How to repair a CX-75 LightPen?

[luckybuck]

Hi together!

 

Someone gifted me a CX-75 LightPen, please see the picture attached.

We have tested the pen at the NOMAM meeting, but it seems to be out of order. :-(

Does anyone has ever repaired an Atari LightPen?

 

Thank you so much in advance.

 

All the best.

Edited April 18, 2019 by luckybuck

[Mathy]

Hello guys

 

I sat next to luckybuck (and Cash) when we tested luckybuck's light pen. "Horizontal" worked just fine, but "vertical" didn't (or was that the other way around?). We tried two different computers (IIRC an 800XL and an 130XE) and two different CRT monitors.

 

We also tried a light gun, it worked as expected.

 

Sincerely

 

Mathy

 

PS I was under the impression that the light pen is basically a light sensor and a switch. GTIA does all the counting, comparing and registering. So how come one direction works and the other doesn't?

Edited April 18, 2019 by Mathy

[luckybuck]

Thanks Mathy for the update. Interesting, I found out, that the Atari 400 had problems with the light pen... But we have tested all followers.

 

Found here a picture on how to disassemble the pen:

http://atariage.com/forums/topic/278690-i-think-this-is-fixable-atari-lightpen-cx75/?do=findComment&comment=4026041

 

But still no solution. :-(

[+CharlieChaplin]

Thanks Mathy for the update. Interesting, I found out, that the Atari 400 had problems with the light pen... But we have tested all followers.

 

Found here a picture on how to disassemble the pen:

http://atariage.com/forums/topic/278690-i-think-this-is-fixable-atari-lightpen-cx75/?do=findComment&comment=4026041

 

But still no solution. :-(

 

From the A8FAQ:

 

Subject: 6.5) What light pens were produced for the Atari?

A light pen or light gun requires a cathode ray tube (CRT) television ormonitor.  Only one light pen or light gun may be used on the Atari at a time.Controller Jacks 1-3 on the 400 do not support a light pen / light gun.

In other words, lightgun and lightpen only work on port 4 of the Atari 400 and therefore most software will not work with it at all (unless you patch it)...

Edited April 19, 2019 by CharlieChaplin

[luckybuck]

Thank you CharlieChaplin, we tested with a crt TV and an 800 XL and a 130XE. One axis worked, the other sadly not. There may be a need for disassembling the pen...

[Rybags]

I would expect vertical should be much less of a problem since a hires pixel distance is ~ 1/16,000th of a second.

Whereas horizontally a pixel is about 1/7,200,000th of a second - though of course all that said the provided resolution is 2 scanlines and 1 colour-clock.

[Gunstar]

I have come across one or two articles lately (last few weeks) while reading through old Atari books and mags that tell how to build your own light pen. I will see if I can find them again. I'd think these would shed some light on how to repair the Atari one.

[luckybuck]

Thank you all.

 

Have found:

https://www.atariarchives.org/creativeatari/Build_Your_Own_Light_Pen.php

http://atariage.com/forums/topic/278690-i-think-this-is-fixable-atari-lightpen-cx75/?hl=light+pen&do=findComment&comment=4026063

https://www.atariarchives.org/ecp/chapter_6.php

 

But not how to disassemble the CX-75. I mean the take away part by part instruction. So just the damaged parts can be replaced.

[Mclaneinc]

Thankfully lightpens are very very simple, we did one as a kit for the Atari, needless to say it wasn't very good but it worked if the tv was at the right brightness and contrast..The photo diode should be available still in some form, will be probably be normal led size though ie not mini led which might make funding a replacement a little hard..

[luckybuck]

Thank you Mclaneinc, yes, that is a good point, altering brightness and contrast plus Rybags' tipp. We should give it a try at the next meeting. Maybe, the pen is very sensible in this way. Great help, thank you all. :-)

 

BTW, did anyone discovered the same, as we had found out, with his own light pen? Meaning, one axis working and the other not?

Edited April 19, 2019 by luckybuck

[Rybags]

Possibly it uses some intermediate lensing and the plastic has deteriorated?

[luckybuck]

Yeah! Good point Raybags, will check this. Thank you. :-)

[BigO]

I'm curiously following this just because it runs contrary to the little knowledge I have of light pens and I expect that I'll learn more from the ultimate fix of this problem.

 

As I understand it, the sensor in a light pen detects light of a given brightness or it does not. Only the driving host device knows how to interpret the position based on the timing of the light detection. Only the pixel being currently drawn should be bright enough to trigger the sensor. The light gun itself shouldn't have any clue where the pixel is that it just detected.

 

So, I'm really curious how the light pen itself could be responsible for making one axis function but not the other. Is not the sensor itself is a single element, one-dimensional device?

 

What was the failure mode? Did it appear to detect the exact same position all the time on the broken axis? Or did it appear to detect light at every position on the broken axis? Or did it appear to just be off by a few rows/columns in the broken axis? Would the mismatch in position change if you rotate the barrel of the light pen as though the input optics were not reading directly out in front of the sensor?

 

Maybe the polling logic actually somehow/somewhy (<< should be a word) reads the sensor twice to determine X and Y position and a pulse for one of those reads is too short to activate a weak, failing component? I'm really just making stuff up here but am interested in seeing what the real reason turns out to be.

