Task: Purchase an Oscilloscope for Troubleshooting Problematic Atari 8-Bit Computers
Needed: An oscilloscope
Time: 30-60 minutes
Introduction
As I reviewed in my previous post about multimeters, I have been learning how to troubleshoot and repair Atari 8-bit computers. Central to this effort is an oscilloscope that can be used to measure the communication signals being transmitted throughout the computer. These signals are in the form of voltages (usually high and low) that alternate at a frequency determined by the crystal clock on the motherboard. The clock on an Atari 8-bit computer pulses at 1.79 MHz or 1.79 million times per second. This is the cadence of the computer and is set specifically to be in synchronization with the 60Hz refresh rate of a CRT television screen as explained in the ROM magazine article from 1985. This rapid fluctuation of 5V and 0V pulses can be measured with an oscilloscope and used to diagnose faulty chips. I give a few examples below in the instructions.
I learned a lot about troubleshooting from Adrian Black of Adrian’s Digital Basement and Jonthan Halliday or FlashJazzCat. I highly recommend watching their videos and have previously posted about the FlashJazzCat YouTube channel. I also learned a lot from the AtariAge forums.
Instructions
Step one is to purchase an oscilloscope. There are several popular brands including Rigol and Siglent. I went with the Siglent SDS 1104X-E because it has good ratings and reviews and was in stock when I went to Micro Center. The Siglent model I got (see photo below) can measure signals up to 100MHz which is plenty for retro work (Atari is 1.79 MHz and the Apple II is a little over 1 MHz!) and can measure four channels simultaneously. There are cheaper models that can go up to say 50MHz with only two channels which is probably all I needed. There are also Chinese-made brands such as Hantek that many seem to like. The model I purchased is $475 on Amazon at the time of this post, has an Amazon’s Choice badge with an average review score of 4.8/5 from over 900 reviews. There are cheaper handheld oscilloscopes if you are on a tight budget. For example, here is a review of one by Adrian Black on YouTube. He has reviewed some others as well.
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 about what can cause personal harm or damage to your computer or oscilloscope.
Step three is to calibrate the probe. Most models like the one I purchased have a calibration signal built in with pins to connect your ground and probe to (see bottom center of unit below). Note that I connected a black alligator clip extender to the ground cable of my probe to make it longer and easier to attached when I am testing signals. I added some electrical tape to prevent any shorts if the exposed clip were to touch a hot pin. The exposed black alligator clip is attached to the right pin of the calibration unit. The signal probe is connected on the left. The screen shows the alternating high and low voltage. This particular calibration signal provided by the Siglent is alternating at 1 KHz or 1000 times per second. There is a little flathead screw embedded in the handle of the probe that can be adjusted to make the signal square as I have done here.
Step four is to measure the clock signal on the Atari to make sure it is pulsing at the required 1.79 MHz. The 6502 CPU has a clock input pin (#37) labeled Φ0 on this chip diagram. I attached the ground clip to the RF modulator case and touched the probe to pin 37 on the 6502. Below is the signal I got. Note that I am using the Eres acquisition option for a smoother and less noisy sampling of the signal. The frequency of the signal shown in the upper right of the screen is 1.789 MHz which is pretty close the expected 1.79 MHz. This indicates that the clock is working and finding its way to the 6502 without trouble. Similar clock inputs can be found on other chips (e.g. pin 35 on ANTIC). Note that Φ1 is the clock signal output that goes to other ICs. It should read the same.
Step five is to check the address and data bus pins on the 6502 that should be quite busy with alternating high and low voltages during normal use of the Atari when it is powered on. According to this pinout diagram, there are 16 address pins (#9-25) and 8 data (i.e. 8-bit!) pins (26-33) that can be checked. A healthy 6502 should have activity on all these lines. A pin that is stuck low or high or with in-between voltages could indicate nonworking 6502. Here is the activity on pin 33 or data bit D0 followed by a photo of pin 20 or address line A11. These patterns were typical for all the data and address lines I tested while a Defender cartridge was running. Each was 5V at the maximum and 0V at the minimum as expected.
Comments
It took me awhile to take the plunge into more detailed testing of the communication signals of the Atari 8-bit computers. I am still learning but feel like I have the basics down. $500 was a lot of money to shell out for one of these. However, I now feel like it was worth it after fixing several computers. I have also learned a lot about how these machines work. Hopefully I don’t need to use it too often!
All the different chips have signals like this that can be tested. Get to know what normal looks like on a normal machine before testing a nonworking unit. I have completed a more in-depth post about the expected signals from each pin of the 6502 CPU. Included are videos to see the signal in action.
I have posts on removing bad chips by desoldering and lifting using a hot air gun.