Re: Sharp MZ-80K help needed on repair
 

I see, it looks like there are four holes in a square for it to fit in.

No matter, plan 'b', as you suggested - with the computer switched off, monitor pin 26 of the Z80 on the scope and turn the computer on. The main thing we are looking for is that it should be normally 'high' (>3.5V or so) when the computer is on, and not stuck permanently low.

As you are using only scope channel 1 for this, the 'mode' switch needs to be set to 'CH1', that turns on channel 1 and turns channel 2 off. The 'INT TRIG' switch below that also needs to be set to 'CH1'. The 'Source' switch, upper right, also needs to be set to 'INT'.

When you've looked at that, try scoping those two clock signals mentioned earlier.

Re: Sharp MZ-80K help needed on repair
 

First go with the scope this morning first picture is pin 8 ic7 picture 2 pin 8 ic9 picture 3 Z80 pin 26 sometimes when I turn it on only get a black screen or sometimes get the garbled screen then it goes blank then as in picture 4 get something completly different.

Re: Sharp MZ-80K help needed on repair
 

You're using the scope in dual-trace mode which makes the screen unnecessarily 'busy' when you are only looking at one thing.

As I mentioned

Set 'Mode' switch to 'CH1' to switch off the second trace.

Your clock signals appear present and OK - I'm at work at the moment so we'll have a think about where to go next.

Re: Sharp MZ-80K help needed on repair
 

Hi
I was wondering about the second line but it is in CH1 mode does that mean there is an issue with the scope.

Re: Sharp MZ-80K help needed on repair
 

Just had a play with the scope and if I press the CH1 ALT MAG button it gets rid of the other line and gives an overlay of information of the settings and tracking cursor.

Re: Sharp MZ-80K help needed on repair
 

There are at least two switches which have 'CH1' written on them, can you verify that the 'MODE' switch (just below the centre of the front panel) is set to 'CH1'?.

If it is, it may be that the MODE knob has been taken off at some point and put back on with the indicator line pointing in the wrong direction, check that when the 'MODE' switch is turned anticlockwise as far as it will go, the line on the 'mode' switch points to 'CH1'.

Re: Sharp MZ-80K help needed on repair
 

Ah, OK, it sounds as though that scope has an unusual feature which shows part of the upper trace, magnified, on the lower trace.

Re: Sharp MZ-80K help needed on repair
 

If you are turning the power off and back on the power on reset might not work correctly if the power is off for only a short time, then the z80 could start executing from a random address and could cause it to start writing data to every memory location. Repeat call instruction could give the pattern you showed in picture 4. If you always wait 15 or more seconds before turning on, does it always briefly show the random characters and then a blank screen?

Re: Sharp MZ-80K help needed on repair
 

No sometimes just get blank screen other times random for a shortime then blank screen also get some popping from the speaker.

Re: Sharp MZ-80K help needed on repair
 

But you don’t see #102 picture 4 if you wait before turning back on?

So I think with the new MROM the screen clear is completing ok, but not going far enough to display any message.

Re: Sharp MZ-80K help needed on repair
 

As I understand it though the image posted in #93 was the result of turning the machine on from a cold start, or was that not the case?

Pudwink, switch the machine on and off as many times as it takes to get it to show something other than a blank screen, then form a metal hoop from a stiff piece of wire (or get a fine tipped pair of tweezers) and with the power already on, try momentarily shorting each of the four reset switch holes to the other three in turn. (I'm assuming the switch holes have solder pads on the top side of the PCB, it is difficult to see from your original image).

What happens if you do that, does the screen clear, or does it change in some other way, or does it have no effect at all?

Edit: Cross posted with pudwink's post #112.

Re: Sharp MZ-80K help needed on repair
 

Picture in 93 was after a cold start picture 4 in 102 could just be a one off it just if I get a black screen then I turn it off and on again.

Re: Sharp MZ-80K help needed on repair
 

The service manual posted by pudwink in #3 mentions a RAM diagnostic ROM, which would be useful both from the point of view of having a RAM diagnostic test and also just something else to try booting from.

