How can I test the oscillator? I have a multimeter with frequency test.
I see that the oscillator has 4 pins, where should I attach those pins?
If I want to test if the CPU is getting clock, which pins should I test?
Good call to check it since it's been messed with in the past. The four pins are ground, 5V, clock out and no signal. I never remember which is which, but it's easy to use an oscilloscope to check each pin, it's pretty obvious when there's a valid signal.
You generally can't use a multimeter for this as the frequency measuring range of these are usually about 2MHz or less. The A2000 oscillator is 28MHz.
Agnus forms the clock divider in the A2000. The CPU receives 7MHz on the clock input, pin 15.
Keep in mind that almost anything can cause a non-booting fault. There's a few basic checks you can do to narrow things down further to see if the hardware is still alive - which I'm not re-typing yet again, see one of the hundreds of other threads on the same subject.
If the hardware seems mostly alive (i.e. it's executing stuff from ROM) then a quick progression from there is to use John H's DiagROM to see what's going on.
