Pots for Y+ and Y- / X+ and X-

[catalanjo]

I spent a few hours driving around looking for pots and nothing. A ton of singles and some duals, but no dual linear pots anywhere that I could find.
Would this work?
$1 each from Futurlec!
Gene

Looks OK to me..... go for it!
Much cheaper than "driving around" I suspect.

Cheers

[chad]

"Looks OK to me..... go for it!
Much cheaper than "driving around" I suspect."

I totally agree those will be just fine. It might be a little harder to find a knob but should be no big deal.

chad

[Phredy1]

Finally got it built and working.

Question: When I turn down (add more resistance) either the x or y, the image gets distorted some. A little wrinkled up is how best to explain it. Before I built it I measured each side of the dual gang pots and found there to be up to 1k difference. Would single 1k pots work to fine tune each side? I'm thinking the distortion is coming from the difference in resistance between x+ and x- in each dual gang pot and 2 single pots would even it out.

[allthatwhichis]

Check your connections. DZ built me a splitter box with similar pots that had something of the same problem and I found a bad solder joint after awhile. I re soldered the connection and the issue cleared.

[dave]

You would be better off using a differential amplifier to convert to single ended, then thru the pot,and driving the scanners single ended..

Finally got it built and working.

Question: When I turn down (add more resistance) either the x or y, the image gets distorted some. A little wrinkled up is how best to explain it. Before I built it I measured each side of the dual gang pots and found there to be up to 1k difference. Would single 1k pots work to fine tune each side? I'm thinking the distortion is coming from the difference in resistance between x+ and x- in each dual gang pot and 2 single pots would even it out.

[catalanjo]

Finally got it built and working.

Question: When I turn down (add more resistance) either the x or y, the image gets distorted some. A little wrinkled up is how best to explain it. Before I built it I measured each side of the dual gang pots and found there to be up to 1k difference. Would single 1k pots work to fine tune each side? I'm thinking the distortion is coming from the difference in resistance between x+ and x- in each dual gang pot and 2 single pots would even it out.

I find the (add more resistance) comment a bit worrying here!
Each pot should be wired with input at one end and 0vdc at other end so you are never "adding resistance" you are effectively "choosing" the size of the signal which is progressively smaller as it travels through the pot resistance from max at one end to zilch at the other.

Are you sure you have them wired correctly ?

As for adding a trim pot... you can do this but if the problem is linearity not overall resistance you would have to readjust your secondary trim pot for every new position of the primary pot which is a bit of a hassle to say the least and rather defeats the idea of using double gang pots in the first place.

You can check the linearity by using a couple of batteries (less than max signal voltage obviously) where the centre 0VDC would be where the +of one battery connects to the - of the other. When outputted to the scanners if the laser waves around when you "attenuate", then the pots are NOT linear, if beam stays in same place then they are linear. If the beam slowly moves to one side it just means that the batteries are not identical voltage so if you swap them you should get the reverse movement meaning that the pots are linear.

Hope this helps!

Cheers

[Phredy1]

Are you sure you have them wired correctly ?

The wiring diagram I found online showed pin 2 and 5 as input, so I have X+ at pin 2 and X- at pin 5. Then measuring each output I found that pin 3 & 6 is normal size at full turn on the pot knob to the right (all the way up) and smaller as I turned the knob to the left (down). Using pins 1 & 4 as output was the opposite.

So, I wired it:

X+ input pin 2
X+ output pin 3
X- input pin 5
X- output pin 6

and the same for Y.

My first attempt at wiring had grounding issues, so it ended up taking the 2 pin 25 wires tied together and then attached to one of the nuts holding the DB25 connector to the metal case and that seemed to solve the problem. The pots are only grounded by the nuts holding them to the case now and that is working best I can tell.

I'm guessing my wiring is messed up with a capital F?

[catalanjo]

X+ input pin 2
X+ output pin 3
X- input pin 5
X- output pin 6

and the same for Y.

I'm guessing my wiring is messed up with a capital F?

Well sort of ........ .. or to be honest YES !

This may be just a terminology problem here!
The numbers are almost invisible on your post but blowing the pic up a bit I could see that pins 2 & 5 are both wiper (middle) pins.

These should be the OUTPUTS of the pot (but they are also equally the INPUTS to the scan amps).
Unfortunately this causes confusion ,since just about any wire is an OUTPUT from one thing but an INPUT for something else, which in turn causes the capital F occasionally when someone takes the name as gospel without thinking much about the circuit.

The end pins 3 & 6 should be the X+ and X- coming from the DAC so that the wiper (going to the scan amps), when fully clockwise, connects to these pins (= full on).
Pins 1 & 4 should be 0(zero) VDC (not ground) so that the wiper connects to them when pot is full anticlockwise (= full off) or just a point (a bit dangerous this, so perhaps your extra trim pots would serve as a preset "minimum" size limit. The zero volts should connect through them to the main pot (wired as variable resistor = just one end and the wiper).
I think you should go a bit bigger than 1k for this however (eg.10k) since otherwise min size would be less than 1% normal size) which could be dodgy for an eyeball.

That wiring you used would neatly explain the "wrinkled up deal" as you moved the wiper toward the zero volt end you were inadvertently shorting out the output from your DAC (not nice for the DAC's output op amps!). They will be much happier when they have a full 100k between them and zero volts so they don't have to bust their balls trying to maintain their output voltage with an ever increasing current drain as you turn the size down.

The idea here is not to modify the DAC signal just to choose how much of it you want to use.

I hope this explanation helps!

Cheers

[Phredy1]

Ok, where do I find a 0v wire in the 25 wires? I don't remember seeing anything like that on Pangolin's ILDA pinout for DB25 on their web site. I'm on my phone now so I can't double check now.

If I wire it correct, what adjustment will I get - X & Y size or position? If its one or the other, is there a way to be able to adjust both size and position on X & Y?

[Phredy1]

I went back to Pangolin's QM2000 pinout page and the only thing I can see that could be 0v would be either the cyan, deep blue or yellow laser 0 - 5v control wires. Just guessing that any of those would be 0v if they are not being used. Everything else seems to be used or grounded to pin 25.

My incorrect attempt at wiring was thinking reostat (variable resistor) instead of wiring it as a voltage divider (I'm guessing this is the goal).

FYI, I know just enough about electronics to be dangerous....