Unfortunately the source which provided the code for the CGROM and the MROM does not appear to have a file copy of this diagnostic ROM, does anyone else have it? I found the source code for a RAM tester here

https://forum.sharpmz.org/viewtopic.php?t=180

but it is obviously intended to be RAM resident and uses subroutine calls which supposes that the machine must be 95% working already. What we'll eventually need is a minimal ROM resident RAM test which can be plugged in as an alternative to the MROM, and does not itself use RAM.

However, I am not wholly convinced that the machine is running any code at this moment.

Re: Sharp MZ-80K help needed on repair
 

I 'borrowed' the attached image from an article elsewhere explaining how to add a reset switch to the MZ80K, it usefully shows which of the pads for the absent switch need to be joined together momentarily to reset the machine - the pads marked '1' and '2' in the image. To reset the machine, momentarily join those pads together with a wire link or the points of a pair of tweezers. (We can clearly see from this image that the holes for the switch do have solder pads on the PCB top side).

There are two distinct inputs to the Z80 reset on these, one is from the reset switch via IC31 (74121), and the other is from the power-on reset circuit derived from R28/C19, the associated 2SC373 transistor and one gate of IC4, (7414).

It may be that the power-on reset circuit is not working properly, so this is why I want to see what happens if we can force a manual reset when the machine is already powered on.

Re: Sharp MZ-80K help needed on repair
 

I will give it a go in the morning at work at the moment I have also got hold of a ram checker file so will put that on a rom and see what happens.

Re: Sharp MZ-80K help needed on repair
 

Do we know the history of this mz80k? Has it worked in the past with 48K ram? Just wondering if we need to verify the links are set correctly for three banks of 16k, or was the extra ram fitted by a previous owner without setting the links?

Re: Sharp MZ-80K help needed on repair
 

Know nothing about the history but as far as I can tell the links are set correctly.I have just got home from work and quickly burned the ram checker rom and it has booted up into a ram test screen which I hope is a positive sign.Iwill try and work out how to run it in the morning unless anyone knows the procedure.

Re: Sharp MZ-80K help needed on repair
 

If the ram checker is the one described on page 10 of the service manual then I think we just need to interpret the result shown on the display.

Re: Sharp MZ-80K help needed on repair
 

Yes, this sounds like a giant leap forward, if the RAM checker code is able to run from ROM then we can say that probably 95% of the system is working.

MROM is expecting fully working RAM and will be trying to use it, so it will most likely fall over if there is a RAM fault. The RAM checker ROM probably avoids using any system RAM itself, so it is able to work.

You might also want to try programming another MROM just in case there happens to be a problem with the first one.

pud, can you link to where you found this RAM checker ROM code?

Re: Sharp MZ-80K help needed on repair
 

Also - in addition to making another MROM try programming an EPROM with the 'new monitor' code from that repository of ROM code on the original site, just in case the SP-1002 (original monitor) code held on that site is faulty.

Re: Sharp MZ-80K help needed on repair
 

This is what I am getting with the ram tester

Re: Sharp MZ-80K help needed on repair
 

been playing with it this morning I either get the ? screen most times or slightly corrupt screen or just plain black screen.

Re: Sharp MZ-80K help needed on repair
 

I would have thought that the fact the RAM tester program runs and produces a sensible display implies that :

The Z80 CPU is running, and can execute machine code programs

The Z80 can write to the video memory

The video memory is basically working

The video display hardware can read out the video RAM and display it correctly

(The video monitor is working too)

That means that quite a bit of the machine is working correctly, which is good.

What happens if you switch off, pull out one of the RAM chips and power up again. Do you get the same display or does the '?' change to something else?

Re: Sharp MZ-80K help needed on repair
 

Looking at the above I agree with Sirius it looks like huge swathes of the machine are working fine... I think we can perhaps suspect the address decoding circuitry - I will try and explore specifics later when I get back home and can look at the circuit again.

Re: Sharp MZ-80K help needed on repair
 

Taking out a ram chip made no difference still got ? I did once get this display as shown in the pic which I am guessing what the icons should be showing but I dont think it would be a reliable state of the memory.

