Help needed with 506 scanners and modified DT40 AMP

[Fearless]

Position scale is turned fully clockwise as per Bill instructions. No oscilloscope. The DC offset was set with the DAC unconnected, very close to 0 millivolts, although when the DAC was disconnected, I got a strange X on the wall. When I connected the DAC the X went away, and I got a nice dot. I do not know what you mean by single or double ended input--there is only one input to the DAC. I did all the modifications on the DT40 board, with the exception of the parallel 470pf cap near the top of the board closest to the heatsink. That cap was so tiny that I could not place the thing--it kept sticking to the soldering iron. When I turn down the scanning speed, say to 20K, everything is fine, and I can move the image around and expand it in software--it's only when it is running at 30K that I run into problems with the scanner tripping. Another thing I found strange is the scanner galvo seems to prefer one stop over the other--I initially left the stops off the scanner so it could find it's preferred position--I then adjust the mirrors to the preferred position. 11.5 hours? You are not up my way in MA by any chance?

[mixedgas]

No where near MA, but I know a PLer that is, and he has a design lab with scopes.

Ok, differential means the input wiring to the amp from the ILDA connector uses both the normal and inverted signals for X and Y to drive the amp. Thus you have three wires to each amp input, X and X prime and Ground. Then Y and Y prime and ground. Differential is used to get the full drive for calibration.

I've found the 470 pF cap to be a critical component for stable amps.

OK, I'm 750 miles from my bench. We need to measure your position sensor "Volts per Degree". It could be done with a good DC voltmeter. I haven't measured it for my amps yet. I have not needed to.


Ok, This may/ will make your DT40 amp unstable and you may loose your tuning. Set up a small image, say the test grid, at 10 degrees or less of angle. While I never recommend changing position scale without an angle test rig, you should try it. Turn position scale about three full turns and see if the image size increases. If it increased, you should be at the correct end of the potentiometer travel. If it decreased, something is very wrong.

NOTE TO OTHERS READING THIS POST: Normal users should NEVER touch position scale after the galvo factory
has set the potentiometer. Doing so may cause you to loose your tuning and/or cause severe oscillation with certain images.

Steve

[Fearless]

Thanks Steve, I'll try paralleling the 470 pf cap tomorrow and try the position scale test.

[Fearless]

Well, paralleling the caps did absolutely nothing, in fact it made it worse. The position scale pot does work in enlarging the image in both the X and Y plane. I eventually got the amps somewhat stable by zeroing the input offset with the galvos attached and the DAC attached but producing no output--what that did was reduce the oscillation with the X and Y galvos to an acceptable level. The image still does dance around a bit, but it will not trip the AMPs. I triple checked all the modification on both boards, and the resistive values are correct. I really wished I had got one of Steve's modified amps. After New Years, I'll send the galvos and amps to Steve or Pangolin to see if they can create a stable platform--or at least tell me what I did wrong with the modifications.

[lasermaster1977]

NOTE TO OTHERS READING THIS POST: Normal users should NEVER touch position scale after the galvo factory
has set the potentiometer. Doing so may cause you to loose your tuning and/or cause severe oscillation with certain images.

I'm seriously considering buying a pair of 506's and I understand that the DT40 is one of the compatible scanner drivers with Pangolin's recommended modifications. That said, am I correct in understanding the context of Steve's (mixedgas) comment above applies only when the driver and galvo are sold together?

My own research brought me to the conclusion that for the 506 scanner pair the DT40 seems like a decent choice for a driver to be mod'ed, but I am very interested in the experience and wisdom of those on this forum.

Being a newbee on this forum I'm not yet familiar with those of you who may have products to sell. I just noticed a posting where mixedgas and others refer to his red boards or amps.

Thanks in advance.

[mixedgas]

Position scale needs to be set EXACTLY according to the 506 PDF modification instructions for the DT amp. No test rig is needed. Then it should never be touched.

For any other scanner amp, position scale is matched to the Galvo using a angle test jig and should never be touched. The amp is designed so Galvos are reasonably interchangeable once scale is set for a given model number of Galvo. However they do not interchange from manufacturer to manufacturer.

My warning is simple, once configured, POS SCALE turned more then a small amount will change the feedback loop characteristics of the amp. A possible need for a retune or in extreme cases servo loop oscillation will result.

ScannerMax 506's position sensor has a much greater output then its competitors, and for good reasons outside the scope of this post.

SIMPLIFIED EXPLANATION:

Position Scale calibrates the Volts per Degree of Deflection for the position sensor. That turns into Volts per Degree Per Second (Slew Rate) when the Galvo moves. If the slew rate is wrong, the sensor can be viewed misrepresenting the Galvo shaft position with respect to other adjustable time constants in the amplifier. That leads to interesting behavior. If too low the Galvo is sluggish. If too high, the system can oscillate. There is a small band of adjustment where the feedback loop works as designed.

So the first thing calibrated at manufacture or when high precision is needed is the Position Scale. It is then left alone. By convention, no one touches it.

Steve

[Fearless]

I got my 506's working with the modified DT-40 amp. I still have a minor oscillation when everything is hooked up including the dac. The Position Scale control is turned all the way clockwise, and Input Gain control turned all the way counter-clockwise. I found the most critical thing is making sure the Input Offset is set closest to zero as possible, with everything hooked up. I'm still trying to get them stable--certain wide angle beams will trip the scanners. If you do not mind the time invested and the frustration involved in trying to tune them yourself then 506's are worth it.