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MZ-80K displaying garbage chars on screen

Posted: Sun Sep 19, 2021 10:27 pm
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

Re: MZ-80K displaying garbage chars on screen

Posted: Mon Sep 20, 2021 11:51 am
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.

Re: MZ-80K displaying garbage chars on screen

Posted: Tue Sep 21, 2021 9:55 am
by rowanhall
Hi, Thanks for getting back to me.

I'll check this out later and report back!

Cheers

Re: MZ-80K displaying garbage chars on screen

Posted: Wed Sep 22, 2021 8:39 pm
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!

Re: MZ-80K displaying garbage chars on screen

Posted: Wed Sep 22, 2021 11:00 pm
by hlide
No, no difference. You could put a CMOS Z80, it also works.

What is the frequency of the pulses on pin 6?

Re: MZ-80K displaying garbage chars on screen

Posted: Wed Sep 22, 2021 11:14 pm
by rowanhall
I only have a simple logic probe unfortunately so I can't tell.

Re: MZ-80K displaying garbage chars on screen

Posted: Mon Sep 27, 2021 12:06 pm
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

Re: MZ-80K displaying garbage chars on screen

Posted: Mon Sep 27, 2021 2:05 pm
by hlide
And a cheap LA1016 too.

Re: MZ-80K displaying garbage chars on screen

Posted: Mon Sep 27, 2021 2:15 pm
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.

Re: MZ-80K displaying garbage chars on screen

Posted: Sun Oct 10, 2021 12:46 pm
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?