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richard at home
May 8th, 2014, 03:39 AM
I picked up a lovely Sharp MZ-80k at an auction the other day. DOA but after replacing the tant caps in the psu, it has burst into life.

I managed to dig out a cassette recorder to get BASIC onto a tape and it loaded fine.

After a few days however, I now have a video RAM fault where D0 of every 16th byte is stuck.

The video ram is just two 2114 chips, soldered in unfortunately.

I tried chilling them just to see it the fault went away but no, it's staying there.

Odd that it worked fine for days and suddenly the fault appeared.

Any ideas, other than tracking down some new chips?

I have the schematics and I have a logic analyser which I might hook up, just to see what's going on...

Looking for some ideas...?

jltursan
May 11th, 2014, 11:19 PM
First of all, welcome to the board :-)

I suppose your diagnostic is good, maybe some photos of the fidplay full of chars could be of help anyway.
The RAM ICs are prone to faults, specially while messing with PSU and such. Being only two, it's a relatively easy task to socket the ICs.

The 2114 is an easy to find RAM, eBay is full of offers, maybe not cheap; but you'll find enough supplies for sure.

richard at home
May 12th, 2014, 12:51 AM
First of all, welcome to the board :-)

I suppose your diagnostic is good, maybe some photos of the fidplay full of chars could be of help anyway.
The RAM ICs are prone to faults, specially while messing with PSU and such. Being only two, it's a relatively easy task to socket the ICs.

The 2114 is an easy to find RAM, eBay is full of offers, maybe not cheap; but you'll find enough supplies for sure.


Hi jltursan,

Thanks for the welcome!

I have a pair of 2114's enroute as we speak! They were pretty cheap off ebay (99p)!

I'll socket them and see what happens.

It's an odd fault as it seemed to get progressively worse over time. Initially fine and now covering most of the screen.

It's also gone from Spaces turning to 'A' (char code zero changing to char code one) to some more characters.

This is what I see now on power up:

18678

and after Clear Screen:

18679

richard at home
May 14th, 2014, 11:08 AM
All sorted!!!!

I cut the old video rams out, put sockets in and plugged in some new chips from ebay and she's working a treat!

I'll leave her on for a while to burn in.

The motherboard is completely mint, no mods anywhere. It's a great machine and I can still remember dreaming of owning one when I was at school!

I had to make do with a zx81 in kit form, which I soldered together. It worked first time!!!

jltursan
May 18th, 2014, 11:22 PM
Good to hear that your MZ is alive!.

Some time ago mine also needed maintenance. The PSU weaks over time and it usually ends failing, I replaced it with a PC one and now I suppose that it will last for some more years :-)