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I'm repairing my Nintendo Donkey Kong arcade PCB and I have no pulse when tested with my logic probe. I found out that the crystal oscillator (2 pin HC-18U) was not connected to the board, since the legs have rusted completely and broken. The 2 pin crystal is a 61.44 MHz HC-18U model. It generates the clock for the Z80 processor.

Can I use a crystal with a different frequency, without damaging the components or affecting the game?

I can't find any two legged crystals, only 6 legs flat VCXO type... could that work?

Thanks

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  • \$\begingroup\$ Not familiar with the board you're talking about but very often other frequencies, such as those required to generate video, are derived from the main crystal frequency. Changing it could result in garbled video, sound with incorrect pitch, etc. \$\endgroup\$ – Unimportant Jan 3 at 15:13
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    \$\begingroup\$ For a Z80, it's much more likely to be 6.144 MHz, which is a commonly-available value. \$\endgroup\$ – Dave Tweed Jan 3 at 15:19
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    \$\begingroup\$ In addition to having misread the value, you need to pay attention to the difference between a crystal and a crystal oscillator. Are there no original or reverse engineered documents for this board or its general form available anywhere? And you probably don't want to do something that could result in a fixed-frequency monitor being sent the wrong scanning frequencies (assuming this was raster and not vector) \$\endgroup\$ – Chris Stratton Jan 3 at 15:32
  • \$\begingroup\$ I doubled-checked: the PCB value says 61.44, the original Crystal says 61.44, the schematics says 61.44 and the part list in the manual says 61.44 MHz. \$\endgroup\$ – Luke Jan 3 at 15:52
  • \$\begingroup\$ arcade-museum.com/manuals-videogames/D/dk-tkg4u.pdf Page 18, (last item on that page) and the schematics at last page 32, coordinates C-16. \$\endgroup\$ – Luke Jan 3 at 15:56
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This crystal (61.44 MHz) is used in video board generator. It has no relationship to Z80, which uses a different 6 MHz crystal, so your reference to CPU confuses readers.

As shown in the schematics,

enter image description here

the crystal is a part of a sophisticated HF-oscillator, so a VCXO-type programmable crystals/oscillators will unlikely serve as direct replacement.

You have two basic options, either to find this vintage crystal (or similar one) and hope that its ESR will match the original, and the circuit will start oscillating. Or replace the entire generator with a modern programmable oscillator, program it to 61.44 MHz, and drive the isolating transformer T1 directly with modern electronics. You might need to keep the Q2 as power amplifier.

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