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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 4th Sep 2021, 10:29 am   #1
pudwink
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Default 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.
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Old 4th Sep 2021, 11:20 am   #2
Timbucus
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Default 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.
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Old 4th Sep 2021, 12:06 pm   #3
pudwink
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Default Re: Sharp MZ-80K help needed on repair

I have the service manual if that helps
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Old 4th Sep 2021, 12:29 pm   #4
Timbucus
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Default 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.
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Old 4th Sep 2021, 12:37 pm   #5
pudwink
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Default 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.
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Old 4th Sep 2021, 12:52 pm   #6
Timbucus
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Default 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.
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Old 4th Sep 2021, 4:38 pm   #7
Mark1960
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Default 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.
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Old 4th Sep 2021, 6:24 pm   #8
pudwink
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Default 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.
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Old 4th Sep 2021, 7:06 pm   #9
Timbucus
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Default 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.
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Old 4th Sep 2021, 7:11 pm   #10
Timbucus
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Default 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
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Old 4th Sep 2021, 8:25 pm   #11
Mark1960
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Default Re: Sharp MZ-80K help needed on repair

Quote:
Originally Posted by Timbucus View Post
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.
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.
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Old 4th Sep 2021, 8:35 pm   #12
Timbucus
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Default 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.
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Old 4th Sep 2021, 8:44 pm   #13
Mark1960
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Default 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.
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Old 4th Sep 2021, 8:51 pm   #14
Timbucus
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Default 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.
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Old 5th Sep 2021, 8:52 am   #15
SiriusHardware
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Default 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?
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Old 5th Sep 2021, 9:41 am   #16
Timbucus
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Default Re: Sharp MZ-80K help needed on repair

Quote:
Originally Posted by SiriusHardware View Post
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?
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...
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Old 5th Sep 2021, 10:10 am   #17
SiriusHardware
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Default Re: Sharp MZ-80K help needed on repair

Quote:
Originally Posted by SiriusHardware View Post
I haven't seen any mention of the the +5V supply and -5V supply voltages being checked yet...?
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.
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Old 5th Sep 2021, 4:15 pm   #18
pudwink
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Default 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.
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Old 5th Sep 2021, 6:18 pm   #19
Timbucus
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Default 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.
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Old 6th Sep 2021, 10:56 am   #20
SiriusHardware
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Default 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.
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