Tutorial: Melles Griot HeNe Head disassembly

Ok I have searched the internet for a tutorial on disassembling a Melles Griot head to remove the plasma tube. I found a couple that were very vague and had no pictures so I decided to give it a try myself and document it with pictures as i went...

Ok so the specimen in question is a 05-LHR-121 laser head as seen below:



So the first thing I decided to do was to remove the end caps starting with the shutter at the aperature end cutting about 1cm back from where the plastic meets the aluminum being VERY careful not to cut too deep into the housing with my dremel. Please be sure to wear the proper protective eyewear. :



Then we move to the back end of the housing where the power for the tube enters the housing. I also chose to cut about 1cm back from where the plastic meets the aluminum and this worked nicely as it was far enough back to get past the epoxy but not so far as to get past the plastic cap so the cap could serve as a guard to keep from cutting too deep:






Now with the tailcap removed I noticed there is only one lead going to the laser this is because the housing serves as the cathode lead to make the connection to the front of the tube. I now cut the anode wire close to the tailcap to leave enough wire to allow for an easy connection later. Now as you can see below there is a potting material in the tailcap. This potting material contains the ballast resistor and the cathode connection to the housing. The material is most likely and RTV silicone and is somewhat easy to break up and crumble to get to the ballast resistor which will need to be removed to be resoldered into the circuit later.






You'll notice the Ballast resistor calls for 68Kohms out of curiosity i tested it to be sure it was in spec and everything seemed to be in order:



Now to the fun part... Removing the tube!

If you'll look closely on the sides you will see two rows of little black spots circumnavigating the tube. Every other one in each row is alternating one being a nylon set screw (hard and broke off) to position the tube in the housing and the other is a hotglue (obviously the softer material) which is used as the potting material.



First you will need to remove the nylon set screws. I accomplished this by cutting slots in the screws and removing them with a flat screwdriver afterwards:





Now to take care of the hotglue potting... If you look in the picture below you will see and end on view of the tube with the potting in view and luckily it is minimal.



Now there have been a few suggestions on Sam's FAQ about how to take care of this problem such as guitar/piano string and others. I chose to use a piece of aluminum flashing. I cut it into a narrow strip and heated it with a lighter to break the connection of the glue to the housing which worked relatively easy. I tried very carefully to keep the strip close to the housing as not to scratch the tube. After cutting all of the seals I used a piece of plastic to push the tube out of the housing being especially careful not to bump the mirrors. Once the tube is out the hotglue peels off really easy.



...and after a little cleaning



TADA! now to see if it works




Sucess!!!!

Here is a cross section from Melles Griot's website that may prove useful:

Re: Melles Griot Head disassembly

Killer ---

I held my breath all through that string of pix !!!!! Are there 2 ballast resistors? Looks like 2 in series.
This is a good post if someone needs to take a laser assembly apart. Go slowly, have mind turned on and again -- go slowly !!!

+1

HMike

Re: Melles Griot Head disassembly

Nice work. I donno if I'd have the guts to remove mine from it's housing.

Re: Melles Griot Head disassembly

Hemlock_Mike said:

Killer ---

I held my breath all through that string of pix !!!!! Are there 2 ballast resistors? Looks like 2 in series.
This is a good post if someone needs to take a laser assembly apart. Go slowly, have mind turned on and again -- go slowly !!!

HMike

Click to expand...

Nah its just the one ballast resistor... I looked at the photo and it kinda looks like a second resistor in the depression of the potting material lol But yeah its just one.

Re: Melles Griot Head disassembly

Kenom said:

Nice work. I donno if I'd have the guts to remove mine from it's housing.

Click to expand...

Each time is definitely an adventure, Kenom

Good work KiLLrB

+1

Peace,
dave

Added a couple of pics.

If anyone has something to add to this please feel free.

Ouch! It's actually fairly easy to remove the end caps with no cutting at all, saving the destruction of a nice head. This is useful for getting to the mirrors to align them or fixing electrical connections. I've never tried removing a tube just for the sake of removing it though, heads are worth more than bare tubes and a lot easier to use.

Anyway just heat up the aluminum near the cap with a heat gun, taking care not to use so much heat that it melts the plastic. Rotate the tube or the gun as you go so that it heats evenly. The heat will expand the aluminum slightly and soften the adhesive. Now take a sharp knife and gently force it in between the aluminum and the plastic cap. You should be able to form a small gap as it starts to separate and then work your way around. After this the cap will simply pop out if you tug on it and can be stuck back in easily. Take care not to touch the hot aluminum with your fingers while doing this.

The ballast resistor is almost always 75K and usually it is made of several smaller resistors in series both to better cope with the high voltage, and to fit better in the confined space than a single long rigid HV power resistor.

To choose an appropriate resistor, calculate the power dissipated in the ballast resistor using Ohms law V=I*R where V is the voltage across the resistor, I is the current in Amps, and R is the resistance in Ohms. Once you have that, V*I will give you the wattage, then add 25-50% as a safety margin. For example, 4.5mA tube current, 75k ballast resistor, V = .0045 * 75,000, giving 337.5V drop, 337.5*.0045 = 1.52W, so a 2W resistor would be a good choice. Note that if you have several resistors in series, the dissipation will be spread over all of them so each one can be a lower wattage. I like to use two 30K and one 15K in series. These are standard values, cheap and easy to find in 1-2W sizes and will work for nearly any HeNe tube.

Thanks for the info. This will be very helpful. I have 6 JDSU 35mW tubes to do this on. The endcaps are held on with setscrews so at least I won't need to use the dremel to remove the endcaps.

M-