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Are there factory bodges on MZ-80K PCB?

Posted: Wed May 18, 2022 11:01 am
by TheByteAttic
Hi all,
I'm in the process of restoring a previously very heavily modded MZ-80K. Videos here: https://www.youtube.com/watch?v=NiAL5LC ... eByteAttic.
I'm now troubleshooting the machine, and noticed that there is nothing connected to the shift-clock input of the shift register responsible for the display bit stream (the 74LS165). The schematics call for the 8MHz clock to be there, but there is no trace on the board leading the clock to that pin. My current hypothesis is that there was a factory bodge there, which I removed along with all the other bodges made by the previous owner. Does any of you know whether there were factory bodges in the original MZ-80K PCBs, European version?
I've noticed other inconsistencies between the schematics in the service manual and the particular PCB I own. Is it known whether the schematics are inaccurate?
Thanks for any feedback!

Re: Are there factory bodges on MZ-80K PCB?

Posted: Wed May 18, 2022 11:08 am
by hlide
That model were heavily modded and had a 4MHz turbo clock which means several cuts on te motherboard to make it happen. Just removing such addition won't restore the original clock circuit.

Re: Are there factory bodges on MZ-80K PCB?

Posted: Wed May 18, 2022 11:11 am
by hlide
Try to see if the mod is that one: https://www.sharpmz.no/articles/the-mz- ... m-support/

I believe there are several methods though.

Re: Are there factory bodges on MZ-80K PCB?

Posted: Wed May 18, 2022 11:35 am
by hlide
Another set of schematics here at the end of the page: http://www43.tok2.com/home/cmpslv/Mz80k/EnrMzk.htm

Re: Are there factory bodges on MZ-80K PCB?

Posted: Wed May 18, 2022 11:42 am
by hlide
Ah yes, you probably had a 40/80 column switcher so the clock circuit should have been modified as well.

Re: Are there factory bodges on MZ-80K PCB?

Posted: Wed May 18, 2022 11:50 am
by TheByteAttic
Hi all,
Yes, it had an 80-column mod. And yes, I bridged all the cut traces I could find, including one from the oscillator. But there seems to be no traces, cut or otherwise, bringing the 8MHz clock to the clock input of the 74LS165. The PCB seems to have nothing connected to that pin, unless it's hidden underneath the IC (I may pull it to resolve the mystery). That's why I wondered if there were factory bodges.
Thanks for all the feedback and new set of schematics, much appreciated!

Re: Are there factory bodges on MZ-80K PCB?

Posted: Wed May 18, 2022 12:55 pm
by hlide
My assumption is the clock of LS165 was modified to output at 40 or 80 columns so there was probably a more complex circuit due to addition of VRAM from 1KB to 2KB. I don't know the mod though.

Re: Are there factory bodges on MZ-80K PCB?

Posted: Wed May 18, 2022 4:03 pm
by TheByteAttic
I found the trace. It weaves through several VIAs and then under the chip. It was indeed cut open during the modification. The cut is now bridged and one more MZ-80K is back to life. Thanks for the feedback!