Sharp MZ-80K help needed on repair
 

Hi All
I have a Sharp MZ-80K which when turned on I only get vertical lines that move from side to side.
I checked the voltages from the power supply and the 12v was giving out 19v so replaced a few capacitors and back to 12v but still the lines so looking at the board and the socketed ICs had some corrosion so I have replaced the sockets and put in a brand new Z80 CPU but still exactly the same if anyone has any ideas out there what to try next I would be very grateful.
Here is a link for a video of what the computer is doing
https://youtu.be/1CaQtj-JQvE
But ignore the flashing that is how my phone has took it.

Re: Sharp MZ-80K help needed on repair
 

Hmm strange - if I was a betting man from other systems that looks like perhaps a flashing bit being set and the same value returned for all screen memory addresses - so I would start assuming either the 2K video RAM is not being scanned, has failed or the default power on value displays that pattern and the main machine never runs to clear it.

I do not know the machine but looking quickly at a tech description (https://original.sharpmz.org/ it seems it has the usual multi rail RAM's they need +5v, 12v and -5v - any deviation will usually fry them. The easiest way may be one of the these cheap RAM testers and a desolder. If you can point us to some confirmed circuit diagrams then perhaps we can help more and identify a some tests that would pin it down a bit closer.

Re: Sharp MZ-80K help needed on repair
 

I have the service manual if that helps

Re: Sharp MZ-80K help needed on repair
 

Do you have a scope so that you can look at various waveforms as per the manual?

I would concentrate to make sure that the Z80 clock is working as a first step and then confirm that the circuit for the Video on Page 31 is working as you would expect.

A failure of the CG-ROM could also provide the effect you see along with RAM (Less likely to have failed as a static single rail but, still possible) or whatever cycles the address lines for display - but, if the address lines on IC41/42 are incrementing consistently (higher frequency the lower the address line) then you will know that the RAM is being scanned. The upper address lines (A10 down to A4) on the CG-ROM would display the static high/low values of the ASCII for the vertical line character if they are not cycling - also check that A3-A1 (Pins 6,7,8) are cycling for the character rows. If it is socketed then a small arduino code as per other threads to cycle each address line could confirm its contents is another good start.

Re: Sharp MZ-80K help needed on repair
 

I do not have a scope but I do have a logic probe if that could be used to find anything out.

Re: Sharp MZ-80K help needed on repair
 

Indeed - the probe would allow you to see if the pulses are present on the scanning lines I suggest - even though you can't see the frequency you can work out what they have on them - so start with Pin 6,7 and 8 of the CG-ROM (In the area around IC39-IC43) they should all be pulsing at slightly different frequencies as the characters are generated.

Next see if A0-A9 on IC42 or IC41 which are Pins 5,6,7,4,3,2,1,17,16 and 15 are also pulsing - that shows that the RAM is being read out...

Finally back to the CG-ROM and give the binary value of D0-D7 9,10,11,13,14,15,16 and 17 - if they are pulsing then that is also a result.

You might be able to judge the speed of pulsing on the RAM A0 should be pulsing fastest and A9 slowest but, that is a guess it could be the other way around as I do not know the RAM layout.

Re: Sharp MZ-80K help needed on repair
 

It might also be helpful to post pictures of the main board so we can see what is in sockets as that might help us to suggest some tests.

For example if CG ROM is socketed you might easily remove it, then ground each of its output pins and verify a different pattern of vertical stripes on the display.

Do you have access to a prom programmer that could read the contents of CG ROM.

Re: Sharp MZ-80K help needed on repair
 

Hi
I have tested all the points Timbucus sugested with my logic probe and on every point all three lights were lit which the instructions for my probe does not say what that means.
The CG Rom is socketed and I have just purchased a TL866II plus programmer hopeing it might help in this repair but not sure if this will read the roms or not maybe someone can tell me.

Re: Sharp MZ-80K help needed on repair
 

I don't actually use a probe so can't help on that - I suppose we should establish what that situation means - perhaps an indication of a square wave? What shows if you probe the Z80 Clock pin (pin 6)? - Also confirm that the Ground (Pin 29) and 5v (pin 11) show an expected Low and High to prove the probe is working.

