Task: Purchase a Multimeter for Troubleshooting Problematic Retro Computers
Needed: A multimeter
Time: 15-30 minutes
Introduction
It is difficult to collect and use retro computers without developing some repair skills because they do fail. I recently decided to level-up my repair skills and have successfully fixed several Atari 800XLs and Commodore 64s. I have also fixed several disk drives including a Rana 1000 and an Indus GT.
An essential item for testing the voltage paths and electronic components (e.g. resistors, capacitors, etc.) is the multimeter. This is important because it is possible for individual failed components to bring a system down even though the specialized integrated circuits such as the 6502 processor and the ANTIC graphics chip work fine. The opposite is of true as well.
I learned a lot about troubleshooting from Adrian’s Digital Basement and The Retro Channel on YouTube. I find both of their videos to be clear and informative.
Instructions
Step one is to purchase a multimeter. There are many good ones to choose from on Amazon. I purchased a Chinese made Kaiweets HT118A from Amazon about five years ago for around $50. This is currently an Amazon’ Choice with over 22,000 reviews and ratings that are mostly positive. The Kaiweets are also available on eBay. I have been happy with mine and think it works great. There are plenty to choose from. Here is a nice video review of the Kaiweets multimeter I have that includes a good demo of how to use it.
Step two is to read the instructions if you have never used one before. Also find some instructional videos online. Note all the safety instructions, cautions, and warnings that can cause personal harm or damage to your computer or multimeter.
Step three, use your new multimeter to test the voltage on your circuit board. You will see in the photo below that I have the black lead attached to an alligator clip connected to the metal box of the RF modulator to serve as ground. I can then use the red probe to test the voltages on different components and chips. Make sure you are using a trusted power supply such as those you can buy newly made. A good first test on this Atari is to make sure the 6502 Sally CPU is getting 5V on pin #8 (labeled Vcc in this schematic). You can also probe pin #1 (labeled Vss) that is the ground for the chip. This should read very close to 0V. Follow the pinout diagrams from this website to test Vcc and Vss on the other main chips such as ANTIC and GTIA.
Each computer is a bit different. For example, the Commodore 64 has some chips that take 5V and some that take 12V.
Comments
Test as many chips and components as you can to get some practice.
Testing the voltage with a reliable power supply should be the first order of business when troubleshooting a nonworking 8-bit computer.
Be sure and read all the warning and cautions about using a multimeter that are supplied in the documentation of the unit you buy. For example, exceeding the maximum voltage of the unit can damage it.
Also be careful not to shock yourself! Touching a power line can provide a shock. Capacitors are also notorious for releasing larger voltages.
Another tip is to avoid touching more than one contact at a time with the probe. This can create a short and damage your components.
This is a repost from Atari Projects.