Odd Issue with CT-6215s, Galvo Guru Needed!

[dkumpula]

Hey folks,

I think that I have created a problem that I know neither how it was caused nor how to address it, so I'm asking for your thoughts and guidance.

Long story long, I'm building a projector for a friend (Kecked). It is a PBS combined RGB 1.1W single mode with some glow in the dark stuff added in as well. He had a set of 6215s with MicroMax 671 amps sitting in a box, so we decided to incorporate these. During initial testing, things were looking good, but the 405nm was mostly going right through the CT stock mirrors. Doh, but I've seen and fixed this before by modifying new mirrors (using stock 3mm Pangolin mirrors that I had available which reflect 405nm well.) I did this for a set of 6210s last year and ended up with a very well tuned, stable galvos after. I figured I could do this trick again, so I removed the mirrors by placing a soldering iron at 278 degrees at the point of contact of the mirror and galvo shaft adding a little pressure to the mirror until the mirror started to move, then removed heat and removed the mirror and epoxy carefully from both with a razor. During this process, I noticed that one of the Y galvo bumpers was loose. I glued it back in place after removing the old epoxy from the galvo shaft.

The mirrors that I made (a long process of slowly diamond filing down the larger set of mirrors) were nearly identical in shape (under 0.1mm), thickness and weight (within 1 or 2 micrograms) to the stock CT mirrors. I then glued them on with Gorilla brand 2 part clear epoxy. After 36 hours of curing (way longer than it should need as it was '5 minute' epoxy) I tested and noticed a distinct 'wobble' that I had not seen before. (Damn!) What was unusual is that the 'wobble' increases in waves of intensity and the time between the waves decreases as I speed up the display speed starting at 18kpps and going to 60kpps. I have seen a variety of power and signal interference issues in my ~15 years of building projectors, but I've never seen a change that varied with scan speed, so I was convinced that it was a galvo or mirror balance or mounting issue, especially as that was the only thing that changed. The tuning still looks good to me and didn't change with the other set of mirrors that I could see.

After some attempted analysis, I swapped the mirrors back to the stock CT mirrors, using high end two part epoxy (JB Weld) and had the exact same result. The thought occurred to me that the wobble may have been there previously (although I highly doubt it). These galvos have ~10 hours of use. I've attempted to remove all possibility of electrical and signal problems including:

• Swapping out the scanner PSU (I'm using one 3.2A per channel PSU from Pangolin that I am making a standard in all my builds).
• Removing power from the 12V PSU that drives the diodes and fans (I used a laser pointed mounted in place to fill in for the powered down lasers).
• Building a new signal wiring harness for X and Y along with one color channel.
• Checking all power wiring and wiring between the amps and galvos.
• Removing power from each amp separately. The wobble appears to occur on both the X and Y separately. If I run just the X galvo (Y amp power removed) there is wobble on the X axis. When I run a pattern on the Y galvo (with power removed from the X amp) there is wobble on the Y axis.
• Removing signal from each galvo separately (same results as above).
• Testing with signal ground cross connected between the amps and not connected. I also tested with the X/Y signal grounds grounded to pin 25. There was no change with any combination.
• Swapped the ILDA cable
• Tested multiple DACs
• Tested the same cables/DACs with other projectors that can do 60Kpps (both have Saturns) and the pattern is rock-solid on those.

Finally, I called Cambridge Tech customer support which was most unhelpful. First, they only service galvos that are three years or younger. These have been in a box for ~8 years. The person I spoke with seemed generally disinterested at listening to the issue and when she did, she had no ideas for a cause that matched the symptoms although she was quick to tell me that using a set of Pango mirrors (despite being nearly identical in shape and weight) was 'bad'. I didn't tell her that I made the mirrors, but only that they were the same shape and weight as the stock CT mirrors. . . my guess is she would have lost her mind. She also had no idea who could/would service the galvos and suggested googling.

So I'm stuck. The only thing I haven't done is try the stock PSUs (which put out about 4Amps per channel I believe) which I will try. I also haven't physically removed the galvos from the projector housing and tested them separately. Review of the mirror mounting looks perfect to my eye. I also haven't replaced the cables between the galvos and the amps although I think that would be a long shot as I moved the cables around and wiggled the connectors to no avail. I also have yet to pray to one or more deities for divine intervention. (Is there a god that represents mechanical/electrical repair issues??)

Here's a video of the behavior:


Can anyone suggest a path forward? A used set of these babies are around US$1500 if you can find them and a new set is $2500. Even if I damaged both galvos, it shouldn't be a complete loss, but CT didn't even offer to sell me the galvos. (Note to self: Don't buy expensive galvos from companies that have poor customer service.)

Any knowledgeable suggestions are most welcome.

-David

[rns0504]

Can you 'feel' any radial play if you grab the mirror and try and rock it forward and back / side to side? (there should be none)

[dkumpula]

Can you 'feel' any radial play if you grab the mirror and try and rock it forward and back / side to side? (there should be none)

Thanks for the reply, Ryan. I just checked and both galvos feel solid when attempting to shift them in any other direction than the normal rotation. There is no 'grittiness' in the galvos as I rotate them either, so epoxy residue falling into the bearings strikes me as unlikely as well.

