Task: Diagnose and Repair the Clock Oscillator Circuit in an Atari 8-Bit Computer
Needed: oscilloscope, clock circuit components, soldering iron, desoldering iron, hot air rework station
Time: 1-2 hours
Disclosure: Note that as an eBay Partner Network Affiliate, I may earn from qualifying purchases.
Introduction
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 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.
I recently encountered an 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. However, the oscilloscope 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. 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.
Instructions
The following are some things to try if you suspect the clock circuit might be faulty.
First, use an oscilloscope to check the clock signal on the GTIA, ANTIC, and the 6502 CPU to make sure it is the expected 1.7897725 MHz (1.7734470 MHz for PAL). Below is what the clock signal should look like on pin #3 of the 6502.
Second, use a multimeter to test the voltage coming into the clock circuit. You can do this on the left pin of resistor R111 that is connected to the 5V power rail. It should read 5 volts.
Third, use an oscilloscope to measure the signal on the pins of the crystal clock. There should be a regular repeating signal of 3.579545 MHz.
Fourth, use a multimeter to test the resistors and capacitors in the clock circuit. Below is this portion of the circuit diagram for the NTSC clock. The resistances and capacitances for each component are listed on the diagram.
Fifth, use a multimeter to test the two transistors in the circuit.
Sixth, replace any faulty components. Note that I bought my new crystal clock from Amazon. However, my understanding it that these rarely fail. The fault is likely to be one of the other components.
Seventh, if replacing faulty components doesn’t work, check for broken traces and repair.
Comments
These steps allowed me to repair my clock circuit and bring an Atari 800XL back to life. Although I replaced several components, I think the bad resistor was the culprit that brought the entire system down. This was the first time I have seen the chip signals look completely dysfunctional on the oscilloscope.