Are there factory bodges on MZ-80K PCB?

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TheByteAttic
Posts: 3
Joined: Tue May 17, 2022 12:40 am

Are there factory bodges on MZ-80K PCB?

Post 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!
hlide
Posts: 660
Joined: Thu Jan 25, 2018 9:31 pm

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

Post 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.
hlide
Posts: 660
Joined: Thu Jan 25, 2018 9:31 pm

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

Post 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.
hlide
Posts: 660
Joined: Thu Jan 25, 2018 9:31 pm

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

Post by hlide »

Another set of schematics here at the end of the page: http://www43.tok2.com/home/cmpslv/Mz80k/EnrMzk.htm
hlide
Posts: 660
Joined: Thu Jan 25, 2018 9:31 pm

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

Post by hlide »

Ah yes, you probably had a 40/80 column switcher so the clock circuit should have been modified as well.
TheByteAttic
Posts: 3
Joined: Tue May 17, 2022 12:40 am

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

Post 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!
hlide
Posts: 660
Joined: Thu Jan 25, 2018 9:31 pm

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

Post 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.
TheByteAttic
Posts: 3
Joined: Tue May 17, 2022 12:40 am

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

Post 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!
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