Introduction
I friend brought me his Atari 800XL that was not generating any video signal. I confirmed with a multimeter that the chips were all getting good voltages. None of the chips were overly hot, and I confirmed with a working machine that they were all good.
Case Study
Given all the main ICs were good, it was time to break out the oscilloscope for further testing. The scope showed that the data and address lines of all the main ICs were not even close to being normal. Further, the clock signal was also not correct. This led me to the evaluate the clock circuit itself. The oscilloscope showed the crystal clock was not outputting the correct 3.579545 MHz signal (see scope photo below with correct clock signal output after processing). I replaced the clock component with a new one and started testing the resistors and capacitors in that circuit. I replaced a capacitor that was out of range and found a 470 Ohm resistor (R113) that was reading well over 1000 Ohms. Once the new resistor was installed the Atari started working correctly. Any significant deviations from the required resistance or capacitance can yield a faulty clock signal which is what happened here.
Comments
The crystal clock oscillator is used by the Atari 8-bit computers to serve as the metronome for the computer to keep all the chips in sync for proper operation. These simple electrical components have a quartz crystal that expands and contracts in regular intervals when an electrical current is applied. The clock used by the Atari pulses at 3.579545 MHz with an electrical current supplied by a small circuit of transistors, resistors, and capacitors with 5 volts coming in from the power rail. Note that the PAL computers have an additional clock circuit. This 3.579545 MHz signal is further processed by the GTIA that outputs a clock signal of 1.7897725 MHz used by the rest of the system. If these clock signals are not correct the entire system will not work.
This is reposted from my Atari Projects blog.