RGB Laser Power Board Issue

[partyanimallighting]

Hi all, I have a 1/2 watt RGB laser that was purchased quite a few years ago and it was DOA. Because of this, and a lack of spare time to fiddle, it's been packed up in it's flight case all this time and I basically was using the flight case as a trolley/workbench for other units being repaired.
Anyway......I decided to take a stab at it to see if it could be repaired. The layout is pretty basic, there's a rear panel PCB that handles POWER IN, RED, GREEN and BLUE INTENSITY, X x Y SCANNING WIDTH, ILDA INPUT, KEY LOCK and INTERLOCK. This PCB receives 24VDC from one of three AC/DC power supplies, another outputs 5VDC to each of the laser drivers and the last outputs 18VDC to the X / Y scanner motors. All the power supplies are functional but....the 5VDC power supply is not outputting sufficient voltage to the diodes. When it's connected directly to 120VAC, it powers up fine, but it's 120VAC input if fed from a small voltage control PCB that is connected to the rear panel PCB.
Here's the thing, when the 5VDC power supply is NOT connected to this control PCB, the 120VAC output from the PCB is fine. Once the power supply is connected to the control PCB, the voltage drops down to 1.5VAC so the power supply does not power up the diodes. I gave the control PCB a basic look through and I'm not seeing anything untoward with my untrained eyes. I also removed the relay that controls the 120VAC feed and tested this off the PCB and it's fine, clicking and continuity is there. i then connected the 5VDC power supply directly to a separate feed and all the diode PCB's are receiving proper voltage so it's definitely an issue with either the voltage control PCB or the rear panel PCB that feeds 24VDC to this control PCB.
The RED diode is also working fine but the GREEN and BLUE are not functioning so I swapped out connectors on the driver PCB's and the RED diode is working fine on all of the other PCB's so I'm assuming that the GREEN and BLUE diodes have died so I was also wondering if laser diodes will naturally fade or die over time, even if not in use.
Here are some pix. One of them has the relay removed. I'd really appreciate some assistance with this!!

 

[kellys_eye]

All the AC outputs are paralleled so if 'one' output (the 5V DC power supply feed) drops to 1.5VAC, why don't they ALL? Maybe they do - you didn't say.....

It can point to a poor connection between the plug/socket leading to the 5V DC power pack (resolder all pcb joints and test reliability of plug/wires) or, if ALL outputs drop to 1.5VAC then the relay contacts are (perhaps) failing under load which seems to be the real reason.

 

[kellys_eye]

I'm assuming that the GREEN and BLUE diodes have died

Sounds logical - they are 'only' diodes so can be tested like any other LED using (most) test meters - note, some diode test meters don't have sufficient Vf to show the higher volt drops across green/blue diodes.

 

[partyanimallighting]

Hi kellys_eye, all the outputs are paralleled so I didn't check the other outputs because, although there are three outputs, only one is used so I will test that later but logic dictates that I should get 1.5V on all. I'll resolder all the traces before I test also. I checked all the harnesses beforehand to the power supply and they are fine, continuity is OK. Without the load applied from the 5VDC power supply, the output was 120VAC but once the power supply was connected, it dropped down to 1.5VAC. I'll check and update the thread later. Also remember that I did not test the GREEN or BLUE diodes with a meter but with the functional output from the RED diode driver PCB and got no output so I'm guessing "definitely dead" but would they deteriorate and fail just from lack of use? Remember this is a new DOA unit so I found it strange that two out of three diodes are dead.

 

[kellys_eye]

No, they dont deteriorate when unused. Voltage can't 'disappear' without going somewhere. If there was a fault (as such) the fuse would blow. Since the voltage I just dropping it's indicative of a poor connection i.e the relay contacts.

 

[partyanimallighting]

OK, did some further testing. There's a definite drop across all the outputs once the power supply is connected, even after I resoldered all the traces and checked everything for continuity and everything's fine on that side. I have the original PCB's and two replacements that the supplier sent out years ago when I first encountered the problem so I'll do some further testing. I'll also disconnect all the diode driver PCB's that are receiving 5VDC from the power supply and test them one by one to see if there is still a voltage drop and I'll update the thread later.

 

[partyanimallighting]

OK. latest update. I tried one of the replacement rear panel PCB's with the voltage control PCB and the problem is still there. So I did some digging and found a similar relay in a parts bin and changed it out and re-tested the unit. It powered up with no voltage to the 5VDC power supply then, after about 15 seconds, the control PCB acted as it should and the relay sent the 120VAC to the power supply so that problem's solved, thanks to your assistance in pointing me in the right direction. There's only one more problem left. The non-functional BLUE and GREEN diodes. From what I've figured out from looking at the unit, the diodes are powered by 5VDC and are simply switched on and off via DMX control. It's the scanning mirrors that produce all the patterns and beams via X Y scanning. I've attached pictures of the diodes and I'm wondering if there is any way I can check them directly via a 5VDC feed to be sure that they're really dead of if it's a driver PCB problem. There are cooling fans built into the rear of each of the diode housings and these probably run on 24VDC. Any help will be welcome.

 

[kellys_eye]

Measure the current to the LEDs - DC clamp ammeter would be easiest but whatever you have that can do it will suffice. Note that there can be 10A+ (20A? 30A?) so make sure your meter is up to it. Use the working Red LED as a convenient comparison.

You said you'd put the Red LED on the Blue and Green drivers and the Red LED still works thereby proving the drivers are ok so it's very likely the Green and Blue diodes are shot.

 

[partyanimallighting]

Yup!! These diodes are dead. I am getting new diodes to replace these burnt out diodes but these new diodes come with a separate driver PCB and they operate on 12VDC so I just want to confirm one thing. The existing power feed to the original diodes is via one 5VDC P/S so all I will have to do to power up these new diodes and drivers is replace the existing 5VDC P/S with a 12VDC P//S. Do you foresee any problems with this?

 

[kellys_eye]

None at all - just ensure the PSU can deliver the required current AND have some overhead (say 50%).

 

[partyanimallighting]

Of course I'll use a large enough power supply. However.....each individual diode driver PCB is switched on and off via outputs on the main control PCB (Picture 1, three small terminals on the bottom in a row). These three terminals run to each driver PCB (Red, Green, Blue diode drivers) and connect via the yellow/black wire harness (top of Picture 7) and white/black wire (top of picture 8) and one other (no picture). I have attached a picture of the new driver PCB's and I would think that one of the terminals on the new PCB's have the same function. I'm not sure so I stopped by on the forum to get some feedback.

 

[kellys_eye]

Do you know if the new device is 'active low'?

Simple enough to measure the potential on the old wires to see what you get. Worst case is you'll have to 'invert' the signal (and maybe clamp the level if it exceeds TTL voltages), in which case use the appropriate inverter-driver chip (LS04?) and some zeners.

 

[partyanimallighting]

I will ask the supplier. I can't do any testing right away as the unit is packed away in pieces back into it's flight case so I'll try to make some time, reassemble it and take some voltage readings off of those terminals on the main PCB. That's the right track, correct?

 

[kellys_eye]

One terminal may be 5V, the other ground - in which case it will reflect the 'pull low' requirement to switch it on (i.e. short the pins). Easy to check and the most likely of designs.

 

[partyanimallighting]

OK. I'll check tomorrow and post an update. Thanks!!