CD-ROM/CD lens/tracking adjustments - only works when oscilloscope is attached??

[CkRtech]

I have something that baffles me.

I am attempting to do a minor tweak to an old video game console that has a CD drive. Most games I try work OK. One in particular has to track to the very outside edge of the disc to playback a video, and it never seems to load properly. It will track out there, read for a solid 5 seconds or so, and then track inward and back outward to the same spot and attempt to read the disc again.

I imagine I can simply tweak one of the pots available in order to get it to track/read the entire disc without issues.

What is odd is that I attached my scope to one of the test pins (PCLK), grounded it to the device, powered up and two things happened:

1: I had a bit of video interference when scope was attached (no big deal)
2: The game worked!

I pulled the probe during the video playback. It finished, went to the next video, and wouldn't load. I powered down, started up again with no probe attached, and it wouldn't load.

I decided to probe a different test pin. Powered up, and it worked again. I just left the probe on there and started playing the game. It pretty much worked the entire time.

What is happening? Am I adding resistance or something with the probe hooked up to a test point that the tracking/lens is suddenly happy enough to read that track?

[German_EE]

Try attaching a 10pf capacitor with a 1M ohm resistor in parallel between your test point and ground, that's roughly what the input of your scope will look like.

[SeanB]

Where are you attaching the scope ground lead? there might be a poor ground connection to the board and the scope ground is providing enough extra current flow so it can read the data. Check the board for cracked solder joints on the power pins.

[CkRtech]

Thank you for your replies.

The ground clip is connected to a metal shield surrounding the port used for card based games. Not exactly close to the signal, but easy to use with that ground lead clip.

[mikerj]

Is that a PC Engine DuoR?  Don't think I've ever seen one in the flesh.

[CkRtech]

Is that a PC Engine DuoR?  Don't think I've ever seen one in the flesh.

It is.

Many calibrate the laser and tracking by throwing a CD on and adjusting by ear. I would prefer to use the oscilloscope if I can find a good guide to walk me through it.

I adjusted a regular Duo by ear. It works, but too much guess work for something that could probably be done much more efficiently with proper tools.

[leblanc]

Where SeanB was going with the ground of the oscilloscope is that your oscilloscope is connected to the wall and so is your console, so they share the same ground. The scope probe's ground is shorted to the ground of the scope.

So when you connect the probe's ground clip, you're putting whatever you're attaching it to to ground. If the thing you're attaching it to had a lose ground connection (from a cracked solder joint or other), you're making that ground connection better.

But since you have it attached to a shield, that is probably not the issue.