Re: Sharp MZ-80K help needed on repair
 

Tried it again this morning still the same this is what I get on pin 6 of the Z80

Re: Sharp MZ-80K help needed on repair
 

Hi
I have replaced IC 36 and 37 again and no difference just the corrupt screen so at least we know it's not that.

Re: Sharp MZ-80K help needed on repair
 

The clock looks ok to me.

Most likely cause of an intermittent fault would be an IC socket connection, could be IC pins corroded, socket connection corroded, maybe IC pin bent under with a turned pin socket, or a missing gold insert on a turned pin socket.

As it seems to be in a permanent fail it might be possible to find the fault by measuring resistance between IC pins and the pad on the pcb, but take care not to apply pressure that might temporarily improve the connection.

Another possibility is failed through hole plating, I have seen this with boards from the 80s when the pcb manufacturing process was not so well developed. I have a reject z80 pcb from that time that I fitted links through each through hole to get it working. I would have expected this type of fault to have caused a much earlier failure in the life of the mz80 though.

Is there any signs of corrosion in any area of the board? Perhaps close inspection and checking continuity of through hole vias in any areas of corrosion might show a fault.

Re: Sharp MZ-80K help needed on repair
 

I am just wondering why when it goes corrupt that the reset side fails to work correctly and I get a lot of black screens where pin 14 of the 8255 stays at 0v until the picture comes back.
Now you will probably tell me this is correct just trying to narrow down what is causing this it was so sweet the brief moment we got it working would like to see it working again.

Re: Sharp MZ-80K help needed on repair
 

Now I have been continually pressing the reset button (not that it does a lot) and its started working again and most of the keys are working this time, but after about a min the top line text starts pulling to the left which is what it did last time before it stoped working but if I turn it off leave it a short time then turn it back on again the top line is perfect again for a short time and starts pulling again.

Re: Sharp MZ-80K help needed on repair
 

No, this is a problem.

Pin 14 (V-Gate out from the 8255) is not correctly set up in the RAM test code to output a logic '1' to make V-Gate high - as you are in contact with the author of the RAM test EPROM maybe you could alert him to that problem or just draw his attention to this thread - he obviously has very good knowledge of these machines so we'd welcome his direct input. However, the resistor you added should compensate for that issue in the meantime.

It is possible that in the circumstances you describe the 8255 is being part initialised with that port pin set to be an output and then driven low, which will have the effect of driving the V-Gate line low and turning the video off, hence blank screen.

There are only officially two things connected to that line, 8255 pin 14 and IC25 pin 1 (plus the added pull up resistor). First check the resistor you added, make sure the soldering is good and that you did connect the other end of it to a known good +5V point.

To eliminate the 8255 as the source of the problem remove it, carefully bend pin 14 out to one side so it no longer goes into the socket and then re-insert the 8255 still with that pin hanging out to one side. Does that allow the screen to stay on consistently?

Re: Sharp MZ-80K help needed on repair
 

At the moment it is booting up corectly but not leaving it switched on long so the 5v on pin 14 is constant so untill it stops booting dont think bending the pin out is going to prove anything at the moment.
with the resistor just did a temporary measure.

Re: Sharp MZ-80K help needed on repair
 

Actually just tried something else now it is booting I have disconnected the resistor and still getting 5v on pin 14 so I don't think the resistor is making any difference and not needed.

Re: Sharp MZ-80K help needed on repair
 

Whew, this is a tough one because we seem to be trying to hit a moving target.

Just to clarify, are you saying you now think this is time related so that if you leave the machine on the video becomes distorted at the top and then eventually goes off?

If I have that right, let it happen so that you have a blank screen and then measure or scope the logic level on IC25 pin 1 and also scope IC17 pins 1, 2, 13 and 12. What waveforms or steady states do you see on those pins when the 'blank screen' fault is present?

Re: Sharp MZ-80K help needed on repair
 

The next time it boots up and seems to be running the RAM TEST code normally with the 1-2-3 rolling in the corner, try pressing and flexing the PCB - does that make it crash, does the number in the corner stop rolling?

Looking at the broader picture we can say that it didn't work, then it did very briefly, now it doesn't again. It would be unusual (although not impossible) for an IC to be faulty, fix itself and then break down again so it is more likely that you are looking for a physical-intermittent fault such as a bad IC socket contact or a bad VIA (a through hole connection between the upper and lower side of the PCB) or a fractured or damaged track. You'll only find a problem like that by exercising extreme patience and measuring from every point on each circuit node to every other point on the same circuit node.

