MZ 80K repair project

ozzybetts
Posts: 7
Joined: Sun May 31, 2020 11:33 am

MZ 80K repair project

Post by ozzybetts » Sun Jun 07, 2020 2:34 pm

Hi all,
First time posting here.
Hoping you are all well.
I recently aquired a MZ80K with the hope of getting it working, so my students can have a go with using it.
Apart from being filthy, the 'first' problem I have found is that the PSU does not work.
Image
I gave it a quick dusting, then fired it up, intending to check the voltages coming out of it; 2 fuses blew. (bottom right, by the transformer, and top right by the black/red/blue wires)
Upon taking the board out and desoldering the transformer to allow easier access, I found the board was coated in some sticky, tarry substance (not sure what that was...) I managed to get most of that off with alcohol and elbow grease.
I have also replaced all of the electrolytic capacitors apart from the 4 larger ones (which I am having trouble sourcing...)
On that note, this is the closest I can find :
https://www.mouser.co.uk/ProductDetail/ ... 4I1w%3D%3D
But it mentions they are 'Low ESR Electrolytic Capacitors'; I'm not sure what that means...so am unsure whether they would be suitable?

After replacing the other capacitors and blown fuses and cleaning the board up, fired it up and the same fuse (far right by the transformer) keeps blowing straight away.
Any thoughts? :?:

I have noticed there is some sort of deposit on these components, but had assumed it was some sort of glue, could be wrong?
ImageImageImage
Worst case scenerio, as a temporary measure, has anyone had success using a modern power supply on old equipment like this? Perhaps using similar method to the one described here:
https://www.electronics-tutorials.ws/bl ... upply.html

Looking forward to hearing your suggestions!

hlide
Posts: 403
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ 80K repair project

Post by hlide » Sun Jun 07, 2020 3:22 pm

I posted your post in FB groups dedictated to SHARP MZs to broaden the scope.

One answer from Melissa Wells for the worst case: "i replaced mine with a meanwell one and it works a treat".

hlide
Posts: 403
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ 80K repair project

Post by hlide » Sun Jun 07, 2020 3:24 pm

May I suggest to check if there is no short-circuit due to the motherboard when powering the machine on. Unless you tried it without the motherboard to make sure it is an issue from the PSU.

ozzybetts
Posts: 7
Joined: Sun May 31, 2020 11:33 am

Re: MZ 80K repair project

Post by ozzybetts » Tue Jun 09, 2020 9:18 am

Thanks for your reply, and also for passing it onto others :)
hlide wrote:
Sun Jun 07, 2020 3:22 pm
One answer from Melissa Wells for the worst case: "i replaced mine with a meanwell one and it works a treat".
It is great news that others have had success using modern power supplies to run these computers; very promising.
hlide wrote:
Sun Jun 07, 2020 3:24 pm
May I suggest to check if there is no short-circuit due to the motherboard when powering the machine on. Unless you tried it without the motherboard to make sure it is an issue from the PSU.
My first action when opening the case was to isolate and remove the powersupply. I got that idea from a very good youtube tutorial where the creator found that the voltages were way off due to warn out capacitors. With mine however I get no voltages at all onces the fuse blows :roll:

hlide
Posts: 403
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ 80K repair project

Post by hlide » Tue Jun 09, 2020 10:28 am

Ok. I also think they are glues in the last photo.

hlide
Posts: 403
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ 80K repair project

Post by hlide » Tue Jun 09, 2020 10:40 am

https://original.sharpmz.org/mz-80k/download/sm80k.pdf

Look at page 36, 37 or rather 38 if your PSU is a UK version.

So F101 and F401 blew. By replacing them, only F101 blew.

Honestly your first photo is too small to spot something wrong. You should make a higher resolution of it.

Alos from page 12, you have the steps to check for the PSU.

ozzybetts
Posts: 7
Joined: Sun May 31, 2020 11:33 am

Re: MZ 80K repair project

Post by ozzybetts » Tue Jun 09, 2020 11:34 am

Thanks for the link to that file. Looks very interesting.

Here are links to the original resolution images already posted :
https://drive.google.com/file/d/15zwgyC ... sp=sharing
https://drive.google.com/file/d/163l0Qx ... sp=sharing
https://drive.google.com/file/d/16130gS ... sp=sharing

This is an image of the back of the board : https://drive.google.com/file/d/1699NaC ... sp=sharing

This is the full board from a different angle : https://drive.google.com/file/d/167Xypi ... sp=sharing

hlide
Posts: 403
Joined: Thu Jan 25, 2018 9:31 pm

Re: MZ 80K repair project

Post by hlide » Tue Jun 09, 2020 12:09 pm

In all honesty, your board looks clean, much cleaner than the MZ-700 I had to repair once.

Back of the board: I can see some scratches but not sure they are an issue. It doesn't look you have cold soldering points but I'm not a good judge.

Full board from a different viewpoint: The only dirty spot I can see is at the bottom of the big blue cap.

Interesting enough, several researches about MZ-80 K PSU failures are related to tantalum caps (four blue ones) issues. Here is one link: https://www.nightfallcrew.com/13/01/201 ... ly-repair/.

And beware, the board is a little different because I think yours is a UK version.

S_U_C
Posts: 32
Joined: Sun Feb 17, 2019 6:41 pm

Re: MZ 80K repair project

Post by S_U_C » Wed Jun 10, 2020 2:20 pm

Check that capacitors C407 c406 c503 or c504 are OK.
I can remember repairing a K power supply (that kept blowing fuses ) for MH in Weymouth many years ago by replacing the blue capacitors which are part of the voltage setting circuit. I think one was short circuited but we replaced all 4.

ESR is short for Equivalent Series Resistance.
All PS that power computer equipment are low ESR
Search for PSs with 4 output voltages +/- 12V and +/-5V and ignore the –12V if you need a new replacement unit.

Gortho
Posts: 27
Joined: Thu Mar 22, 2018 3:53 pm
Location: Poland

Re: MZ 80K repair project

Post by Gortho » Wed Jun 10, 2020 9:28 pm

If I look at the schematics correctly, fuse near cables you are talking about is F401, on the +12v rail. Did it blow *after* you removed it from mainboard? Because there is not much there to cause it when it's not connected to anything.

That gunk is probably just kind of glue used during assembly/soldering.. or whatever was left from anti-vibration silicone/glue ;)
Leaked caps are much more... gross :)

*edit*
And +12V rail is used only in display unit. So you might have to look for shorts there.

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