MZ-80K displaying garbage chars on screen

rowanhall
Posts: 5
Joined: Wed Sep 01, 2021 7:52 pm

MZ-80K displaying garbage chars on screen

Post by rowanhall »

Hi all,

I'm new to this forum and was hoping to get some help identifying how to sort this fault.

The machine had been working perfectly until recently when the PSU failed. I traced this to a bad tantalum cap which was replaced.

However now the machine displays garbage text on the screen when you turn it on, as shown in the attached pic.

I have been looking at this issue on and off for the last while and I'm none the wiser how to sort it.

I do know that if you start the machine with the processor removed, the same screen appears. So I'm assuming this is some kind of video initialisation screen before the processor takes over?

Anyhow any advice where to look to continue troubleshooting would be greatly appreciated, its not a computer I'm overly familiar with at a hardware level.

Regards,
Rowan
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S_U_C
Posts: 46
Joined: Sun Feb 17, 2019 6:41 pm

Re: MZ-80K displaying garbage chars on screen

Post by S_U_C »

Hi, this is the stardard cpu not running screen display as the Z80 clears the screen using code.
As you said, same result if no cpu fitted.

First try a manual reset, a quick short on pin 26 Z80 to ground (0V) as the reset circuit may have failed.

If you have a logic probe check to see if there is a clock signal on pin 6 Z80, fed from pin9 IC 35.
also check Pin 11 IC35 if no clock on pin 9. The clock is running else there would be no display.

Then try replacing the Z80 chip.
rowanhall
Posts: 5
Joined: Wed Sep 01, 2021 7:52 pm

Re: MZ-80K displaying garbage chars on screen

Post by rowanhall »

Hi, Thanks for getting back to me.

I'll check this out later and report back!

Cheers
rowanhall
Posts: 5
Joined: Wed Sep 01, 2021 7:52 pm

Re: MZ-80K displaying garbage chars on screen

Post by rowanhall »

Hi,

Got some time this evening to have a look.

- Tried the manual reset, no difference.
- Z80 pin 26 probed as low.
- Probe showed pulse on Z80 pin 6, also on IC35 pin 9 and 11.

Source a Z80, or are there further tests?

Is there a difference between the currently fitted Sharp LH0080 Z80 and the commonly available LH0080A which would cause an issue here?

Appreciate your patience, cheers!
hlide
Posts: 602
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ-80K displaying garbage chars on screen

Post by hlide »

No, no difference. You could put a CMOS Z80, it also works.

What is the frequency of the pulses on pin 6?
rowanhall
Posts: 5
Joined: Wed Sep 01, 2021 7:52 pm

Re: MZ-80K displaying garbage chars on screen

Post by rowanhall »

I only have a simple logic probe unfortunately so I can't tell.
rowanhall
Posts: 5
Joined: Wed Sep 01, 2021 7:52 pm

Re: MZ-80K displaying garbage chars on screen

Post by rowanhall »

Hi,

Received an LH0080A Z80 today and popped it in. No change unfortunately. So presuming that the original Z80 is alright?

Where would be the next thing to check? Is it worth getting a simple scope to do some deeper diagnostics?

Rowan
hlide
Posts: 602
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ-80K displaying garbage chars on screen

Post by hlide »

And a cheap LA1016 too.
hlide
Posts: 602
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ-80K displaying garbage chars on screen

Post by hlide »

A scope and a logic analyzer are ALWAYS helpful. A scope will help to see if a signal has a good shape (video line or clock). LA will allow you to watch the timings of several logic signals and their coherency.
AndyR
Posts: 8
Joined: Tue Mar 17, 2020 2:40 pm

Re: MZ-80K displaying garbage chars on screen

Post by AndyR »

I have the same issue with an MZ-700, random characters on startup (in colour)
Could it be the RAM used for video perhaps?
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