This will be made even harder if the fault is connecting and disconnecting whenever it feels like it so it just happens to be working at the moment when you measure it.

Re: Sharp MZ-80K help needed on repair
 

it is a difficult one I have it running the MROM at the moment I had left it off for about three hours turned it back on to a corupt screen press reset cleared fine to monitor screen.
The bending of the screen at the top if I press reset will not clear it but if I turn it off for about 30secs then back on we have straight text again but this could be a seperate issue with the monitor side.
Noticed that the keyboard sometimes we get most of the keys working then we lose that to only a couple working so wondering if thats a clue.
Put the ram tester rom back in and its running fine but still get the bending screen but computer still running at the moment the only constant I can think of is that when it does crash we lose the 5v on pin 14 on the 8255.

Re: Sharp MZ-80K help needed on repair
 

When it's doing that in a repeatable way, that is the time to try disconnecting pin 14 because that will tell us whether that mysterious 0V on V-Gate is being output by the 8255 or whether there is something else in play here - intermittent low resistance or short between V-Gate and ground.

From your most recent observations it appears that the V-Gate line can be at 5V even without the pullup resistor - this is the condition which we and the RAM TEST code expect, that is why the RAM test code does not bother to set up and use 8255 pin as an output. We need to know where and why low voltages sometimes appear on that pin.

Re: Sharp MZ-80K help needed on repair
 

Took out pin 14 of the 8255 Ram program still running.

Re: Sharp MZ-80K help needed on repair
 

Leave it out for now and check the voltages on the 8255 pin 14 and on IC25 pin 1, which are currently disconnected from each other, from time to time.

Re: Sharp MZ-80K help needed on repair
 

8255 pin 14 0v IC25 pin 1 5v

Re: Sharp MZ-80K help needed on repair
 

While it's in that condition use your little resistor rig to connect the disconnected 8255 pin 14 to nearby +5V (via the resistor). Now what voltage do you see on the 8255 pin, still 0V or some other voltage, with the resistor connected?

If you see 5V with the resistor connected leave the resistor in place and keep checking the 8255 pin voltage to see if it stays at +5V. If you see 0V on the 8255 pin even with a pullup resistor connected, report back.

Re: Sharp MZ-80K help needed on repair
 

Hi
sorry to be mr stupid and you are welcome to point out how stupid I am but if I connect 5v to pin 14 with the resistor then measure the same pin it will remain 5v.

Re: Sharp MZ-80K help needed on repair
 

Not necessarily, if the 8255 pin somehow gets switched to an output and drives low then the voltage on the pin will be about 0V or at least very low even when there is a pullup resistor connected to +5V.

The drive downwards to 0V by the 8255 pin in output mode would be stronger than the modest pullup to +5V by the resistor, so the 8255 output would win.

The purpose of the resistor is to allow us to see if / when the 8255 pin does go into output mode and drive low, for reasons unknown.

Re: Sharp MZ-80K help needed on repair
 

sods law it wont crash at the moment so dont know if thats good or bad but still get the pulling screen and the keyboard at the moment has decided only the few keys that are responding this time are not showing what they are surpose to show for example the bottom row instead of CR space etc are producing numbers instead.and not getting a flashing cursor.

Re: Sharp MZ-80K help needed on repair
 

When you are seeing these keyboard faults I expect that's with the MROM in and with the 8255 inserted normally?

While the keyboard fault is present, try scoping the following IC6 pins:- 15, 14, 13, 12.

Re: Sharp MZ-80K help needed on repair
 

I think that's down to the telly, safe to ignore for now.

Re: Sharp MZ-80K help needed on repair
 

The CRT display is actually an integral part of the machine, I don't think it is easy to connect to any other display as the video and combined sync signals are TTL-Level. It could be down to malformed sync pulses as well.

Re: Sharp MZ-80K help needed on repair
 

picture of pin 15 scope the other pins are simular and alter when a working key is pressed but pin 12 has 1v on it and does not move.flashing cursor has returned.

Re: Sharp MZ-80K help needed on repair
 

I think you meant that pins 15, 14, 13 of IC6 have activity on them but 12 is stuck at 1V.

