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Cheap Scope Troubleshoots Commodore

[Adrian] had a Commodore computer to fix and decided to see how his latest tiny portable scope would work. He paid $57 for the tiny little test instrument although the current price seems higher. It claims to have 120MHz bandwidth along with 500 megasamples per second. There are several versions with different claimed specs, but we did find a similar device for under $60. You can see the unboxing and how it worked in the video below.

Of course, these kinds of instruments often overstate their specs, and [Adrian] was also suspicious. One odd feature of the device is it can echo its output to an NTSC video output so you can send the screen to an external monitor.

If you want to skip the scope unboxing, forward up to about 19 minutes to see the inside of the Commodore 64. The scope was easily sufficient for scanning the chips in the computer and revealed a suspicious address line. The line went to a PLA and a mux chip, neither of which were in sockets. He clipped the PLA out of the circuit, and the address line started looking normal. So the conclusion was the PLA was dead.

After that, it was straightforward to remove the chip and replace it. Well, technically, replace it with a socket to make a future repair easier. Will a $57 scope replace your big benchtop instrument? Maybe not. But it was a useful tool for troubleshooting.

Even if you don’t want a cheap scope, you can learn a lot from [Adrian’s] thoughtful troubleshooting and analysis if you are faced with any digital repair project. We do like cheap scopes around here. It is amazing how much scope $100 will buy now compared to just a few years ago.