CO2 laser instability diagnosis – tests completed and request for your technical opinion
Posted: Thu May 21, 2026
Hi everyone,
I’m trying to diagnose a very unusual CO2 laser problem and I’m hoping someone here has seen something similar.
The laser has **2 distinct operating states during normal firing**:
* **Stable state:** tube is quiet, PSU is quiet, and output power is good
* **Noisy/unstable state:** tube and PSU both make a high-pitched / buzzing / crackling sound at the same time, and output power drops
Important behavior:
* sometimes the laser starts stable and quiet, then after about **5 seconds** the noisy state appears
* once the noisy state starts, it **does not go away by itself** while firing continues
* I have to stop and restart
* sometimes restarting brings it back to a stable quiet state, sometimes not
* when the system is in the noisy state, the abnormal noise is present across the **whole usable current range**; the sound changes with current/amperage, but it does **not** disappear
A key observation is that the **tube and PSU are always synchronized**:
* when the PSU is noisy, the tube is noisy
* when the PSU is silent, the tube is silent
I uploaded a short video here that clearly shows the **noisy state** and how the **sound changes as amperage changes**:
https://youtu.be/r6nxMVo3qdk?si=PKgK9KwMYaSIh5p5
## What I tested
* Original tube: problem present
* New replacement tube: same problem
* Another new tube: same problem
* Original PSU: same problem
* New PSU: same problem
* Tube connected directly to PSU output (anode + cathode), bypassing the machine’s internal extension wires: same problem
* HV connector replaced: no change
* HV wires removed from the machine and physically separated/isolated: no change
* Cooling tubes separated from the HV wires: no change
* Cathode wire completely replaced with a new one: no change
* FG connected directly to main chassis ground: confirmed
* Ground continuity between PSU case, chassis, signal G, and main machine ground: confirmed
* AC input to PSU: about **117V to ground on each leg** and about **201V across AC1-AC2** (machine is on **120/208V service**)
* Ruida **L** signal: about **4.52V idle**, about **0.257V during firing**
* Ruida **IN** signal: about **0V idle**, about **4.24V during firing**
* **L / IN** behavior in quiet vs noisy firing: **same in both cases**
* **P** signal: no meaningful change
* Conclusion from controller signal testing: the controller signals look normal and appear **identical in both the good and bad states**
## Important note about the PSU TEST button
I also tested the local TEST function directly on the PSU.
What I observed:
* I do **not** see the tube visibly light up
* I do **not** measure any actual laser output power at the end of the tube
* at low and high knob positions, I hear nothing unusual
* at mid-range, I hear a slight crackling/buzzing from the PSU/tube area
So in my setup, the PSU TEST function does **not** seem to create a real laser firing condition. It seems to create some internal activity or partial HV behavior, but not visible tube glow or measurable output power. Because of that, I do **not** treat the PSU TEST result as a true standalone reproduction of the main fault.
## What I think so far
At this point I feel I have mostly ruled out:
* a bad tube alone
* a bad PSU alone
* external HV connector/wire routing alone
* a simple grounding omission
* an obvious controller signal issue
My main remaining theories are:
1. **PSU instability during real firing under load**
2. **Bad PSU/tube interaction under load**
3. **Machine-level electrical issue affecting multiple PSUs**
4. Possibly some form of **internal corona / partial discharge**, although I now doubt the external HV wiring is the main cause because I replaced or isolated so much of it already
## My question
Has anyone seen a CO2 laser behave like this:
* sometimes starts stable
* then flips into a noisy unstable state after a few seconds
* once noisy, stays noisy across the full usable range
* tube and PSU always make noise together
* controller signals look normal in both the good and bad states
Any thoughts are welcome. I’m deep into this one and trying not to keep replacing parts blindly.
I’m trying to diagnose a very unusual CO2 laser problem and I’m hoping someone here has seen something similar.
The laser has **2 distinct operating states during normal firing**:
* **Stable state:** tube is quiet, PSU is quiet, and output power is good
* **Noisy/unstable state:** tube and PSU both make a high-pitched / buzzing / crackling sound at the same time, and output power drops
Important behavior:
* sometimes the laser starts stable and quiet, then after about **5 seconds** the noisy state appears
* once the noisy state starts, it **does not go away by itself** while firing continues
* I have to stop and restart
* sometimes restarting brings it back to a stable quiet state, sometimes not
* when the system is in the noisy state, the abnormal noise is present across the **whole usable current range**; the sound changes with current/amperage, but it does **not** disappear
A key observation is that the **tube and PSU are always synchronized**:
* when the PSU is noisy, the tube is noisy
* when the PSU is silent, the tube is silent
I uploaded a short video here that clearly shows the **noisy state** and how the **sound changes as amperage changes**:
https://youtu.be/r6nxMVo3qdk?si=PKgK9KwMYaSIh5p5
## What I tested
* Original tube: problem present
* New replacement tube: same problem
* Another new tube: same problem
* Original PSU: same problem
* New PSU: same problem
* Tube connected directly to PSU output (anode + cathode), bypassing the machine’s internal extension wires: same problem
* HV connector replaced: no change
* HV wires removed from the machine and physically separated/isolated: no change
* Cooling tubes separated from the HV wires: no change
* Cathode wire completely replaced with a new one: no change
* FG connected directly to main chassis ground: confirmed
* Ground continuity between PSU case, chassis, signal G, and main machine ground: confirmed
* AC input to PSU: about **117V to ground on each leg** and about **201V across AC1-AC2** (machine is on **120/208V service**)
* Ruida **L** signal: about **4.52V idle**, about **0.257V during firing**
* Ruida **IN** signal: about **0V idle**, about **4.24V during firing**
* **L / IN** behavior in quiet vs noisy firing: **same in both cases**
* **P** signal: no meaningful change
* Conclusion from controller signal testing: the controller signals look normal and appear **identical in both the good and bad states**
## Important note about the PSU TEST button
I also tested the local TEST function directly on the PSU.
What I observed:
* I do **not** see the tube visibly light up
* I do **not** measure any actual laser output power at the end of the tube
* at low and high knob positions, I hear nothing unusual
* at mid-range, I hear a slight crackling/buzzing from the PSU/tube area
So in my setup, the PSU TEST function does **not** seem to create a real laser firing condition. It seems to create some internal activity or partial HV behavior, but not visible tube glow or measurable output power. Because of that, I do **not** treat the PSU TEST result as a true standalone reproduction of the main fault.
## What I think so far
At this point I feel I have mostly ruled out:
* a bad tube alone
* a bad PSU alone
* external HV connector/wire routing alone
* a simple grounding omission
* an obvious controller signal issue
My main remaining theories are:
1. **PSU instability during real firing under load**
2. **Bad PSU/tube interaction under load**
3. **Machine-level electrical issue affecting multiple PSUs**
4. Possibly some form of **internal corona / partial discharge**, although I now doubt the external HV wiring is the main cause because I replaced or isolated so much of it already
## My question
Has anyone seen a CO2 laser behave like this:
* sometimes starts stable
* then flips into a noisy unstable state after a few seconds
* once noisy, stays noisy across the full usable range
* tube and PSU always make noise together
* controller signals look normal in both the good and bad states
Any thoughts are welcome. I’m deep into this one and trying not to keep replacing parts blindly.