[BigO]

You might also "fix" it by buying one for $7.95 on eBay.
https://www.ebay.com/itm/Atari-CX-75-Light-Pen-800-XL-XE-No-Software-no-Manual/132876977520?hash=item1ef015c570:g:XtsAAOSwa1ZcBgjD

[Rybags]

It detects light - but is more sophisticated than the old light guns of the late Pong clone era - those things generally reacted to any light source sufficiently bright.

 

The (later) light pen and light gun depends on the transition from dark to light then back to dark. Actually this is looked after by Antic.

If you plug a joystick into the correct port and monitor the PENH/PENV registers you can see what happens by pressing the stick button. Either transition type latches the position but of course in reality you'd be getting a reading from the light to dark one.

 

There's usually a bit of lag in the system (esp horizontally) which is why lots of software gives you the calibration option.

 

The light pen itself - very simple in that all that's needed is the photo sensor and (possibly) lensing and a barrel to keep out peripheral light. And of course the button to signal when it's drawing or shooting.

[luckybuck]

Thanks BigO,

'Is not the sensor itself is a single element, one-dimensional device?' I assume so, that is why I asked for a take away part guide.

'What was the failure mode?' We tested many different light pen programs, including Atari Graphics cart and that SALT 2.07 cart. All have that problem.

'Did it appear to detect the exact same position all the time on the broken axis?' Yes!!! Totally forgot, thank you so much, it was frozen in the lower left corner! Good point!

Have made a photo of the light pen head, please see below.

[luckybuck]

@BigO: $24.2 plus customs plus tax for overseas... will result in something like > $30... But the price is not the problem, but they why, especially for others. For the computers we have:

Technical Reference Notes or Manuals, else Hardware Manual. But not for the light pen. :-( Would love to add this for the Wiki.

[BigO]

@BigO: $24.2 plus customs plus tax for overseas... will result in something like > $30... But the price is not the problem, but they why, especially for others. For the computers we have:

Technical Reference Notes or Manuals, else Hardware Manual. But not for the light pen. :-( Would love to add this for the Wiki.

Understood. If I had a use for a lightpen, I'd probably pick up that eBay one since I'm in the US. But, I wouldn't pay $30 either.

 

I was searching around a little and found almost no useful information on the CX-75, so I respect what you're doing in terms of discovering and documenting. Too bad you can't pick up a working version for reference, though.

 

I assume someone can or has disassembled/decompiled software to see exactly how it's reading from the pen in case there are any clues there.

 

Good luck with your efforts.

[luckybuck]

Thank you BigO, you know, I never give up and I never will surrender... So, it is just a question of time. Happy Easter.

[Irgendwer]

...

We also tried a light gun, it worked as expected.

...

 

 

That is suspicious. How did you test the light pen? With a light gun game? I guess you had a horizontal problem then?

While quite similar, light gun and pen have two differences:

* The trigger pin/signal is different and

* the light gun reports the beam hit with a delay while the pen does so immediately.

 

So you aren't able to play/test a light-gun game with a light pen...

Edited April 19, 2019 by Irgendwer

[BigO]

Thanks BigO,

'Is not the sensor itself is a single element, one-dimensional device?' I assume so, that is why I asked for a take away part guide.

'What was the failure mode?' We tested many different light pen programs, including Atari Graphics cart and that SALT 2.07 cart. All have that problem.

'Did it appear to detect the exact same position all the time on the broken axis?' Yes!!! Totally forgot, thank you so much, it was frozen in the lower left corner! Good point!

Have made a photo of the light pen head, please see below.

Hmm...frozen in the lower left corner does not equate in my mind to working in one axis.

 

That sounds like working in no axes, possibly not detecting the light at all. Which would lend credence to the theory about the brightness of the screen. Or maybe the sensor became less sensitive due to optics or electronics. Maybe the sensor failed entirely.

 

If you knew more about the electronics, you could probably build a static test harness fairly easily. Knowing the pinout of the device would probably be enough to give that a try.

[luckybuck]

First, thanks for TURBAN, even more important! :-) As written above, with many programs, AtariGraphics cart, SALT 2.07 and so on. All have the same problem, horizontal position. AtariGraphics is for light pen, not light gun. The SALT cart tests both, of course.

 

Selecting with the pen trigger did worked with AtariGraphics.

Edited April 19, 2019 by luckybuck

[luckybuck]

@BigO: 'Maybe the sensor failed entirely.' That is what I have in mind. Even electronic elements get old. After > 20 years, here > 30 years, some parts may fail. That is why I search for the technical reference manual or take apart guide.

[_The Doctor__]

There really isn't much in the pen, it's a good light pen if the pen works on a 130XE etc , and as you know the 400 doesn't work with the light pen except possibly port 4... you would have to fix the 400, there were such write ups back in the day, so with todays hardware hacking software creating wizards you can be sure it can be done again.

 

The pen works with real crt's only, and back around the 2000's some televisions did weird display methods that weren't always so great for accuracy...

Edited April 19, 2019 by _The Doctor__

[+Allan]

I don't recall Atari putting out repair manuals for either light pens or the touch tablet. There are available for the track balls, the 5200 controller, but not the other controllers.(that I am aware of).