If so, can you do the same check on pins 4, 3, 2, 1 of the 8255, especially pin 1, which generates the signal you should be seeing on IC6 pin 12?

Re: Sharp MZ-80K help needed on repair
 

Checked 8255 pins and they are fine but then rechecked pin 12 on IC6 and that is showing fine it might have been my fault it is in a hard to reach area.

Re: Sharp MZ-80K help needed on repair
 

If you read 1V on IC6 pin 12 originally you must have been making contact with it, or do you think that maybe you might have been on the wrong pin?

I've just been back through the thread and while I remember you chose to replace the 8255 because the original had badly corroded pins I didn't see anywhere where you said you had also changed the 8255's socket, sorry if I missed it?

If the connection between the 8255 pin 1 and IC6 pin 12 is coming and going that will have a direct effect on the scanning of the keyboard keys.

Re: Sharp MZ-80K help needed on repair
 

Yes I have replaced the 8255 socket. And I may have found the issue I have discovered if I press the 8255 down flexing the board I am losing then gaining pin 12 on IC 6 so I will in the morning get the board out and take a look.

Re: Sharp MZ-80K help needed on repair
 

Excellent, if you can get to the bottom of that, that will be one less intermittent problem. Hopefully that will be progress of a sort.

Re: Sharp MZ-80K help needed on repair
 

Sorry to have to suggest this, but it may be worth taking the 8255 out just to make sure that pin 1 is not folded over. Still happens to me from time to time, even now.

Re: Sharp MZ-80K help needed on repair
 

And ... also check the IC socket pins arent folded under, and soldered correctly.
You can normally check for this type of fault with torch and magnifier.
Good luck.

Re: Sharp MZ-80K help needed on repair
 

Had a look this morning and resoldered some of the pins still having issues even the slightest touch on the 8255 and it effects it just wondering if it's worth changing the socket again for a turned pin one.
But Most of the keyboard works most of the time and it has not crashed since so brings me back to the screen pulling any thoughts on what might be causing it something must be breaking down when it gets warm then fine once it cools down.

Re: Sharp MZ-80K help needed on repair
 

If the socket and its connections are OK it should not matter whether it is a turned pin one or a conventional one. If you are unlucky you have somehow managed to spread / open up the pin 1 contacts in the socket, but replacement of the socket would fix that.

The one potential drawback with turned pin sockets is that they are very, very difficult to remove from double sided PCBs like this without causing damage so if you do fit a turned pin type be absolutely sure that you will never want to remove the socket again.

Your actual problem may be on the PCB - an intermitted connection between the pin 1 pad and its associated track, or as suggested earlier the pin 1 pin of the socket may be folded underneath the socket and not through the hole as it should be - although if it was, the pin 1 pad on the underside would be suspiciously flat with no pin sticking out through it.

There is 'a thing' called freezer spray which is designed for finding thermally progressive faults, you let the fault appear and then go around spraying suspect components - when you hit the right one, bing, the fault disappears so you know that's the guilty component. That's the theory anyway.

Have a good look at the waveforms on IC26 pins 8, 9 and 10 and also at the video waveform on IC3 pin 2. Those waveforms should be clean and sharp with good vertical edges, no rounded corners at any point. Then let the fault appear and have a look at the waveforms again. See anything different?

Re: Sharp MZ-80K help needed on repair
 

scoped the pins but they never changed when the fault appears.

Re: Sharp MZ-80K help needed on repair
 

Images 1 and 3 didn't really catch what we are looking for, maybe try again on those ones, they are likely to stay mainly in one state and only pulse to the other state for a very short time, so try to catch a whole one of those pulses on each of those pins.

Are the controls, especially the 'H-Hold' controls exposed on the rear of the monitor, in other words can you adjust them from outside? If so try adjusting 'H-Hold' gently up and down from its present setting when you have the 'bent top' problem on the display. If turning it left or right a bit cures the bent top line, leave it like that. If it makes no difference return it to exactly where it was.

Re: Sharp MZ-80K help needed on repair
 

With pictures 1 and 3 there are no wave formed but the two lines do show pulses when I alter the time/div.

Re: Sharp MZ-80K help needed on repair
 

Sorry, I think maybe my terminology leaves something to be desired - for me a digital 'waveform' is any signal which has a mixture of high parts and low parts in it, not necessarily 50-50 high-low like the ones you saw on the address lines in the NOP test.

