ILT 5500A Questions and Help

[dhg6]

revision my mistake the faceplate wasn't on I was measuring the top left (power) to the middle left (cathode). I went down took new readings and from the left most cathode jack to the left most anode jack it is setting rock solid at 213.5 vdc. respectively. so all looks good. do you have further ideas for isolating the problem when hooked up right (shut down). I also added a high cfm fan at the rear of the tube to add extra cooling (I like to be on the safe side).

[mixedgas]

Tube voltage is a function of tube gas pressure, external magnetic field, and current. It is NOT user adjustable.

The tube has a slight negative resistance characteristic, it is fed by a current source, not a voltage source.

Increasing tube current results in its effective series resistance going down in value. DC Plasmas do that.

If the PSU does not limit the current flow, the tube becomes a virtual short and something melts down, either in the PSU or in the Tube.

Think of it as a neon lamp or led analog, it needs a current limiting resistor. Only in this case its a active resistor in the form of the power supply. A pure resistive limiter would cause it to oscillate.

There should be a series current sense resistor in the head, or the current jacks are fed by the Hall Effect sensor in the PSU. One way or another, these determine your I calibration for the jacks. If its the series R, you can calculate the ratio. If its a Hall Effect sensor, you need to see what the ratio for the sensor is. Most small lasers use a sense resistor in the head, usually a .1 Ohm 2o Watt Ohmite AL cased resistor. Some times a parallel set of .1 Ohm resistors is used to get a given scale factor with larger tubes.

The cathode heating should be consuming some place between 65 and 125 watts, depending on size they used. (65,75,80,100, 125 Watt are common sizes) If the tube is healthy, the cathode watts should INCREASE when the plasma lights. Older tubes do not show the increase as they get closer to dying/low pressure. The Cathode is usually fed by some form of constant wattage transformer, but these do not have 100% regulation, so you can see the changes. Cathode power is best deterimined by using a AMP-CLAMP if you have one.

Steve

[dhg6]

revision my mistake the faceplate wasn't on I was measuring the top left (power) to the middle left (cathode). I went down took new readings and from the left most cathode jack to the left most anode jack it is setting rock solid at 213.5 vdc. respectively. so all looks good. do you have further ideas for isolating the problem when hooked up right (shut down). I also added a high cfm fan at the rear of the tube to add extra cooling (I like to be on the safe side).

[dhg6]

According to the manual the measurements are within reasonable tolerance. I get a feeling that perhaps the circuit may have some comparator circuits and possibly someone turned a pot at some time. I see black marks and I haven't moved anything other than the external power pot. But those marks mean someone was careful as they have a mark on the screw and two marks on the potetiometer body. If only I had a schematic to this board it's driving me crazy because I'm so close and I know the unit is healthy and has many hours remaining on it. Please keep this out in the laser network as I really want to fix it correctly. If anything dawns on you please reply back. I truly thank and appreciate all the help both you and Stoney3k have given me. I will continue to chase pins and keep you posted if I discover new ground.

[Stoney3K]

213.5V at 13.6 amps sounds like a respectable enough value. If it makes 1,6W at that current, you've got a ship-shape tube there with some proper optics -- we had to put some effort into a SP168 making that kind of power at at least twice the current!

If you really want to, you may be able to swap out the entire board with one from a (dead) non-medical ILT 5500 head. You may need to make some creative adjustments to allow for the water flow sensor in that case though, but it would allow you to run the tube in CW mode without any inexplicable shutdowns. It would mean you'd need to compromise the original, collector's condition of your rare, prototype medical laser though.

Obviously, the tube will shut down when you try to run it with the case off. Kept me searching for an unexplained tripping Spectra-Physics interlock for countless times, until I discovered a tiny switch on the head which was activated by the cover.

[dhg6]

I moved the normally opened pos to normally closed to allow running without the hood and I put an extra fan on the resonator rails to help cool even more. Thanks for the Idea I will keep trouble shooting this board. I'm so close.