RF excited CO2 lasers

Well I have just tested just about every obvious connection on the board and I just can't seem to find the problem. I know the power has been connected the right way, I triple-triple checked. All grounds are common, all voltages are constant, and still no RF OK light. I am having an extremely difficult time tracing the 5V circuit as the traces on the board travel underneath large components that are flat to the board. Though I did test the LED and it works so that's at least one thing less to worry about

I'm starting to think the maybe one of the IC's got toasted somehow, everything seems to lead back to them. I guess I'll order some and start replacing them since they are dirt cheap.

Any other ideas anyone?

EDIT: Alright, I found and ordered some replacement ICs, as they only cost a few tens of cents each. There are three on the board, two of them are optocouplers and one is a comparator, whatever that is : I'm hoping the problem isn't another specific part which I believe is the RF signal generator. The part is labeled LM2000B 27.12M. A google search leads me to believe that the 27.12M means 27.12MHz, indicating to me that it has something to do with the RF signal that isn't working, either a generator or an amplifier. But I have yet to find a source for a replacement. But I'm going to replace every component on this darn board if I have to! (Unless Chris has more he is willing to sell) This laser WILL live.

Alright, well I have replace both Optocouplers (six pin ICs) and the Comparator (eight pin IC). Here is an overview of my board after my crappy soldering jobs on the ICs and transistor.


Unfortunately the thing still does not work. Same problem as before, zero draw on the 5V input and no RF OK light, and no lasing. Only two major components remain on the board, pictured here:



It must be either that LM2000B 27.12M thing or the can transistor directly below it. I'm going to try and track down these parts, but I am still open to suggestions as to alternatives.

I might be able to get my hands on an analog oscilloscope sometime soon, if I did perhaps someone could show me where to take measurements and what to expect. I know how to use the oscilloscope though so it shouldn't be hard.

Any further help here would be greatly appreciated!

EH ---
That 27.12 can is the oscillator and the transistor is a driver (I think) ---
They should be working because they are somewhat protected devices in the circuit. The O'scope can be helpful but there are power limiting circuits in there related to the TTL input. Something is shutting down the driver circuit. Do any of the BIG transistors get warm at all ??

Mike

As far as I can tell no major components on the board get warm at all. Everything stays cool to the touch. The 12V input is only drawing about 60mA and the PWM input is only drawing about 30mA. Considering that a green LED is running on the board that's not much current flowing through the components at all.  :-/

What might cause the driver circuit to shut down? Because it seems to me that somewhere the power just isn't getting through. Possibly a broken connection?

Yes the small circuit will shut down if there is no or unstable voltage to the big tube transistor. Make sure it's really getting 32-38V

I just triple checked the voltage input to the laser, and its sitting nice and stable at 32.4 volts. But you see, the preamp board used to work even when the laser wasn't even connected. I did do this at least twice. It didn't seem to affect anything at the time.

Also my voltage source for the laser when it died wasn't quite as stable. I had some laptop power adapters conencted in series-parallel and some of the supplies were trying to force current backwards though the slightly weaker ones, so the result was laser input power that was fluctuating between 33 and 30 volts or so. Could this have caused a component failure? If so, what component should I be looking at next?

If it is the fet epoxied to the laser that died, how would I go about testing it?

EDIT: I think what I'll do is buy that last one Chris has up on eBay and use it to figure out whats wrong with the existing one. I'll take measurements of voltage at each of the major components and find out which one is busted. Then I'll fix the bad one and relist the good one on eBay. Piece of cake.

EDIT 2: Whoops, I wasn't paying attention and missed the end of the auction. So scratch that last edit. Still need suggestions.

The big FET on the laser tube is not likely at all to die- they are beasts!

Hmmm well I'm going to try and get ahold of my friends oscilloscope and take some measurements. I'm thinking of testing the output signal of the oscillator and along its circuit to see if I can find where it stops.

Any ideas as to what specifically I should look at? If not then I'll just have to wing it I guess.

Event Horizon:
Sorry to hear about your troubles but with your persistence I'm sure you'll work it out one way or another!

In the mean time I've got a little problem myself that I would appreciate if someone could help me out with.

Take a look at the attached picture.
The green ceramic capacitor seem to have lost a leg and what I wonder is where it is supposed to be soldered back. My guess would be onto the big metal thingy sticking up just above the BNC-connector. Could one of you guys check your amp-board to make sure?

Thanks!
/Tottish

tottish that is correct the big metal thingy is it Lynn one leg to the plastic insulator the other to the metal stem with the phillips screw on the board nightstick

Thanks NightStick! I will just solder it back then.
I will (hopefully) finish my housing for the lasertube today, It's looking real nice. I will probably post some pics later tonight (7:20 AM here).

Have a nice day fellow laser-enthusiasts! =)

Regards
/Tottish

Event, I too am sorry to hear about the hard time your having, and Im not able to be of help on this paticular board setup I have two of them and havent even started messing with them. if I do figure anything out I will definatly let you know, I also have one of the original type power supplys (amps) and It works great but its a totaly different setup.. there is a way to fire these tubes up by conecting them directly up to my 100 watt ham radio tuned to 27.1200 mhz or a 100 to 150 watt cb amp with an old cb tuned to that freq. but that dosent come cheep by the time you get the amp and the radio.... but if all else fails that is an option. wish you the best Nightstick also if you want one of my boards I can sell one pm me if you want one

I was just thinking about the temp-endurance of the lasertube. Does anyone have any idea about how hot it can get without being harmed? I will cool mine with air from a compressor and I guess that should do fine since I'm guessing most of the power is wasted in the driver/amp judging from there grotesque heatsinks! The amp-board will be cooled with two 120mm fans that I guess will do the job but a hint of the recommended max-temp of these would be much appreciated as well.

Regards
/Tottish

I probably couldn't give you an exact figure, but the tube is made of ceramic and so it looks like it should be able to handle high temps. I've got a bunch of small heatsinks epoxied to mine and I'll cool it with a single 120mm computer fan. During the time mine was working I didn't find it getting hot at all.

Nightstick, I've sent you a PM. I'll certainly buy one of your boards if you don't need it! It would be very helpful in diagnosing what the actual problem is! I'll hold off on borrowing my friend's oscilloscope until you get back to me.

Ok Thanks EH, I sent you a PM hope this will help its nice to see other co2 enthuist in here talk to you all soon NH