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Thread: AL15/AL20K + Adapting galvos to work with another scanner amplifier?

  1. #1
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    Default AL15/AL20K + Adapting galvos to work with another scanner amplifier?

    Hey guys,

    I have a set of AL15k and AL20k by goldenstar that I completely reverse engineered 1 year ago or so when I never was able to get the information of the manufacturer through goldenstar.
    Both sets are practically the same minus the amp. (The High frequency damping is missing on the AL15k and you need to wire the ilda X+,X-,Y+,Y- to the header pins). I bought these scanners because they were cheap for practicing tuning and just fiddling around and practicing tuning (I can tune really fast now and really enjoy it).

    When I figured out the potmeters on my own I retuned the AL20k to scan 30kpps at 8 degrees from memory without any issues really.. The AL15k despite having the same galvo' s couldn't be tuned up properly because of the missing HFD.. it can be added though. These scanners can work on as little as 12volts differential and scan just fine with minimal heatsinking but will require some heatsinking for the amplifiers. These scanners benefit more if you provide them with a higher differential voltage to obtain a higher gain.. The weakpoint of these scanners are vibration caused by their thin mirrors but benefit from having a low inertia.
    I have done multiple experiments with these and both are in tact and they were very educational for experimenting.

    The AL15k costs $50 ex shipping making it one of the cheapest galvo set out there.
    I recommend the AL20k at the moment if you want to practice tuning as it's really fun and not as scary and hard as people made it seem those years ago (I can tune without having to crank back to 0)

    Before I start my question I wanted to share my efforts with everyone here:
    AL20k:
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    AL15k:
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    note on image 1: Installing new pots did not provide HFD.. the HFD circuit was not present in this amplifier.

    Galvos (on both the same):
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    AL20k Scanner in action: laser beam fed from another projector: https://www.youtube.com/watch?v=uRvs-p1L51M
    I dont think I still have the ilda test pattern pics.. It' s been a while I will look if I have them still.

    So now here is my question: Is it possible to modify a galvo to work with a amplifier not designed for it?
    Say making this galvo work with a scanner amplifier designed for the Cambridge 6800 galvos... The pinouts are different and it seems like some ground lanes are merged into one lane to safe cost (running along the diodes ground pin?). I hear the IR diode for the cambridge 6800 consumes 12 volts?? that' s a lot compared to ~1.15volts for this.
    Last edited by masterpj; 11-29-2014 at 15:59. Reason: more galvo pics

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    I am sure you could make these work on camb. amps. Note: the IR diodes need a current source not voltage source. It should be possible to add HF damping to your amps. You could add a sense resistor, opamp, pot and a few passive parts on a piece of prototype board. Find the input to the power amp on your boards and "inject" the HF damping there. The circuit is very simple.

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    Quote Originally Posted by logsquared View Post
    I am sure you could make these work on camb. amps. Note: the IR diodes need a current source not voltage source. It should be possible to add HF damping to your amps. You could add a sense resistor, opamp, pot and a few passive parts on a piece of prototype board. Find the input to the power amp on your boards and "inject" the HF damping there. The circuit is very simple.
    No need to add HFD the AL20k board had it and I have the original cambridge schematic which showed the HFD circuit. It should be similar to what was done on that spencers projector thread.
    Thanks though this is a very usefull note to take!

    I wanted to share it because if someone wants to buy these because they are cheap a retune will be adviced.. they are badly tuned and can perform much better but the manufacturer wont respond and thus as an educational experience I went out figuring it out on my own.

    What do you mean with the current source? Are they like laser diodes where they only pull as much voltage as then need and as much current as they can?
    Would the difference in resistance of the coil compared to real cambridge scanners not possibly damage the camb amps?

    I don't really care if the galvos break that much but I do if the amplifier damages.

    My only problem is to figure out which sensor is the A and which one is the B. and which drive coil is positive and negative
    Putting a voltage on the coil wires with a small D-cell flops the mirror one direction depending on polarity.. figuring out the diode polarity wasn't hard either.

