Troubleshooting Laser Diode (From Color LaserJet)

[made2hack]

Hello all,

I am trying to troubleshoot the laser scanner assembly on my laser jet. The fault codes indicate a problem with the laser assembly (more specifically the Yellow diode - Code 51.23). I wanted to try to probe the diodes and see if they work.

How would I do this? Can I simply probe with my multi-meter on diode check? What output should I expect? I mean, I take it it is invisible light. Should it "burn" a piece of paper? I have no specs on the diodes in use.

Also, what probe points should I use? I take it LD is Laser Diode, COM is GND but what about PD?

I've attached a picture of the board.

Thanks in advance,

[tomato]

I take it LD is Laser Diode, COM is GND but what about PD?

Photodiode?

[made2hack]

I guess, but how should I try to probe?

[Kleinstein]

The 3 pin laser diodes have an internal photodiode.  One could do the normal diode test, but this would not tell much, as a broken laser diode is likely still a working LED, just a little dimmer.

Laser diodes are also very ESD sensitive - about as bad as small unprotected MOSFETs. So un-soldering them is not a good idea.  Even the DMM in circuit can be a problem.

The next problem is that the optics needs to be aligned.

So I am afraid, that the laser diode removed from the board essentially means it is beyond a realistic repair.

The realistic check would be to reverse engineer the driving circuit part and than compare the driving current and PD current from a working on the problem channel.

[made2hack]

Hi Kleinstein,
Thanks for your input. So the photodiode is there to excite the laser? To get it to "lase" ?
This was more a trip down a rabbit hole until a replacement laser scanner arrives.
As for alignment, they are still in the original factory holder. The PCB you see was soldered in place after the diodes were fixed.

For anyone interested, I've attached a generic schematic of other laser diodes.

[tomato]

Hi Kleinstein,
Thanks for your input. So the photodiode is there to excite the laser? To get it to "lase" ?

No, the photodiode is just used to monitor the output.