Re: Sharp MZ-80K help needed on repair
 

In terms of reading the ROM then it looks like it is a standard single rail 2716 so should readable in the TL866II

Re: Sharp MZ-80K help needed on repair
 

If all three leds are lit then you have a signal pulsing high and low, sometimes from relative brightness you can tell if the signal is mostly high or mostly low.

Re: Sharp MZ-80K help needed on repair
 

Well if they are all about the same then that seems like a square wave with a regular period which would seem to indicate that the memory is being read out and so is the ROM. In that case the relative brightness might show the RAM address line difference as with a slow wave on the +Ve or -Ve on the higher numbers and more even on the lower - or vice versa.

I think knowing the content of the ROM and as Mark said perhaps forcing a low or fixed pattern on all its upper addresses will be the best approach to prove the main video and allow us to concentrate on the Video RAM, CPU and Logic.

I am going to predict some main RAM failures are they are the multi rail Dynamic RAMS that die with excess voltages.

Re: Sharp MZ-80K help needed on repair
 

While waiting for the programmer try removing the CGROM, that should show a white screen. Then if you ground each data pin in turn you should see vertical bars in eight different positions.

You could also connect A1, A2 and A3 to a data line and see a different pattern in the vertical bars.

Might also be possible to see something useful by connecting A4 to A11 to one of the CGROM data lines.

Re: Sharp MZ-80K help needed on repair
 

When you ground each pin in the socket use very thin wire and just push gently as otherwise the socket will be widened and need to be replaced. Again as Mark says well worth trying this even while waiting for the programmer. Especially proving the vertical lines.

Re: Sharp MZ-80K help needed on repair
 

Just hovering... I agree that the 19V from the 12V regulator may have damaged anything which was running on it, including the RAM. I haven't seen any mention of the the +5V supply and -5V supply voltages being checked yet, though?

Re: Sharp MZ-80K help needed on repair
 

Calm collected project manager thinking there - I just assumed that they were OK as the 12V had been repaired but, we all know what assumptions lead too...

Re: Sharp MZ-80K help needed on repair
 

Well, this would have been pretty hard to take from the person who spectacularly failed to notice no +5V supply to his RAM on another recent project.

It did go to show, though, that everything, no matter how unexciting and unlikely, needs to be checked and crossed off the list of suspects.

Re: Sharp MZ-80K help needed on repair
 

The power supply is showing 12v +5v and -5v I also tried the logic probe on the Z80 and pin 6 all light lit again but pin 29 and 11 showing low and high as expected.

Re: Sharp MZ-80K help needed on repair
 

That is great news the Probe is working as we expect, there is a clock signal and all the power rails are OK. So I think if you try the tests in #13 and #14 next we can pin down what is happening until you can read out the character ROM.

Re: Sharp MZ-80K help needed on repair
 

Pudwink, forgive us if we initially suggest things which are obvious or which you have already thought of (Ref: Supply checks, etc).

Because we haven't really 'met' you in this section before, it is difficult to know how basic or complex any suggestions need to be. We'll get a better feel for that as time goes on - for example, we can already see that you obviously do know how to use a meter.

Let us know how you get on with those checks Mark and Tim asked for. I have a feeling you are going to need some way of testing the RAMs as well, due to the likelihood that they have been overvoltaged.

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.

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.

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.

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.

Re: Sharp MZ-80K help needed on repair
 

The first bank looks to be the original sharp memory the other banks must of been added later.

Re: Sharp MZ-80K help needed on repair
 

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

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.

Re: Sharp MZ-80K help needed on repair
 

I think the -3 is indicating 300ns access times, though I haven’t found a sharp data sheet online to confirm. I was a bit surprised that the service manual didn’t quote the access time.

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.

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.

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.

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.

Re: Sharp MZ-80K help needed on repair
 

Does the display look different with the new CGROM?

Re: Sharp MZ-80K help needed on repair
 

No different so I am hopeful that is a good sign.