Since I see this 'wobbling' on both the X and Y axis even when there is no power on one or the other amp makes me think that physical damage is less likely than an electrical issue of some sort. I've just never seen an electrical / signal issue that changed the waves of wobbling as scan speed changed. Furthermore not noticing it when doing early testing struck me as unlikely albeit I was scanning on a near wall, so the wobble wouldn't have been as pronounced as when scanning across the room.

I suppose removing the scanners and putting the original PSUs on the amps is the next thing I'll try, but otherwise I'm running out of ideas.

-David

[laserist]

Any chance the soldering iron tip isn't isolated? If there's an AC or DC leakage current it might fry both the same way...

[rns0504]

send you an email!

[dkumpula]

Thanks to those of you who reached out with ideas here and especially to those who offered to help personally. You guys are a generous, helpful, and knowledgeable community!

After eliminating all of the electronic / electric issues I could find, today I finally tried a third cable (6 feet) run with a third DAC and a laptop . . and Voila,, a rock solid test pattern. I then narrowed down the problem by swapping out the third DAC for one of the first two I tried and everything was still good.

I highly doubt it is a computer issue as all of my other projectors have been running shows without issue. My assumption at this point is that this set of CT-6215s is more sensitive to signal degradation than any other set of scanners I have used. The run between my 'server closet' in the disco and my workbench is a 75 foot cable run through my roof, but I haven't had any other scanner sets have an issue with signal degradation in my last ~12 years of custom builds. This experience includes CT-6215s with the same amps and a dozen other galvos/amp combinations.

The same test pattern using the same cable runs looked solid with the two graphics rigs I keep back there (both of which are Saturn 1b builds), so I had eliminated this as a consideration early in the testing process. I don't know why this set was flaking out, but I'm really happy that I didn't ruin a set of 6215s!

Again many thanks to all of you who took the time to read the thread and offer your assistance!

-David

[kecked]

I also thank all of you but now I’m more determined than ever to dump ct scanners forever. I had weirdness like that with a power conditioner on 6800syears back. Never figured out why.

[swamidog]

I also thank all of you but now I’m more determined than ever to dump ct scanners forever. I had weirdness like that with a power conditioner on 6800syears back. Never figured out why.

i love the 6215's in my projector... sure, i'd love saturns more, but budget.

[Pangolin]

Hi David,

After hearing back from you in email to Ryan, I think the problem must be "common mode rejection" of the 6215 servo driver. If you really want to troubleshoot it further than just knowing that, what you will need to do is trace the signal input connections where you will find four resistors of identical value. On our Mach-DSP I think we use 10K, but on the Cambridge driver they could be higher, like 100K or even 200K. Two of the four resistors could be measured in the circuit, but unfortunately you will need to desolder at least one end of the other resistors to check their value. What you will find is that at least one of the resistors is "off".

For good common mode rejection, all resistors must have exactly the same value. On the Mach-DSP we use resistors matched to within 0.1% of the value. If you find one that is off by 2%, then it would cause this problem.

There is another possibility, which is that you've not connected the servo driver as I recommend in my Projector Connections article.
http://www.pangolin.com/resguide12.htm

If you don't connect the signal input using differential signaling, then that too would cause this problem.


Bill

[dkumpula]

Thank you for this, Bill.

In regards to common mode rejection, I saw 'the wobble' on both axis. Since I removed signal inputs and power to each amp to ensure that it wasn't a single galvo or wobbly mirror causing the problem to both axis at once, doesn't this mean that a resistor would have to be out of spec in each amp? One thing I failed to mention previously is that I had (carefully) soldered position feedback onto each amp. Could that have had negative repercussions on both amps? (We're talking a fraction of a second soldering onto large-ish connection points on the amp.)

Regarding following the signaling and power connections paper you linked, I keep a copy of this paper at my work area! I've certainly seen similar wavy distortions in prior builds due to electrical or signal interference, but I've never seen the 'waves' of interference change in frequency with the scan speed. (The video I provided was inadequately clear on this.) Combined with my timing of mirror swapping, this led me to believe it was a galvo or mirror issue that I had introduced.

In any case, the signal cabling that came with the 6215s had the signal ground connected between the two amps, but I did not connect it to case ground or pin 25 initially and doing so didn't affect the wobble either way. During debugging, when I made a separate signal harness and removed power from the entire projector except for the single PSU connected to the amps, I did not connect signal ground to anything (or cross connect signal ground between the X and Y amps). Again, there was no change in 'the wobble'.

Finally, as I mentioned to Ryan, getting technical advice/support from Pangolin for a problem with a Cambridge product says something really positive about your company. Not that those of us here on PL who know Pangolin are going to be surprised, of course. Again, thanks to Ryan, you and the rest of the Pangolin team!

Now I'm going to go back and wrap up this build including putting those hand-filed mirrors back on. Hopefully nothing bad will happen this time . . .

-David