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    I don't remember what the 6800 coil resistance is. You scanner is probably very close. You wont damage the amp. If you hook up the coil backwards the motor should just slam the stop and blow the fuse. You should start with the servo gain all the way down. This should keep the motor from a violent reaction. If you have the sensor A and B switched the motor should be forced to the stop also. These sensors control low freq damping. So, make sure it is turned down too. I think the cambridge motor specs designate what direction the rotor is supposed to move with polarity.

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    I was thinking... You can figure out the A and B. Use the D battery and note the direction or rotation then hook the motor back to the original amp with the scanner fuse removed. Next, turn the shaft in the same direction as before. Measure the voltage put out by both sensors. The one with the highest voltage should be A. This assumes the sensors are used differential like the camtech motor. I can't see a reason why they wouldn't be.

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    Quote Originally Posted by logsquared View Post
    I was thinking... You can figure out the A and B. Use the D battery and note the direction or rotation then hook the motor back to the original amp with the scanner fuse removed. Next, turn the shaft in the same direction as before. Measure the voltage put out by both sensors. The one with the highest voltage should be A. This assumes the sensors are used differential like the camtech motor. I can't see a reason why they wouldn't be.
    Thanks! I will do this I personally wanted to use the original amps (AL15k) to figure this out by removing the wires from the coil drive however I'm worried disconnecting the drive on the coil while keeping the rest connected will damage the amps but that way I wouldn't have to find a proper current source to power the IR led *yes I should invest in a adjustable bench power supply someday but the cheap chinese ones suffer from nasty spikes*

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    All you really need for the IR LED is a resistor to limit the current. I can't remember the 6800 LED current. I think its something like 15mA. IIRC its regulated by an AGC circuit.

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    Quote Originally Posted by logsquared View Post
    All you really need for the IR LED is a resistor to limit the current. I can't remember the 6800 LED current. I think its something like 15mA. IIRC its regulated by an AGC circuit.
    Thanks. Here it's about 9.250ma it took a while to climb to that exact value *it was in 0.001 increments).
    Will look if thats the current provided to the 6800 is supposed to be 15ma. If so: soldering a resistor in between will do the trick.

    No fuse on the cambridge amp.
    It's a hybrid G120/cambridge amp.
    Apparently the manual also states a adapter kit is needed for it to work with the 6800.
    Only the pinout of the G120 is listed not the 6800 !
    The 6800 is a 10 pin connector so 5 pins at least are not utilised.
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    If I can get the adapter somehow then with the original cambridge 6800 pinout I can figure out which has to go where on my own soldered 10pin cable for these galvos.
    Last edited by masterpj; 11-29-2014 at 21:48.

  9. #9
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    Logsquared has the basic steps right, but it needs some refinement.

    One thing to be careful of is AGC circuit on Good AMPS (even cheap ones) is not a fixed LED current. It corrects your linearity. Feedback from both sides of the position sensor is used to shape the led current in real time. Not doing that on a complex amp will lead to image distortion at angles greater then say 5' AGC circuit is a active current source, it is not always as simple as "soldering a resistor in"

    Correct method is to use an oscilloscope (worse case multimeter) to check A-B off a test point with the output fuse lifted. Otherwise the galvo will slam to one side and cook if your wrong on powerup.

    A D battery may source too much current and hang the coil on Cambridge clones. Use a LM317 based current source or current limited bench supply to source no more then 100 mA into the Galvo.

    On a class Zero Amp, servo gain all the way down may work. On a class I amp, that is not always the case. Best to be sure before stressing galvos which do not have internal stops. 6800 clone does not have internal stops, you can rip the flag off the sensor or cook the coil if your not careful. The coil is about .002" from the rotor. If you overheat the coil it expands, melts its glue, and drags on the rotor.

    One other thing, some of the clones have the LED polarity reversed. Wrecks havoc when they have it tied to "galvo shield".

    Steve
    Last edited by mixedgas; 11-30-2014 at 05:41.
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    Mother always said I could use some "refinement" HAHAHA

    Seriously though.... I think he said the coil was around 8ohms. 1.5V /8 =187mA. Is that really too much for the china motors if just "tapped" on the cell?

    My older 6800's had the fuse on the motor. Probably why the amp doesn't have a fuse. Put one in series!

    Steve's right these things are pretty delicate. If it were me I would use the lowest possible supply rails and fit a 100ohm resistor in series with the fuse and coil. This will help limit the current available to the motor.

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