Re: Sharp MZ-80K help needed on repair
 

I mean the same

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.

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.

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.

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

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.

Re: Sharp MZ-80K help needed on repair
 

ScottishColin bought a Hantek 6022Bl which he used for quite a lengthy debug of his PET 3016, and this seemed to provide quite good results. The MZ80K looks to be similar technology level so it might be worth looking at one of those. Note these rely on a PC to provide the display and controls, probably needs windows. The difference between the 6022BE and 6022BL is that the BL includes a 16 channel logic analyser, but I don’t remember if Colin used that feature.

Re: Sharp MZ-80K help needed on repair
 

Sirius persuaded me to buy a standalone digital scope a while back and it was the best investment I made. The fact it is not tied to a computer and is light and easy to move makes it very versatile. I was really lucky with an offer on Amazon at the time to get the 100Mhz version by Hanmatek the DS1102 and now they are only around £40 dearer.

https://www.amazon.co.uk/DOS1102-osc.../dp/B0833X3RFK

While just looking for it I notice the OEM OWON (who make a lot of these clones and is the origin of the badged Hanmatek list the slower model - fast enough for older machines like the Sharp) if you want slightly cheaper:

https://www.amazon.co.uk/SDS1022-Osc.../dp/B07RD6FMM7

One thing I have missed on mine is the external trigger which my 20MHz Hanmeg features. No idea on the brand but, these are probably all much of a muchness I spotted this one which has the 100Mhz and an external trigger - probably well worth the extra £30 and a great foundation for developing your electronics skills...

https://www.amazon.co.uk/UNI-T-UTD21.../dp/B092ZB7678

Re: Sharp MZ-80K help needed on repair
 

All of the above is good advice but unless pudwink has deep pockets, why not wait and see if fixing what appears to be a definite RAM fault fixes the machine? As he says, he has already laid out quite a bit on replacement RAM, plus a 2716 Eprom which he did not necessarily need.

Pudwink, when the RAM arrives I suggest you test every one of the new chips in your tester before putting them in the machine. All of us here have had experiences recently of finding out that one out of x chips in a batch of new RAM was dud - hopefully yours will all be OK but it's better to check them independently before putting them in.

Are you planning to replace the sockets that the corroded RAMs were in, or do you think they look OK?

Re: Sharp MZ-80K help needed on repair
 

Agreed - we are not at a stage where we need to bring a scope to bear - it is a luxury investment as he has a logic probe and meter.

I recently found another set of new RAM chips that had 1 of the 8 faulty - as it was the last bit of a 16K I had been chasing an addressing issue for the second bank for ages as the onboard test failed at 7FFF - not 8000 which would have been in the next bank... I just didn't think...

Re: Sharp MZ-80K help needed on repair
 

Dont worry every chip will go through the tester first I dont fancy replacing all the sockets so I will proberly fit the new chips first see what result I get then if the same look at all the sockets then if still the same get some tissue to wipe the tears of frustration and come back to you guys for some expert help.

Re: Sharp MZ-80K help needed on repair
 

Maybe after fitting the chips perform a continuity check to the pins, at least for the supply and ground pins. If one of the three voltages is missing it could possibly damage the new chip.

Re: Sharp MZ-80K help needed on repair
 

Also, with all of the dud chips still out, power on and measure on the pins of the sockets to make sure that you have all of the expected supply voltages on the power supply pins of all of the sockets.

Re: Sharp MZ-80K help needed on repair
 

I didn't want to but looking at the socket pins under a magnifying glass there is some corrosion on some of them so I surpose I am going to have to replace all 24 sockets for peace of mind oh joy.

Re: Sharp MZ-80K help needed on repair
 

You could try some spray contact cleaner first - I have rarely had to replace a socket unless the pins had no grip left or were actually black.

Re: Sharp MZ-80K help needed on repair
 

The ram has turned up today out of the 20 chips 8 passed on my tester 1 failed totally 2 showed only half good and 9 passed the high speed test but failed the retension test so I dont know if they are good or not.
I have put them in anyway and still the screen is the same no different.