Poking inside a vic20 with an oscilloscope

[patrickm]

Hi,

I would like to measure some signals in my 2 prong vic with an oscilloscope.

I assume the 10vac transformer will prevent any ground loops? Checking because I don’t want to risk damaging anything.

Thanks!
P.

[Mike]

Hi, Patrick,

the VIC-20 with original PSU is a Class II appliance (i.e. double isolated), but as soon as you operate your VIC-20 with a Class I TV set or monitor, it is grounded by the display. If you want to prevent ground loops, you also need an isolating transformer with your scope.

Greetings,

Michael

[patrickm]

Thanks Mike.

You're absolutely right. I don't know how I missed that big monitor right in front of me!

Patrick

[beamrider]

does this apply to the DIN type (and 64)?

I've just got a new scope and wondering if I need to disconnect it from the 1702 when making observations?

[JonBrawn]

When poking around with the video, remember that the monitor is part of the circuit. If you disconnect the monitor then the output from the DIN on the back of the VIC-20 will change - I don't know if it's a significant amount or not - but it will change, so if what you're looking at looks a bit funky then try putting a 75-ohm 1% resistor between the video output and ground, which will present the same loading as a real monitor without any ground loop problems.