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-   -   Sharp MZ-80K help needed on repair (https://www.vintage-radio.net/forum/showthread.php?t=183455)

pudwink 6th Sep 2021 12:53 pm

Re: Sharp MZ-80K help needed on repair
 
HI
I dont mind any sugestions at all I am a novice with a love of old computers and I am ahead of you with the ram I have ordered a dram tester which with any luck will arrive tomorrow only issue will be a replacement source for any ram chips that are bad.

SiriusHardware 6th Sep 2021 2:13 pm

Re: Sharp MZ-80K help needed on repair
 
The 4116s (if that is what they are in your machine) were used for the lower (first 16K) of memory in the Sinclair Spectrum for which there are still a lot of spares lying around - or so I thought. This seller did have them at reasonable cost but is currently out of stock.

https://www.retroleum.co.uk/zx-spectrum-chips

As you already have a RAM tester on the way it makes sense to work through what's in the machine and find out whether all, some, or none of the RAMs actually need replacing first. If you have a general interest in retro computers that tester will be a very useful thing to have anyway.

If it seems you do need one or more then you could maybe harvest some RAM from a spares-or-repairs Spectrum PCB, if you can't find any for sale individually.

Mark1960 6th Sep 2021 3:22 pm

Re: Sharp MZ-80K help needed on repair
 
What speed rating of 4116 are needed? Maybe you could tell us the full part number of the ram chips fitted to your mz80k.

From the photo you posted earlier it looks like the ram is all socketed and all three rows populated. If not for the 19v overvoltage I might have expected you to have enough working ram chips for one or two rows at least.

Try and verify the display circuit working first and that the z80 is running before spending money on ram replacement.

ABRA electronics have 200ns available at CA$1.07, and might be worth the shipping charge for the quantity needed.

SiriusHardware 6th Sep 2021 3:39 pm

Re: Sharp MZ-80K help needed on repair
 
Pud mentioned that he already has an independent RAM testing gadget on the way, (#21) so there should be no need to replace any that are not actually dud.

pudwink 6th Sep 2021 4:01 pm

Re: Sharp MZ-80K help needed on repair
 
2 Attachment(s)
The first bank looks to be the original sharp memory the other banks must of been added later.

Mark1960 6th Sep 2021 4:06 pm

Re: Sharp MZ-80K help needed on repair
 
Quote:

Originally Posted by SiriusHardware (Post 1404426)
Pud mentioned that he already has an independent RAM testing gadget on the way, (#21) so there should be no need to replace any that are not actually dud.

Yes, but not sure how many might survive 19v on the 12v supply.

SiriusHardware 6th Sep 2021 4:10 pm

Re: Sharp MZ-80K help needed on repair
 
I think I meant that there was no need to caution against buying RAM unnecessarily since pudwink will soon have the means to determine which ones are or are not dud.

And look, they are even in sockets. How lucky is that? An interesting mix of devices, so as pudwink suggests, probably fitted in two stages.

Mark1960 6th Sep 2021 4:33 pm

Re: Sharp MZ-80K help needed on repair
 
Quote:

Originally Posted by pudwink (Post 1404437)
The first bank looks to be the original sharp memory the other banks must of been added later.

I think the -3 is indicating 300ns access times, though I havenít found a sharp data sheet online to confirm. I was a bit surprised that the service manual didnít quote the access time.

Timbucus 6th Sep 2021 8:01 pm

Re: Sharp MZ-80K help needed on repair
 
I recently repaired a Dragon with a failed -5v rail that was passing well below that onto the rail and 15 of the 16 RAMs survived. So you never know your luck - it was of course the last one I de-soldered that had failed - I was luckily taking them all out on the way to making it a 64K machine so that is the first step anyway.

pudwink 9th Sep 2021 11:10 am

Re: Sharp MZ-80K help needed on repair
 
Hi all
Dram tester came today but when I took out all the chips I did not need it to see they were an issue.
Ram 1 row with the sharp 4116 all passed but the next two banks with the ITT 4116 are all badly corroded and fail I have cleaned one up but still failed.
Now looking at https://www.sharpmz.no/articles/the-...ints-and-tips/
It looks like you can only set it up to 20k min do you think if I set the jumpers to 20k but with just 16k installed would it boot.

SiriusHardware 9th Sep 2021 1:36 pm

Re: Sharp MZ-80K help needed on repair
 
Hmm, awkward as it looks like the only configuration using ONLY 4116 is the top whack, 48K.

I wonder if there is a way to (temporarily) wire in a larger more modern device to substitute for the upper two 4116 banks - just to determine whether the RAM problems are the only problems the machine has.

It's also possible that somebody already makes a plug-in RAM replacement board for these just as they do for the PET. I know pud wants to get it working as-is and would eventually replace all the original faulty RAM if possible, but it is always handy to have a different set of RAM (of whatever sort) to swap in for diagnostic purposes.

pudwink 9th Sep 2021 6:38 pm

Re: Sharp MZ-80K help needed on repair
 
Just ordered 20 4116 drams so hopefully might get it going yet I have also programmed up a new cgrom.

Mark1960 9th Sep 2021 7:05 pm

Re: Sharp MZ-80K help needed on repair
 
Does the display look different with the new CGROM?

pudwink 9th Sep 2021 7:17 pm

Re: Sharp MZ-80K help needed on repair
 
No different so I am hopeful that is a good sign.

pudwink 9th Sep 2021 7:18 pm

Re: Sharp MZ-80K help needed on repair
 
I mean the same

SiriusHardware 9th Sep 2021 8:35 pm

Re: Sharp MZ-80K help needed on repair
 
That is actually good as it means you can retain the original CGROM. You'll want to save as many original parts as you can. It's also good to have a spare, it almost guarantees that the original part will never fail.

Mark1960 9th Sep 2021 10:09 pm

Re: Sharp MZ-80K help needed on repair
 
It might mean there is another fault in the display circuit causing the vertical stripes, it doesn’t mean the original CGROM is good.

I would have expected a memory fail message on the display if only the dram was faulty, or random characters, but I don’t know what is expected for the mz80k.

SiriusHardware 10th Sep 2021 1:33 pm

Re: Sharp MZ-80K help needed on repair
 
As pudwink now has a device reader and presumably the correct CGROM code from some online source it should be possible to read the code from the original CGROM and verify it against the known good code.

I should have suggested that earlier, I didn't know pud was going to be so quick off the mark ordering a replacement device for the CGROM position. But as I said, a spare is never a bad thing to have and can often function as a surreal kind of insurance against the original part failing.

SiriusHardware 10th Sep 2021 4:12 pm

Re: Sharp MZ-80K help needed on repair
 
For anyone following, the CGROM code is here:

https://original.sharpmz.org/

Use the red buttons on the left hand side of the menu to open / expand

Download - MZ80C\K - ROMS

pudwink 10th Sep 2021 5:56 pm

Re: Sharp MZ-80K help needed on repair
 
Yes that's where I got it from ,it will be next week before the new 4116 chips turn up this little project has cost me a bit up to now but would really like to see it running so some kind of oscilloscope might be needed could anyone recommend a very cheap version I could buy that would work on fault finding on this computer.


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