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Thread: Repair/adjust Tarm 2 myself?

  1. #1
    Join Date
    Nov 2019

    Question Repair/adjust Tarm 2 myself?

    I have a Tarm 2 that's probably 3 years old. I used it a couple times then left it with a friend who probably banged it around a bit, judging by the scratches on it. Got it back the other day and after firing it up I see 2 problems. I'm wondering if I have a good chance of fixing either of them on my own. I'm an electrical engineer with lots of hands on experience, so not unfamiliar with such repairs.

    1. The colors don't line up super well in the vertical axis. Projector is 15 ft from the wall in this pic.

    Click image for larger version. 

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    I've popped the cover off and it seems like I could simply loosen the screws holding the R G and B dichroic filters down and adjust their angle. I'd probably adjust just one to keep it simple. If I'm visualizing the beam correctly, adjusting the filters left right should adjust the beams up/down after it passes through the galvo mirrors and fix the issue.

    The question: is this foolish? Will I never get the beams to line up again if doing this by hand?

    2. The vertical spread is very jittery for a while after first powering the laser on. It settles out more or less but does tend to still jump around.

    See it in this video:

    Note that the camera's frame rate conflicts with the laser scan rate to make it seem like it's drawing the image slowly. That's not real, it's the vertical sizing that's bouncing around quite a bit and that's what I want to fix.

    The question: This one is more of a mystery to me. Any idea what could be causing this and what I might do to fix it? Mechanical or electrical? Replace the vertical galvo? Replace a circuit board? Send back to the manufacturer?

    Appreciate any insights from other diy folks...

  2. #2
    Join Date
    Jan 2006
    Charleston, SC


    The jitter you see in the video looks like signal interference on the Y axis. Could be a bad ground, bad cable, or a loose connection. Could also be bad position feedback coming back into the amp from the scanners.

    Couple things you can try...

    First: Swap one end of the cables that go from the amps to the scanners. (So connect the X galvo to the Y amp, and the Y galvo to the X amp.) Doesn't matter if you swap them at the scanner end or the amp end - just pick whichever location is easier to access. Then see if the jitter remains on the vertical axis after the swap or if the problem switches to the horizontal axis.

    If the problem remains on the vertical axis, then you know the problem is isolated to the Y galvo itself. In that case it's most likely an issue with the position feedback signal, although it could also be a bad bearing inside the scanner. Either way, it's not going to be easy to fix , unless you get lucky and it's just a cold-solder joint on the cable connector (see below). Repairing the position sensor is probably not an option, so in that case you'll need a new Y galvo.

    On the other hand, if the problem swaps to the horizontal axis when you swap cables then the problem is in the signal path somewhere between the controller (DAC) and the Y scanner amp. In this case you would start by trying a different ILDA cable, then working your way forward and checking for loose connections on the DB-25 connector on the projector, then look at the connections from the DB-25 to the scanner amp board. If you can, pull the signal input wires off and test each one for continuity and resistance to the DB-25 connector. (Should be less than 1 ohm.) Also look for loose ground wires going to the power input on the scanner amps. Make sure you have a solid ground at the projector and be sure all internal power supplies are grounded to the same point. If you have a ground wire connected at the position input terminals for the scanner amps - remove it, as this can cause distortion in the position input signal. As a last resort, you can look for cold-soldered joints on the Y scanner amp...

    If none of this helps, then you've got something really strange going on. But given that the problem goes away once the unit warms up, my money is on a bad connection (or even a cold-solder joint) somewhere in the input signal path to the Y scanner amp, or a similar issue at the scanner itself with the position feedback signal if the problem doesn't move to the other axis when you swap the cables.

    Note that sometimes you can pull the top off the scanner to access the PCB for the position sensor, and if you can get a tiny soldering iron tip in there you can try touching it to each of the pin connections to the cable connector (where they meet the PCB) to try to fix a cold-solder joint. Be careful you don't accidentally flow solder between two adjacent pins and short them, however! (You can also try this same trick on the under-side of the Y scanner amp where the cable connector pins are located, with the same warning about not shorting across adjacent connections.)

    The alignment issue should be easier to fix, although if the projector doesn't have adjustable mounts for the dichros then it will be a tedious and time-consuming task to get it right. (You don't want to un-bolt the mount from the baseplate unless you absolutely have to! Much better to use the adjustments on the mount, assuming they are present.) But basically your initial idea is correct: you need to re-adjust the dichros so that the beams are aligned in the near and far field. This old thread has some good information on how do this correctly. But before you do the alignment, I'd get the jitter fixed first.

    Last edited by buffo; 11-20-2019 at 04:59.

  3. #3
    Join Date
    Nov 2019


    Thanks a million for the detailed suggestions. It's been a while since I've debugged anything electronic but your ideas brought me right back to those old tricks.

    Funnily enough, when I turned the laser on this time there was zero vertical jitter. That's actually bad news, since an intermittent problem is harder to debug. If it happens next time I can start going through all the debugging options.

    As for the beam alignment, the dichro mounts are not adjustable, so I ended up just unscrewing the green and messing with it for a while until I got it about as close to correct as it's going to get. Of course tightening the mounting screw throws off the alignment, which is what I was afraid of, but some back and forth adjustments and a stroke of luck got it in the right place after a while. Definite improvement.

    There's also a tiny bit of misalignment in the X axis but the mounts are not meant to move in that direction. It's all down to the machining tolerances of the mounts and how flat the dichros are glued to them. A couple pounds force on the dichros with a pen actually flexes them enough to fix the misalignment but machining a bracket and set screw to apply force permanently is a project for a very rainy day.

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