Don Recardo, the first two questions are free. Normally we start charging at the third one, but we&#;ll let it slide this time.
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Based on what&#;s you&#;ve described, you have an older power supply (as do I), but mine uses a 51K-ohm ballast resistor.
One thing that I&#;ll caution you about is the voltage across that resistor. In my case 51K x 20mA = 1,000V &#;so be careful! In fact, I added a vinyl sleeve around the wire from the tube cathode to the resistor (see photo). The existing wire is only rated at 600V, and is in direct contact with the grounded metal chassis, so that sleeve adds a bit more margin.
I don&#;t like Adrian&#;s answer, and I don&#;t follow your measurments that followed.
Joe Alexander&#;s answer is correct (and Don Kleinschnitz filled in details).
(tube cathode) --> (resistor) --> (+ mA meter - ) --> (PS laser return input)
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Now to add to the confusion&#;technical stuff&#;
On NEWER power supplies, the &#;laser return&#; is a terminal on a connector on the power supply, and that is directly connected to the chassis ground inside the HVPS. You could verify this with your multimeter. It should read zero ohms to chassis ground. (These power supplies use an indirect technique to regulate the tube current based on the setting of the laser power adjustment input&#;either a pot or input voltage.)
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On OLDER power supplies (mine and others), the &#;laser return&#; is usually a dedicated wire soldered right to the HVPS board. Inside the HVPS there is an additional circuit that conditions and shapes the Laser Return current and then shunts it to ground. So technically, this wire is &#;laser return&#; and not &#;ground&#;. (With the machine off, if you measure that wire input directly to chassis ground. It should read about 300-ohm&#;could be different on various supplies. I&#;d be interested in this result.)
That internal circuit also gives a signal back to the HVPS control circuit. So it directly monitors the laser current, and makes slight adjustments to the HV(+) output to the tube anode, in order to tightly regulate the tube current. Typically, these older HVPS supply only HV to the tube, and rely on a separate LVPS for the +24V and +5V for the machine control.
In some HVPS supplies, there actually is an internal jumper that allows you to connect the Laser Return wire directly to chassis ground, or to select the tube return current monitoring option. One manufacturer suggested direct ground for better cutting, and current monitoring for better engraving.
The best I can figure is that no two K40 machines are wired the same. At this point there appears to be at least two flavors of the new power supplies, and two of the older supplies.
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