diagnosis were accomplished by exposing only the bottom of the PCB. When I found a component that required replacement, I excavated from the side or top to extract it for replacement.

I've de-potted about a half dozen of them over the last week, most are variations of the same basic design. The Liteon uses a 555 IC for the PWM driver while the others have been self oscillating inverter circuits where a feedback winding on the transformer is used to drive the transistor feeding the primary. All use essentially the same output stage with a Cockroft-Walton voltage multiplier with additional stages to create the starting voltage. Most use an opto-isolator to provide feedback for current regulation, as the current through the load (laser tube) increases, the PWM duty cycle driving the transformer is decreased and vise versa. Most also use a clever little active filter in the output to reduce the current ripple and avoid the need for more (expensive) HV capacitors.

Shorted HV capacitors and open ballast resistors in the output have been the most common faults so far. The classic cold solder joint has been the culprit at least once as well. If I ever find the time, I'd like to build one from scratch based on the Liteon circuit. I see no reason why the same basic design couldn't be scaled up or down to drive any HeNe tube out there.

I agree... once the basic circuit has been drawn it is
just a matter of scaling with the proper components...

I have a circuit and the main transformers to build a
~3-4 Watt HeNe PS somewhere in the Shop...

I believe the design was originally from Meridith Instruments...
but I'm not 100% sure...


Jerry

Nice work there, I may just open up a brick psu I have because even though it still work, it makes a high pitched loud noise, which is impossible to work next to.