Donkey Kong PCB Clock Issue?

xray2

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As most of you know, I'm an Atari Vector head but, a buddy asked me to look at his Donkey Kong board (2 board set TKG4-13) which shows no signs of life. I replaced a couple of bad roms on the CPU board, connected up all the voltages and got nothing.

I started probing around with the scope and was getting erratic frequency readings on pin 6 of the z80 cpu and pin 12 of the 8257. The frequency would drift from low KHz all the way up to 25mhz! I started to work my back back through the clock circuit and, let's just say, it is a rather different design that what I am accustomed to dealing with. Simple 2 page schematic I am working from is a little abstract and a huge pita.

Can anyone out there confirm clock frequency measurements at various points on the board or provide pointers on where to go next? I did some extensive searching last night but after looking over 400 threads, found nothing similar.
 
Clocks

Hi!

The clock frequency of a Z80A is a max of 4 MHZ. Simplest method is to either start at the crystal and work toward the Z80 or start at the Z80 and work backward through the schematics. I would start at the Z80, then check 6E-11 then 6E-12 then 6e-3 then 6E-2 etc... until I found a good clock. As you said the schematics are blurry but it looks like the clock is 6.144 MHz/4 = 1.536 MHz... I might be wrong I just looked really quickly.
I have had problems with 7404s and their variants... there are usually a lot of them in the clock circuitry.

Hope that helps, Kenneth
 
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