Re: Sharp MZ-80K help needed on repair
 

It seems we all agree there isn't much wrong with the machine now.

Interpreting that display - it looks like a very good and useful utility actually - we can see from the 'key', bottom left, that it should indicate a blocked-in circle or 'dot' where the RAM device is good (=OK), and an 'X' where the RAM is bad (=KO).

Instead it's showing '?' which I'll take a guess means 'not found', ie, none of the locations within that device returned a valid value when written to and read back from.

Given that the original RAM, all of it, was subjected to +19V, it would be entirely possible for all of those devices to be damaged so I would be interested to know what happens if some of the recently purchased RAMs are swapped in instead, if that has not already happened.

However there is also the possibility that there is a damaged buffer sitting between the CPU and the system RAM, and that is making all of the RAM look faulty. I haven't looked yet to see if there is such a buffer, just going to have a look at that now. It's also possible that there is a problem with the RAM refresh circuitry or perhaps as Tim suggested, the address decoder

I'm also still interested to know if the RAM test ROM runs correctly every time if the machine is reset by shorting the reset switch contacts rather than by powering it off and on.

Edit: Crossed with pud's last post. Maybe there is one, or several dud RAMs which are messing up the rest. Try methodically replacing one of the existing RAMS at a time to see if you can consistently get that O/X display instead of the '?'s.

Re: Sharp MZ-80K help needed on repair
 

There is a buffer (IC50) between the RAM data outputs Do0-Do7 and the CPU's D0-D7 bus but the outputs of the monitor ROM are also on the Do0-Do7 bus, so the fact that the machine is (mostly) able to run code from ROM suggests that data from both RAM and ROM is passing through this IC OK.

Re: Sharp MZ-80K help needed on repair
 

All the ram chips in the machine fully passed the dram tester I have unless some of the sockets are dodgy I don't know if we get a consensus you think it's the ram then I will replace all the sockets which will be a pain. Unless in the short term someone suggests something else to try first.

Re: Sharp MZ-80K help needed on repair
 

You have a 'scope, I believe. I'd look for 'activity' -- square waves or pulses -- on the pins of the DRAM chips. I am guessing these are 4116-like devices (sorry, I don't have the MZ80K service manual to hand, I have an MZ80B...) in which case :

Power (check these with your multimeter):
Pin 1 : -5V
Pin 8 : +12V
Pin 9 : +5V
Pin 16 : Ground

Address inputs, there are 7 of this on pins 5,6,7,10,11,12,13. All should be showing changing logic states

Data In : pin2 and Data Out : pin 14 should also almost certainly be changing

RAS/ : pin 4 should be changing state, it would even be used for refeshing the RAM even if the CPU is not trying to access it

CAS/ : pin 15 and Write Enable : pin 3 could be high all the time if the machine is not accessing that part of memory, or if it is not writing to memory. I would certainly expect them to do something after a reset, it may be worth wiring a push-to-make switch to the reset switch 'holes' on the PCB so you can reset the machine easily.

Re: Sharp MZ-80K help needed on repair
 

Tony, the service manual (quite a decent manual and a good quality scan) was posted in #3 of this thread.

pudwink: What Tony said. Try scoping all of those pins on every one of the RAMs. You could also turn the power off and patiently use your meter to check that every RAM IC pin is connected (zero ohms) to its associated top side PCB pad - this may avoid you having to replace any or all of the sockets. It would be tedious, but not as tedious or expensive as replacing all the sockets on the off chance that one or more of them might possibly be faulty.

If you do want to replace the sockets (and we are not suggesting you do yet) then the way which has been suggested here before is to snip out the cross spars on the original sockets so that each socket becomes two single inline sockets, then rock those gently back and forth from side to side until all the pins snap, then desolder the pins / clear the holes individually. Note this method should NOT be used to remove turned-pin sockets, but I don't think they will be turned-pin types in something this old.

Re: Sharp MZ-80K help needed on repair
 

Try removing all the ram, check what you get on the display.

