MZ-80K where to start to repair

traindriver69
Posts: 18
Joined: Sun Apr 14, 2019 2:52 pm

MZ-80K where to start to repair

Post by traindriver69 »

Hi all
I have just purchased a MZ-80K from a well known auction site and waiting for it to come but it has got issues I am wondering where to start first to get it up and running this is the picture he has posted for selling it so dont know if anyone can get any ideas from it he describes it as screen full of lines.
I have always wanted to own one of these so would be great to see it burst into life.
many thanks
Attachments
mz80k.jpg
hlide
Posts: 588
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ-80K where to start to repair

Post by hlide »

when you open the case, you can see under the screen some potentiometers to control horizontal and vertical synchronizations.
traindriver69
Posts: 18
Joined: Sun Apr 14, 2019 2:52 pm

Re: MZ-80K where to start to repair

Post by traindriver69 »

Hi all
Just received my MZ-80 and tried altering the v hold and H hold but just moved everything around so I have checked the voltages on the power supply and the 12 v is coming out at 19.19v so that's not going to help is it.
traindriver69
Posts: 18
Joined: Sun Apr 14, 2019 2:52 pm

Re: MZ-80K where to start to repair

Post by traindriver69 »

Hi All
Now sorted the power supply by replacing some of the caps and getting correct voltages so now when I turn it on I get straight lines that move side to side any ideas?.
Attachments
sharp.jpg
hlide
Posts: 588
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ-80K where to start to repair

Post by hlide »

That doesn't sound as regular characters. Could you make a short video to see how they move? I count only 20 lines.
traindriver69
Posts: 18
Joined: Sun Apr 14, 2019 2:52 pm

Re: MZ-80K where to start to repair

Post by traindriver69 »

posted a video here

https://youtu.be/1CaQtj-JQvE
hlide
Posts: 588
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ-80K where to start to repair

Post by hlide »

Can I suggest you to unplug the CGROM temporarily to check what the screen displays?
S_U_C
Posts: 43
Joined: Sun Feb 17, 2019 6:41 pm

Re: MZ-80K where to start to repair

Post by S_U_C »

The video is a self contained unit consisting of a video ram driven by an binary counter the ouput of the video ram + binary counter drives the CG ROM, and output of thr CG ROM is serialised to produce the display.

The 12 volts also powers the dynamic ram on the MZ80K so these could be cooked if supplied with 19V.
But the ram appears to be OK as the screen contains a repeatable pattern filled in by the Z80 by calling subroutines.
Remove the Z80 and power up the screen should consist of random characters (the default state of the video ram).

This repeatable 4 byte screen pattern has been reported before, the pattern is always the same but different on other machines (K, A, 700 have all been reported) I have a K with the same problem and I believe the problem to be with the ram drivers/buffers IC 50 (mon), IC 36 and 37 (Z80), IC 43 (VID RAM).
Sadly my soldering skills have abandomed me (only one good eye) so I have been unable to test this idea.
I included part of the K's start up code to show the subroutines called.
As the start up message is not displayed the Z80 may be hung in the Call silence subroutine.
Attachments
mz80k startup.jpg
traindriver69
Posts: 18
Joined: Sun Apr 14, 2019 2:52 pm

Re: MZ-80K where to start to repair

Post by traindriver69 »

Hi
Thanks for the reply I have since took the board out and found corosion on the socketed ic,s so this could be the problem I am going to replace the sockets but I am struggling to find replacement ic,s just in case for instance the cpu I can find replacement LH0080A but not the original LH0080 will the A version work and the others are they easy to come by.
Gortho
Posts: 36
Joined: Thu Mar 22, 2018 3:53 pm
Location: Poland

Re: MZ-80K where to start to repair

Post by Gortho »

traindriver69 wrote: Fri Aug 27, 2021 5:58 pm Hi
Thanks for the reply I have since took the board out and found corosion on the socketed ic,s so this could be the problem I am going to replace the sockets but I am struggling to find replacement ic,s just in case for instance the cpu I can find replacement LH0080A but not the original LH0080 will the A version work and the others are they easy to come by.
You should be able to use the one you found, or most probably any "modern" Z80 bought on mouser/digikey/wherever ;)
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