Electronics Troubleshooting and Repair - Dosimetric Laser
I have repaired a couple of dosimetric lasers in the past, they are also known as top lasers. These lasers give out a set amount of IR radiation at a particular wavelength, the theory is this can decrease inflammation, pain and aid in tissue repair. The purpose of this page is to explain what I would check if I was to fix this laser or refurbish it for a longer life. I was originally going to add a unit of a similar application to this page, unfortunately I did not have the appropriated probe.
Eriel D7764 Top Laser
This is the third top laser I have acquired from this manufacturer. The very first unit was very similar to this one, I quickly found it had been pulled out of service due to a dry joint on a filter capacitor. That unit was in quite poor condition and had a lot of reworks on the inside, it was scrapped. The second unit was a very modern design and came with a lot of documentation, there were still plenty of design flaws but I was happy to sell it on, I made a decent sum of money on it.
This model is somewhere in between of the two in terms of date. From the outside it looked to be in very good condition and came with a probe, if a little dusty. I opened it up to find a very similar design to the previous two, why change what works.
The power board is the very same as that of the first unit, it had failed because of a dry solder joint. This manufacturer likes the use of linear voltage regulators and older style components, maybe they bought way too many and were using up old stock. It seems they were never happy with the design but resorted to modifying the board instead of replacing it with an updated design. A clear indication of this is the film capacitor above the black electrolytic, it was never in the original design as there is no silkscreen for it, it also doesn't fit properly.
The power board is made up of a torroidal transformer into a bridge rectifier, to a filter capacitor and then split off to two linear voltage regulators. The power switch is a dual pole which isolates the supply completely from the transformer. This switch has a metal tab on the front with the design intention of being soldered down for rigidity, not in this case. the manufacturer chose to cover it in hot glue with the intention of strengthening the switch. In the first unit this switch was completely broken, likely from being dropped on it. The complete weight of the switches action is transferred to four solder joints, to the board and then mounts. The whole flexibility in this system will lead to dry joints and premature failure.
The same large blue filter capacitor used in all the designs, thankfully it is a good brand. Checking underneath the board I could see no dry joints this time, I do see some more modifications though.
I honestly could not work out what the modification was, someone had scraped the silkscreen to solder something. I'm thinking that the potentiometer was never in the original design but because they ordered so many of these boards they instead chose to modify.
I was happy with the condition of the supply so while isolated from the rest of the unit I powered it up. The 12V regulator output just under and the 5V output 7.3V.
So what is very strange about this design is that the 5V regulators ground pin is connector to a potential divider, one component a resistor and the other a potentiometer. Turning the potentiometer will adjust the output, but because a linear regulator has been used it will keep it quite consistent. This supply is actually for the laser itself and nothing else on the unit. On the main units board is another 5V regulator taken from the 12V supply, this powers all of the microcontrollers and IC's.
Another thing I noticed on the front of the unit was a loose knob which controls the probe sensitivity. This is just a basic potentiometer with a loose nut.
Closer inspection shows that no lock washer was used which is a standard part supplied with a potentiometer. I chose to tighten this up again but the best choice would be install a washer or use some locking compound. On a separate note, here is the brain of the unit on the right. Each of these units have a specific code to allow the user to enter or disallow unauthorised people to use the unit. The chip at the centre is that which holds all the data specific to this unit, likely an EEPROM. The little note specifies the revision number and a four digit code, this code is what is used to access the unit.
The chip at the bottom is a Dallas DS1230Y which is a SRAM storage. There are two main types of memory, volatile and non-volatile, looses memory when powered down and the latter does not loose memory when powered down. So normally the fast writes such as SRAM will be volatile and the slow writes such as EEPROM and FLASH will be non-volatile. So get around this, this particular chip houses a lithium cell which keeps the data retained. The problem is that when the lithium cell has past its working life then it will loose memory when powered off. I can only speculate what the need of this chip is but it could hold the units parameters and therefore when it does fail the unit will become totally obsolete even if the chip is replaced.
Here is a closer look on the chip, here the digits "1030" can be seen. It is quite interesting to see this code has been written on the back of the unit, likely for electrical testing.
I gave all the unit a very good looking over and found only a couple of more issues. One of these is not the electrolytic capacitors, the reason is because they're all high quality named components and the working temperature is very low. The biggest concern are the flying leads and the larger components. Some really flimsy wires connect the back light on the LCD, I applied some hot glue to ensure the joints would not break off, at least not for a long long time.
I applied glue to all the wires soldered to a board and also glue to the large electrolytic filter capacitor.
I wouldn't say it necessary to glue down this bridge rectifier too, but better safe than sorry.
The unit cleaned up so good it looks new, the only visible wear is that on the laser probe.
I input the code "1030" in order to access the unit and then punched in some parameters.
The laser held against the IR self-test passed and the probe caused the machine to beep when held against the skin.
I am now happy the unit is in excellent working condition. I have rectified all the possible failure points that I can see and I'll be happy to put it for sale. There still are a couple of weak points such as the power switch on the front, the Dallas SRAM and long term the electrolytic capacitors. These are all factors out of my control and those I cannot estimate, like all equipment it can fail at any point, I am confident this is unlikely for many more years.
Hello, if you have enjoyed reading this project, have taken an interest in another or want me to progress one further then please consider donating or even sponsoring a small amount every month, for more information on why you may like to help me out then follow the sponsor link to the left. Otherwise you can donate any amount with the link below, thank you!