Then fit only one ram after testing it first in your ram tester.

You could also try that single ram in multiple positions to check it gives the same result.

If this seems to show you have a good ram chip, try again with different ram chips, only one fitted, until you have enough to try a full bank.

Re: Sharp MZ-80K help needed on repair
 

With the ram test eprom I got it through the sharp MZ Facebook group and apparently the guy who gave it me says (the upper left digit should progress from step 0 to 3 then reload he says it seems to be stuck and could be an issue with VSYNC polling) whatever that means.

Re: Sharp MZ-80K help needed on repair
 

"Polling" in general means a method whereby the system continually looks at a signal to see what state it is in and responds accordingly.

Vsync is the signal which occurs at the start of every video frame and your advisor is suggesting that the RAM test program uses the Vysnc signal for timing / synchronisation purposes and can not proceed because it is not 'seeing' the vsync signal.

In hardware terms it looks like the MZ80K uses port pin PC7 (pin 10) of IC5, the 8255 IC, to read the vertical blanking signal, V-BLANK, so with the RAM test ROM running, try scoping IC5 pin 10 to see if you have a waveform there. I think it will be mainly 'high' with short 'low' pulses so you may have to 'tune around' with the Time / Div control in order to see them.

On this particular waveform you may get a better 'picture' if you locate the rotary trigger threshold control, upper right on the front panel, and pull it outwards towards you - that tells it to trigger on the falling edges of signals rather than the rising edges.

It might also be worth looking at the V-GATE signal on IC5 pin 14.

Re: Sharp MZ-80K help needed on repair
 

I have hit a problem I took the board out to wire in a reset button all done correctly put the board back in and got the lines on the screen back.I redid the eprom and now all I get is this.I have removed the reset switch as well no difference.

Re: Sharp MZ-80K help needed on repair
 

I took the board out again I have replaced the M-Rom socket again just in case and reflowed the Z80 socket just incase pulling the power supply in and out (its quite tight)might of cracked a joint and turned it back on got my ram-test screen again thought I had sorted it but turned it off and on again and back to the screen in my last post.
I have put the reset switch in again but dont think the reset is working if I press it sometimes it blanks the screen press it again and screen comes back with different garbled screens.I also put the oscilloscope on pin 26 of the Z80 and it just remains high pressing reset does nothing.

Re: Sharp MZ-80K help needed on repair
 

Just checked IC5 pin 10 photo 1 pin 14 photo 2 IC 5 is not in the best condition and I had to replace 2 of the legs because of corrosion.

Re: Sharp MZ-80K help needed on repair
 

That VGate does not look healthy on pin 14 - it should be +5v or 0v not at 1v - it ends on IC24 pin 1 - so perhaps also worth checking the signal on Pin 3 of IC24 - in that state I cannot see how it will act at all well - unless I have missed something. That gate after the inverter on ICT21 ensures the blanking signal is gated correctly on the three input NAND IC17 but, we seem to have working video - so this fault condition could account for the Black video as it is always blanked.

What model EPROM programmer do you have? If it is a TL866 then it has logic gate test ability so removing IC24 and testing would not hurt. While it is out scope Pin 14 again to ensure it is the full 5v - there may be other points it terminates on I have missed.

Re: Sharp MZ-80K help needed on repair
 

Pin 3 of IC 24 shows as per picture I have a GQ 4x4 programmer but as I will destroy the chip getting it out as its not socketed and I have no replacements will have to leave it in.
Do you think it is worth ordering a replacement NEC D8255AC-5 (IC5) to see if it makes a difference as I said I had to replace two legs because of corrosion.

Re: Sharp MZ-80K help needed on repair
 

That as I suspected is the reason if you had a black screen at the time. It is hard to say if it is the 8255 not pulling it up to 5v or IC24 pulling it down to 1v. If the 8255 is socketed then substituting it could be worthwhile but, don't leave it too long if you find it only has the 1v on pin 14.

