RF excited CO2 lasers

Replace both the ocillator and it's bypass filtering cap as a set, it's likely that the bypass cap failed and the RF got dumped and fed back into the oscillator and toasted it.

heruursciences said:

Replace both the ocillator and it's bypass filtering cap as a set, it's likely that the bypass cap failed and the RF got dumped and fed back into the oscillator and toasted it.

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Thank god 51pF Dip Mica caps are common! They're about the only component that is! :

The relevent information for that oscillator is in its data sheet, it should output a square wave with between 60/40 to 40/60 duty cycle (ie, +/-10%), with rise and fall times <6ns.

Since the rf transmitter will be putting out a sine wave, I would recommend running it through a buffer of sorts, any reasonably fast ttl not-gate should work fine (then you also have the right chip for making a crystal oscillator when you get bored of having a rc car remote ducttaped your laser).

Good luck with your laser, but I would definitely look into what was causing the failures of those oscillators. Those things don't just die on their own, and are usually pretty well protected from short/reflected power/etc (they are basically the same thing as the output from any standard ttl logic chip). So the first thing I would check is the 5v supply and make sure there is nothing weird going on there, and when its running make sure that the output of the oscillator still looks like square wave (anything else would indicate something being reflected back into the oscillator, which will probably kill it).

Finally, I find it highly unlikely that 2 of these boards would have had the oscillator of all things fail, so I would bet that you were either feeding them with something ugly, that there is a loose connection somewhere in your setup (I would check the rf cable for breaks, and the connectors on the laser for anything interesting), or that there is a more prone component on the board that is failing and taking the oscillator (and probably a few other parts in the rf circuit) with it.

Thanks for the info. I found a 27.145MHz RC motorcycle controller in my shed, I hooked the antenna up to the input of the oscillator amplifier, and i measured the signal at the output of the FET on the laser head, so the signal seems to be getting through at least. The laser didn't light of course because the frequency is too far off and the RF light stayed out because the signal was a non-continuous sine wave. I'm looking into some new oscillators now, the original manufacturer still hasn't contacted me back yet but hopefully I'll have something soon.

Btw, what are this laser tubes made of? The white stuff is by any chance berylla ceramic?

The white ceramic is zirconia

Excellent, I have enough dangerous toys already.

On the other hand, I have my tube, and, it's not lasing...no surprise here as I don't have a psu  

It took me while but I finally fired up my RF laser. I have been playing a while with red laserdiodes but there is not enough power in them. I found out about these RF lasers more or less by accident.
What I can I say? I just wanted to share this moment of joy. This laser burns and it burns well. And I haven't even got a focussing lens in front of it. Oh, and as a bonus, the ceramic lights up pretty nice! So, even though it is IR, there is still a modest lightshow.

Holy crap guys it almost works. I finally managed to track down a suitable oscillator and popped it haphazardly onto the board. This time I'm powering the 12V supply from a lead-acid battery and the 5V supply from a 5V regulator running of the battery as well so no more of this AC adapter nonsense that likely fried the first two. Anyways after becoming deeply saddened when it didn't work, I pulled out the oscilloscope to check the output pin of the oscillator. Lo and behold, as soon as I pressed the probe onto the pin, the laser fired up and the smell of flaming paper filled the air. I removed the probe and it quit again. I tried again and again it came on. So I guess I pulled the ol rushed-soldering deal and made a poor connection. I'm heading off to work now but by the looks of it this laser will FINALLY be working very soon!

EH --

You deserve a "WELL DONE" on this project. It is a fun laser and when you focus it, WOW. When focused, mine shoots pinholes through 1/4 " pine is a second or two.
Congrats.

Mike

Thanks Mike, but don't give me any praise just yet. Everything so far was mostly you guys. And I can't thank you all enough. I couldn't have done it without you. Sure it's cost me almost twice as much as I originally thought. Sure I've spent the last 5 months yelling at the laser for not working. Sure I've cried myself to sleep over not having a CO2 laser. But screw it, I knew lasers wasn't an easy hobby when I got started and this will certainly be far from the last non-working laser I fix. It's been a learning experience. I now know alot more about RF electronics, and more specifically that I do not like RF electronics!

Alright, stop everything. We have one more problem. I am finding that the problem isn't the solder connection. The RF light will only come on and the laser will only lase when I have the scope connected to the oscillator output pin. It's not a physical connection because nothing happens when I push on the pin with my hand or anything. I am thinking perhaps the stray capacitance of the probe has something to do with it? I'm under the impression the output amplitude of the oscillator is related to the capacitance of the load... It's possible the new oscillator is built for a different load capacitance. Does that make sense?

Cancel that problem. I figured it out. I found that if I make a new connection from the oscillator output to ground through a 16 ohm resistor, the laser fires up every time. I'm not 100% sure why, but I guess original circuit has too high a resistance/reactance for the new oscillator and so its peak to peak voltages aren't high enough. When I give it a low resistance path to ground it can source current more easily and the voltages jump into working range. I'm not sure if that's why or not, but it works this way so I'm going to just accept it. It's time to burn some stuff!

I was going to say that the probe likely gives a 10 pF load and was going to suggest a 10 pF cap !!!

Mike

Hello fellow RF CO2 wielders,
I've picked up one of these now, they are nice and small for the power. However it won't output. I have the single board driver 'sliced' pcb and it seems to be working fine. I measure 60V P-P sine at 27.12MHz at the end of the coax, laser end. Those of you with oscilloscopes, is that what you see? However at the FET gate on the laser the signal is only about 10V max and is no longer a sine. The input resistance of the 'RF in' of the laser measures about 48 Ohms, does that match your readings?

Andrew -- I posted to your PM. You have one of the three drivers that I know about. The closest single FET driver I have appears to be grounded grid (gate) class C amp. My favored drivers have 2 fets in what appears to be class B Push Pull.

The closest one I have with a single output amp shows near grounded gate. Ground potential is through two coils to maintain 27Mhz impedance and 4 resistors for bias.



I am trying to contact Nightstick who I believe had one of these drivers. Maybe Chris will step in here to help.

HMike

These older devices had no SWR protection and are easily burned out if the RF load is open or shorted. Using Class C amps causes a rough wave form which I believe helped cause voltage spikes to ignite the tube and improve efficiency.

DrLava --- I asked Chris to contribute here. He said so many things for me to post here or for me to understand. I hope he can also help. He has worked with all three forms of this etching laser.