Spectra-Physics Millennia Vs SHG tower temp regulation problem

[jesp]

I recently got a made-in-2001 Tsunami with Millennia Vs from a biology research lab. The laser was lightly used for about 4 year as a part of a microscope and left in storage for 18 years. First time power up it had a similar temperature problem: the power was close to the spec at the cold start but faded away gradually. At the power mode, diodes went to overdrive trying to compensate. Eventually, currently limit errors showed up, but the laser never gave me any error other than the current limit error. This thread was the only information I found that gave me some guidance on what to do, so I would like share my experience here.

I guessed the problem was a temperature/cooling problem in the laser head, because the two diodes still have plenty of life left and the laser could always "recover" if I shut it completely down overnight. So, I started with cleaning the chilling water circuit as suggested. The laser was not maintained properly before. The chiller's water filter was black. Tried the recommended Nalco cleaner first. After two rounds, each time one week of circulation and charge filter, there is not significant improvement. Then I switched to an acidic laser chiller cleaner, Citranox. Again twice, each time one day of circulation. By the end of second round, the cleaner solution was finally free of floating debris. The chiller was finally clean but the problem still remained.

At this point I felt I might also had a bad circuit board, so I took the laser head apart. The board in my laser was visually clean and looked OK. It looked like a more refined version compared to iggythebrush's board. There was no soldered wires. Beside the 4-pin heater/thermistor wire bundle to the SHG oven, my board had a second 4-pin heater/thermistor wire bundle on the other edge on the board. The second thermistor was connected to the base of Nd:YO4 tower, but the heated was sticked on the outside of aluminum laser cavity casing. I was bewilddled by the position of the second heater. The laser casing is not in direct contact with the Nd:YO4 tower. The location of the heater is furthest away from the tower/the location of the thermistor. The thermistor reading is displaced on the laser controller info panel as "TWR", but the status of the second heater is not shown anywhere. I came to the conclusion that the second heater is an open loop element. Its only purpose, perhaps, is to heat up the entire cavity during startup so that the laser cavity reach thermal equilibrium faster.

Then I recalled that my laser got hotter and hotter as its power dropped. By the end of day the cavity was often too hot to touch. On a hunch, I disconnected the second heater and put everything else back. It fixed the problem! Now the laser takes about 30 minutes to warm up and reach the spec power, but after that it can hold the power. I think in my laser the second heater failed to switch off and was baking the laser cavity. I don't plan to fix the heater or its control as 30 minuted additional warm up is not big deal at all.

[kecked]

I recently got a made-in-2001 Tsunami with Millennia Vs from a biology research lab. The laser was lightly used for about 4 year as a part of a microscope and left in storage for 18 years. First time power up it had a similar temperature problem: the power was close to the spec at the cold start but faded away gradually. At the power mode, diodes went to overdrive trying to compensate. Eventually, currently limit errors showed up, but the laser never gave me any error other than the current limit error. This thread was the only information I found that gave me some guidance on what to do, so I would like share my experience here.

I guessed the problem was a temperature/cooling problem in the laser head, because the two diodes still have plenty of life left and the laser could always "recover" if I shut it completely down overnight. So, I started with cleaning the chilling water circuit as suggested. The laser was not maintained properly before. The chiller's water filter was black. Tried the recommended Nalco cleaner first. After two rounds, each time one week of circulation and charge filter, there is not significant improvement. Then I switched to an acidic laser chiller cleaner, Citranox. Again twice, each time one day of circulation. By the end of second round, the cleaner solution was finally free of floating debris. The chiller was finally clean but the problem still remained.

At this point I felt I might also had a bad circuit board, so I took the laser head apart. The board in my laser was visually clean and looked OK. It looked like a more refined version compared to iggythebrush's board. There was no soldered wires. Beside the 4-pin heater/thermistor wire bundle to the SHG oven, my board had a second 4-pin heater/thermistor wire bundle on the other edge on the board. The second thermistor was connected to the base of Nd:YO4 tower, but the heated was sticked on the outside of aluminum laser cavity casing. I was bewilddled by the position of the second heater. The laser casing is not in direct contact with the Nd:YO4 tower. The location of the heater is furthest away from the tower/the location of the thermistor. The thermistor reading is displaced on the laser controller info panel as "TWR", but the status of the second heater is not shown anywhere. I came to the conclusion that the second heater is an open loop element. Its only purpose, perhaps, is to heat up the entire cavity during startup so that the laser cavity reach thermal equilibrium faster.

Then I recalled that my laser got hotter and hotter as its power dropped. By the end of day the cavity was often too hot to touch. On a hunch, I disconnected the second heater and put everything else back. It fixed the problem! Now the laser takes about 30 minutes to warm up and reach the spec power, but after that it can hold the power. I think in my laser the second heater failed to switch off and was baking the laser cavity. I don't plan to fix the heater or its control as 30 minuted additional warm up is not big deal at all.

wonder if you can just put a switch on it.

[jesp]

wonder if you can just put a switch on it.

Yes I could. I am thinking about putting an LED on it to confirm my guess.