As the first step of removing IC24 you would plan on snipping the pins so if you snipped pin 1 near the top and bent it out - it should return the signal to full 5/0 movement and indicate if a new 8255 was a good idea. It would be possible to blob solder to restore pin 1 to test the new 8255 fully then - a little destructive but, reversible.

Re: Sharp MZ-80K help needed on repair
 

Revised version of Tim's suggestion above: If the 8255 is in a socket which it sounds like it is, remove the 8255 and carefully bend pin (14) out at an angle so that when you plug the chip back into the socket, the pin hangs out over the edge of the socket so that it is not connected to anything.

Then try measuring the waveform on 8255 pin 14 again - it it still only going between 0V-1V or is it now going between 0V and ~5V?

Your other problem - half the screen showing scrambled characters instead of the correct ones - seems like a different new issue with the video display system, specifically,

-The system is displaying the contents of the screen RAM correctly but is no longer able to write data to the first half of the screen memory.

or

-The system is showing the second half of the screen memory correctly but is no longer able to show the first half of the screen memory correctly.

You will need to fix this problem before fixing any other because you need a fully working display in order to run diagnostics on the other part.

The introduction of this new half scrambled display fault sounds like a case of cause and effect - what exactly did you do just before this fault appeared, did you replace the CPU socket or the MROM socket? If so, you need to do a close inspection of what you did and if necessary meter out all the tracks going from whichever device's socket you replaced just before the fault appeared.

Re: Sharp MZ-80K help needed on repair
 

Regarding post #134

Can you be more specific? What exactly did you redo? Did you reprogramme the EPROM or did you physically change something? I'm asking - what was the very last thing you did before the appearance of this new half scrambled screen fault?

If you reprogrammed the EPROM and immediately afterwards got that half screen scrambled problem that suggests a problem with the reprogramming of the EPROM. If so, can you try programming one of your spare TMS2532 EPROMs with the RAM test code and see what happens then? If the fault didn't appear until you replaced the MROM socket then you have probably accidentally damaged one of the tracks going to / from the MROM socket pins.

Re: Sharp MZ-80K help needed on repair
 

Hi
Took pin 14 of the 8255 out of circuit and get 0v on it going to order a replacement.
With the half screen issue I did nothing really I took the board out wired up a tactile switch to the two points on the board its set up for the reset circuit and put the board back in.
I reprogrammed the rom on the m-rom socket as that got rid of the lines last time and I then replaced the socket as a couple of the socket pins looked a little wide so just in case they were not making contact.
But I did nothing that should have caused the issue.

Re: Sharp MZ-80K help needed on repair
 

That reading you have on the 8255 pin 14 (...nothing) where before you had a 0V-1V waveform is not what we would have expected so don't be too disappointed if a replacement 8255 does not fix the V-GATE signal - it might, it might not.

With regard to the half scrambled screen our observation would be that you did something, and then immediately after that the display was no longer fully working, so you have to rule out the possibility that whatever you did just before the change has introduced a new fault, however unlikely or unwelcome that might seem. It is possible that something new just happened to fail at that exact time, but you really need to be sure that the fault was not one accidentally induced by you while carrying out other work.

As I said, this (half scrambled display) fault needs to be fixed before any other, otherwise diagnosis of any other faults like RAM faults will be much more difficult. I'll have another look at the diagrams tonight, anyone else with ideas for checks please pitch in.

If you have enough spare EPROMs can you make another RAM test EPROM and just try that to see if it makes any difference. If it doesn't, that will rule out a possible corrupt / faulty EPROM as the cause and we can look elsewhere.

Always try the things which are easy, and which you don't have to buy more parts for, first.

Re: Sharp MZ-80K help needed on repair
 

The 8255 is in bad shape so at least it's one less thing to consider I have tried a couple of times reburning the eprom but will try again As I said about the reset button does not seem to reset the system rather more making the screen go black then press it again and screen comes back again.

Re: Sharp MZ-80K help needed on repair
 

I appreciate your patience so far and you obviously are committed to trying to get this machine to work. I'm not sure if, when reprogramming the EPROM, you are erasing and reprogramming the same EPROM or programming the code in an EPROM you haven't used yet. The second approach is better, to rule out a possible intermittent problem with the first EPROM - these devices are getting very old now.

