Problem with MZ80-A CRT

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kbjunkie
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Joined: Fri Jun 17, 2022 8:48 pm

Problem with MZ80-A CRT

Post by kbjunkie »

HI All,

My first post here - and thank you to the admin for activating my account!

I have picked myself up an MZ80-A and have repaired the main board (just some faulty RAM IC's). That part is now working but I have an issue with the CRT. I have replaced all electrolytic capacitors on the CRT PCB - with the exception of the bipolar capacitor (C2017 - 10uf 25v) as I can't seem to find one - but in another post I read, it was advised NOT to change it?

Anyhow.. what I get onscreen is not quite a horizontal line. It shows the monitor prompt, but the text is printed like a sine wave - and it moves up and down about a 1/4 of the screen in the middle (I hope that I've described that okay!) The text starts about a 1/4 of the way down the screen, is printed on the screen like a sine wave and will roll to about a 1/4 of the way to the bottom of the screen before jumping back up to about a 1/4 of the way from the top and rolling again.. It's really hard to take pictures of the screen - I will try though.

I should add that it looked like this before I changed the capacitors.

Has anyone any ideas what to look for here? My forte is really digital electronics..

Many thanks,

Mark.
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mz-80a
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Re: Problem with MZ80-A CRT

Post by mz-80a »

Wow, hope you can get some answers (sadly my hardware knowledge is fairly limited), great to see another MZ-80A user though. Unfortunately, video issues are one of the most common issues with the 80A and can be hard to diagnose I believe.
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kbjunkie
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Re: Problem with MZ80-A CRT

Post by kbjunkie »

Thanks @mz-80a

Not sure what the heck is causing this issue - and like I said, my forte is really digital electronics..

Will just have to wait and see if anyone replies with any nuggets of info!

Cheers,

KbJ
hlide
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Re: Problem with MZ80-A CRT

Post by hlide »

(Answers from https://www.facebook.com/groups/sharp8b ... 1587370272)
I would check around the horizontal drive for a dodgy gnd connection, a faulty resistor/capacitor filter combo or the ic itself. It looks like it may be on the linearity side
This is the spec sheet for the ic and has example circuits so you can get a rough idea comparing with the 80a circuit what components are filters
http://www.unisonic.com.tw/datasheet/PC1031.pdf
One other thing is the caps he has used to replace them are low esr and low ripple ones else could have issues too as not all caps are equal !!
Antxiko
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Joined: Thu Jun 09, 2022 2:23 pm

Re: Problem with MZ80-A CRT

Post by Antxiko »

Hi, I had the same problem with my 80B, y replaced that capacitor with an Audio Pro grade cap, now works like a charm. I used a Panasonic ECEA1HN100U.
kbjunkie
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Re: Problem with MZ80-A CRT

Post by kbjunkie »

Hi Antxiko,

Are you talking about the biploar C2017 cap?

Thanks,

KbJ
kbjunkie
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Re: Problem with MZ80-A CRT

Post by kbjunkie »

Just a quick one to close this thread..

This issue was down to a bad uPC1301H. I replaced it with an H2 after modifying the shorter heatsink tab on the H2 variant, and after a little re-twiddling of the pots on the board, it's now working!

Thanks for all of the suggestions!

KbJ
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mz-80a
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Re: Problem with MZ80-A CRT

Post by mz-80a »

That's great to hear!! :) Another working 80A out in the wild...
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