RF CO2 build

[532 with Envy]

:wtf:

Tektronix 2225 Oscilloscope 50MHz 2 Channel, used | eBay

I suggest you buy this before you fry out some expensive RF transistors!



Also, I have a feeling that somewhere the impedance isn't matched properly. RF circuits are very, very picky.
I also suspect that mosfet met the undertaker.

Thanks for the link. I'v looked at oscilloscopes before but there were so many with different specs and very different prices. This one seems to be a fair price and i'm sure would be more than enough for me.

As for the mosfet, what makes you think it's dead. Everything i'v read says they are pretty tough, and it does heat up now. Perhaps if I can swing the oscilloscope soon I can test it that way?

 

[Hiemal]

All semiconductors have weak points, unfortunately. Any number of things could have resulted in an untimely death, so it doesn't hurt to test it out to make sure it's working properly. You can use a multimeter to test the fet, to make sure it hasn't shorted across all leads. (you have to take it out of the circuit though)

And yeah, RF circuits are completely ridiculous to work with...so many things are picky because of stray capacitance and inductance and everything just gets so whiny and picky that you just eventually yell out and hit it so hard that oh, look it starts lasing...

Out of curiousity, what are you planning on doing with the excess CO2 laser tubes you have?

 

[532 with Envy]

I'll keep prodding and poking.

The other tubes I have, i'm not sure all work. What I want to do is get at least one up and running, then hopefully I will be able to test the others. The one I got from heruursciences I would give the best chance but at least one of the other three should work according to the seller. I also have extra pre-amp boards that came 6 in one main board.

 

[532 with Envy]

So I finally got a day off and wanted to try the set-up again.

heruursciences suggested that I attach the FET to the tube housing as that may be the problem. The thought had crossed my mind and every setup i'v see it has been done that way. So i carefully removed it from the heatsink i'v been using and screwed it into the tube housing.

I got all my wiring hooked up and powered up. I smelt smoke and heard a crackly and quickly turned it off. Seems I have burned a resistor somehow. I'm not sure why it would have gone now that I attached the FET to the tube housing and it didn't go with my original mounting?

If anyone has an idea why this would happen perhaps I can solve the problem.

Thankfully before all this I took close up photo's of all the components. I'v seen the chart for resistor colors but am not sure how to read it.
I have a photo of the resistor that went bad, perhaps someone could decipher for me?:beer:

[/IMG]

The chart I found, but when I compare to one at electronics stores they seem to be labled in a different way. Sorry for my ignorance but I just want to get this correct.

[/IMG]

 

[Cyparagon]

Read it with the silver/gold band on the right (as pictured.) Looks like a 47Ω ±5% to me.

 

[532 with Envy]

Thanks Cyparogon. I just needed to verify how I was reading the chart. Turns out I wasn't reading it correctly.

Now i'm seeing the pattern. I know this is basics but thanks again for your simple reply.

 

[Hemlock_Mike]

When in doubt, start these systems using a 20% PWM
cycle. That limits the current on startup.
HMike

 

[turtlecannon]

Hey guys, I'm in a similar situation to the OP. I bought one of these lasers and power supply from heruur a few years ago and I'm having a shot at getting it to lase. The preamp seems to be working fine. I'm supplying PWM and I can get both green LEDs on the board to vary in brightness as I vary pulse width. I can also see the output on a scope. However, the power amp on the laser doesn't seem to be working. The red LED is on, but the device is drawing no current. I thought the FET might have been dead, but I just bought a couple new ones and replaced it, and I'm in the exact same situation.

Any ideas? It seems like it might be as simple as an open connection somewhere, but I have spent a decent amount of time testing continuity and haven't noticed anything obvious. Would this sort of behavior be reasonable if the circuit wasn't tuned properly? I feel like it would at least be drawing some current if that were the case. I should mention that the FET is not getting warm at all. I have verified that the power supply I'm using is working. Let me know if you guys have any suggestions. I really want to get this thing fired up once and for all.

 

[532 with Envy]

I'm still working out what is going on with my tube and set-up but perhaps your problems will end up being what my problems are..:beer:

All I can tell you is when my FET wasn't getting warm at all it seemed like the problem was with grounding the laser and FET common to the main amp board. After I got it all grounded correctly it started to get Hot, but of course, still no lasing.

Keep us posted.

 

[turtlecannon]

Oh, that's a good point about the grounding. Right now the two units are only connected by a dinky coaxial cable. I had assumed that the sleeve was ground but I'm not even sure if that's correct. I'll ground them properly and let you know how that goes (hopefully well).

 

[532 with Envy]

I have read that the coaxial does act as the ground, but I went over board and just tried to link up everything to everything in an attempt to zero out any problems. At a point I started to burn out resistors on my FET assembly for some reason. I seemed to work fine when I had it detached from the laser but when I attached it to where it is "supposed" to be it burnt out the resistor.

I still haven't figured out what is going wrong with my rig. I have three tubes that may or may not work so that throws another wrench into the equation. I have one other tube that may or may not work as well but I need to make the RF coil for it.

These are not an easy project, especially for someone like myself. If you do get yours up and running please post pictures and as much detail you can about it. I see these threads as learning tools for others in the future and hate it when I'm reading great information and it says...."...I'll PM you the rest of the info..." and that's the end of the thread!

Good luck!

 

[turtlecannon]

Alright, I added another ground connection and it didn't seem to do anything. I tested continuity before doing this and the laser and main board were already grounded to each other, so I didn't really expect it to make a difference. So I know that the laser has those copper strips running down it that are supposed to be at high potential, but does anyone know which part of the laser is supposed to be grounded, and how I can verify that it's connected properly? I'm going to probe the rails and make sure they are actually seeing the RF next time I get access a chance to play around with my scope. I'm kind of concerned that I just have a dead tube. Does anyone with more experience than me know if that would explain the fact that I am seeing no current draw on the power amp?

 

[turtlecannon]

Alright, hooked it up to my scope. I have a kinda dirty 27 MHz sine wave coming out of the preamp board, but the rails on the laser are sitting at a constant 36V DC. So it seems like my power amp is screwed up somewhere. I mean, even if the laser is dead, I should still be seeing the amplified signal on it and not just the DC from my power supply, right? Anyone one have any ideas as to where in the power amp the issue might be? I have a feeling it's not the FET, as I just replaced it.

 

[turtlecannon]

Interesting, the gate lead on the FET seems to just be at ground, even with all power supplies turned on. Does this point to the FET?

 

[532 with Envy]

I would not be much help on the technical side. Perhaps posting some photo's of your rig may help others figure out the problem.

Also I have some extra's of the pre-amp boards shown in my pictures. If you need one let me know.:beer: