Sharp MZ80-A screen upside down issue

[centrespot]

Hi All

I have two MZ80-A computers, one has a screen that just isn't working the other screen appears to turn the computers output upside down and is wavy.

I have tried both motherboards and it is the same issue.

Can anyone advise on what is faulty

Thanks in advance

[PJTech]

Hi,
In this case I think it's pretty likely that there are some dodgy electrolytic capacitors in your machine(s) which are causing PSU issues and in the case of this vertically flipped image, also leading to deflection problems. I think the 'upside down' nature of the text here is an artefact of the wider deflection problems.

There's a useful thread on EEVBlog ( https://www.eevblog.com/forum/repair/re ... rp-mz-80a/ ) which includes a link to a YouTube tear-down video and you'll see the guy in that found some very obviously perished capacitors. There's also a useful link to the Vertical IC's datasheet which includes a reference circuit and may be representative of how Sharp implemented it.

Block Diagram From Service Manual of Sharp MZ80

The text being displayed should be at the top of the screen but as it's in the centre I think your monitor is suffering from a partial frame collapse as well as some very obvious linearity issues.... First check is to see what the main 12 volt supply looks like. Then I think you'd be wise to strip the unit down if you are safely able to and examine all the electrolytic capacitors in both the main PSU and the Monitor, with particular focus on the vertical deflection circuit.

If you have a capacitor tester then this may help you as failed caps don't necessarily have any visual defects. Any capacitors which are deformed, bulging or clearly leaking should be changed. If you don't have a capacitor tester of any kind but you do have a multimeter then if you desolder a cap and it looks okay, you can still put your probes across the legs and look to see that the ohm reading gradually goes either up or down, and then gradually does the opposite when you reverse the leads. This is a very rough test but would at least highlight if you had one that was shorted.

The https://original.sharpmz.org/mz-80k/download/sm80k.pdf full service manual includes some reference waveforms which you could test if you have an oscilloscope.

The subject of 're-capping' is always hotly debated, especially with Monitors and TVs - personally I'm not a fan of just replacing all capacitors in case they might have failed but this is one option for you.

I hope this provides some ideas to get you started

Good Luck!
Jonathan

[centrespot]

Hi,
In this case I think it's pretty likely that there are some dodgy electrolytic capacitors in your machine(s) which are causing PSU issues and in the case of this vertically flipped image, also leading to deflection problems. I think the 'upside down' nature of the text here is an artefact of the wider deflection problems.

There's a useful thread on EEVBlog ( https://www.eevblog.com/forum/repair/re ... rp-mz-80a/ ) which includes a link to a YouTube tear-down video and you'll see the guy in that found some very obviously perished capacitors. There's also a useful link to the Vertical IC's datasheet which includes a reference circuit and may be representative of how Sharp implemented it.

mz80-monitor-block-diagram.jpg

The text being displayed should be at the top of the screen but as it's in the centre I think your monitor is suffering from a partial frame collapse as well as some very obvious linearity issues.... First check is to see what the main 12 volt supply looks like. Then I think you'd be wise to strip the unit down if you are safely able to and examine all the electrolytic capacitors in both the main PSU and the Monitor, with particular focus on the vertical deflection circuit.

If you have a capacitor tester then this may help you as failed caps don't necessarily have any visual defects. Any capacitors which are deformed, bulging or clearly leaking should be changed. If you don't have a capacitor tester of any kind but you do have a multimeter then if you desolder a cap and it looks okay, you can still put your probes across the legs and look to see that the ohm reading gradually goes either up or down, and then gradually does the opposite when you reverse the leads. This is a very rough test but would at least highlight if you had one that was shorted.

The https://original.sharpmz.org/mz-80k/download/sm80k.pdf full service manual includes some reference waveforms which you could test if you have an oscilloscope.

The subject of 're-capping' is always hotly debated, especially with Monitors and TVs - personally I'm not a fan of just replacing all capacitors in case they might have failed but this is one option for you.

I hope this provides some ideas to get you started

Good Luck!
Jonathan

Thank you Jonathan, I will check as you suggest.

Do you know whether I could connect an MZ80k screen to the MZ80a board to isolate whether the issue is indeed the screen/monitor

[PJTech]

Thank you Jonathan, I will check as you suggest.

Do you know whether I could connect an MZ80k screen to the MZ80a board to isolate whether the issue is indeed the screen/monitor

I'm afraid my honest answer has to be that I don't know.

If everything looks practically identical - same connectors, same colour wires etc then it's probably compatible but the only way to be certain is to compare circuits or part numbers.

I'm very certain that nature of this fault is in the monitor. However the monitor could be victim to a bad PSU.
That said, you're clearly getting EHT and the horizontal deflection seems okay, so whilst a PSU issue could still possibly cause it, I'd say it's more likely to be a problem with the vertical deflection circuit itself.

[1024MAK]

I concur that a fault in the vertical (field) defection is most likely.

CRTs are analogue systems, and in most the vertical (field) defection uses a large electrolytic capacitor and an output stage based on a transistor (or chip equivalent). Incorrect DC bias levels or defective capacitors can and do cause weird symptoms.

Mark