I have to admit I am worried about the potential state of the sockets given that so many of the ICs plugged into them seem to have suffered such bad corrosion. You could end up getting to the point where you do have to replace all the sockets just to reassure yourself that the sockets are not contributing to your problems.

I'm off to see if I can work out how / why the system would fail to clear / write to the first half of the screen RAM, as that is what seems to be happening.

Re: Sharp MZ-80K help needed on repair
 

In the meantime maybe you can look at something which is easier to trace, the reset signal path. If it is still the case that you don't see anything happening on the reset pin of the Z80 then try scoping the following points in order and watching what happens when you press your add-on reset switch.

IC31 pin 4 - should be low when switch pressed, high when switch released.
IC31 pin 1 - should be high initially, and go low for a while when you press the switch.
IC24 pin 6 - low initially, high for a while when you press the switch.
IC24 pin 10 - high initially, low for a while when you press the switch.
IC46 (CPU) pin 26- high initially, low for a while when you press the switch.
IC4 pin 9 - high initially, low for a while when you press the switch.
IC4 pin 8 - low initially, high for a while when you press the switch.

On all of the above checks 'high' should be at least 3.5V or more and 'low' should be around 0V. if you don't even see a change of state on the first IC pin (IC31 pin 4) then you need to check your reset switch and its wiring.

Re: Sharp MZ-80K help needed on repair
 

It might be worth mentioning potential issues with UV erasable PROMs. If not fully erased prior to programming then they can give intermittent results, often seen due to variation in supply voltage or over operating temperature range. If you are not sure about your uv eraser, check the device is blank after every 15 minutes, when it reads as blank give it an extra 15 minutes.

I would recommend avoiding the use of so called intelligent programming algorithms on old parts, if there are options on you programmer use the standard programming algorithm. Select the device type on the programmer for the same manufacturer as the device you are programming.

On the 8255 issue, I wonder if the ram check is initialising the 8255 as an output correctly. Check the output with MROM fitted before writing off the 8255. As the output is driving an LS ttl input, in most cases it would float to high logic level, but not guaranteed, so it might have worked in the ram test authors system but not in yours.

Re: Sharp MZ-80K help needed on repair
 

Note the reset on the z80 pin 26 will be very short time low level, and might be difficult to capture on the scope. Try using the logic probe, it should trigger the pulse led. Reset from the switch is short so it doesn’t stop the dram being refreshed for too long, so you can reset a rogue program without corrupting memory.

Re: Sharp MZ-80K help needed on repair
 

Good observation from Mark re: The likely length of the reset pulse coming out of IC31. Analogue scopes like the V525 are good for capturing repetitive signals, ones which happen over and over again at a steady rate. For one-off events a Digital Storage Scope is better but when you don't have one of those the 'pulse' indication feature on a logic probe will flash even when the pulse is so short that it would be difficult to see on an analogue scope. Following the reset signal through the pins indicated in post #146 would be a good way to see how the logic probe indicates high and low states and pulses.

It looks like the display memory (1K, 1024 decimal, 0400 Hex bytes) is mapped at D000 onwards. The display RAM chip enables come from the CSD output on the 74LS154 address decoder IC47, which outputs chip enables for the various memory blocks including the video RAM, at 4K intervals. (0000, 1000, 2000, 3000, 4000, 5000 hex, etc.) The CSD output will be active in any address in the range D000-DFFF.

The CS5 to CSC outputs from the 74154 address decoder go to the system RAM RAS / CAS lines so it seems the system RAM starts at 4000 hex.

Just looking again at the image #134, even the lower half of the screen is not showing what it really should.

Re: Sharp MZ-80K help needed on repair
 

Thank you all for sticking with me on this project I do look forward to the day (I hope) to see this computer fully working but I am going to have to wait until the new 8255 arives as when taking the old one out to bend the leg back I noticed another leg had fallen off it somewhere else so rather than bodging it I will wait for the replacement.