The pulses you see (when you alter the time / div) need to look nice and sharp and square, not buckled or rounded.

What about the H-Hold control on the CRT display, can you get to that to try adjusting it a little?

Re: Sharp MZ-80K help needed on repair
 

H-Hold control moves around but will not correct the pulling of the text.

Re: Sharp MZ-80K help needed on repair
 

Sorry to hear that, would have been a nice simple remedy had it worked. Another two other possible scenarios -

-Thermal (time related) fault in the CRT monitor circuit
-Thermal (time related) fault on the horizontal sync generation circuit.

If you don't have any freezer spray you could try letting the machine cool right down (maybe overnight) so that the 'bent top' fault is not present, then go over the mainboard with a hairdryer set to a fairly hot temperature to see if that makes the fault appear sooner than it normally would. I would start with IC8 and move on to the other ICs in the 'Video Generator' section of the circuit diagram (PDF page 34 / manual page 32).

What about that problem with the 8255 pin 1, did you sort that? If there is a problem with that pin somewhere under the 8255 there could be problems with other pins which are related to the other problems you keep having. If you sort that problem out you could end up fixing or at least finding the reason for some of the other problems as well.

Re: Sharp MZ-80K help needed on repair
 

This might be a question for the CRT TV guys, is the deflection voltage for the monitor higher at the top of the screen and having some limiting effect in the CRT circuits? Can degraded capacitors in the Crt circuits or power supply have this effect?

Re: Sharp MZ-80K help needed on repair
 

Asked someone in the sharp Facebook group who said this.
It's a monitor issue. The flyback transformer isn't settling down properly after a VSYNC. The computer part is working correctly and the display is correct, only mistimed until it settles down.

Check the caps and diodes in the monitor scan control section.

Re: Sharp MZ-80K help needed on repair
 

Make sure the monitor circuits are clean of dust that tends to accumulate on the HT circuits before you do anything else in that area. Take care as they can hold charges at high voltage even after being powered off for quite some time. An insulated paintbrush is useful for removing the dust.

Re: Sharp MZ-80K help needed on repair
 

I am not very good at reading schematics so if anyone can point me in the right area of the board I need to be checking capacitors I would be very grateful.

Re: Sharp MZ-80K help needed on repair
 

This forum has probably the highest density of CRT circuit experts anywhere in the world but most of them probably don't read this (computers) section and I don't count myself as anything more than an interested amateur when it comes to CRT stuff.

You could try a separate thread in 'Vintage TV and Video' with the heading 'Sharp MZ-80K CRT monitor fault', post a couple of images of the fault condition and and point them to PDF page 42 / Manual page 40 of the service manual (Monitor circuit diagram) and PDF page 41 / Manual page 39 (PCB layout of monitor). Also the monitor block diagram, PDF page 19 / Manual page 17. For any CRT gurus who do happen to read this, the service manual which includes the monitor circuit details was posted in #3 of this thread.

Given such good service information and good images of the fault condition I'm sure someone in the TV section will be able to point you in the right direction.

All I can tell you is that the upper left and top of the circuit is the video section, upper right is the audio circuit, the middle-left section is the sync separator, the centre middle to centre right section is the vertical deflection / scan circuit and the bottom section going across horizontally is the horizontal oscillator, drive and line output stage. If the fault is that the scanned picture is full height but the top line is bent sideways I think you are looking at a problem in the lower third of the diagram, if the top of the picture is compressed (all the scan lines at the top squeezed into a smaller space) then more likely the middle (vertical scan) section.

From your point of view it's good that the monitor is powered from 12V so there aren't any mains voltages backed up by unlimited current lurking in there and waiting to kill you, but the monitor circuit itself does generate some high voltages especially the voltage (Kilovolts) fed to the tube via the thick rubber lead and cap going to the tube, and there are also voltages of well over 100V in the area around the line output transformer / tube neck.

Don't forget the simple stuff as well, try scoping the +12V supply to see if it is dipping or sagging every time the display sets off on a new scan down the screen. It should be steady 12V, maybe with a slight sawtooth shape to it but without any big dips or downward spikes.

Don't just plonk the scope probe on any random point in a monitor like this, if you accidentally hit one of the high voltage points you could potentially damage the scope.

With care you could also use freezer spray to try to isolate this fault as long as you kept it away from the high voltage areas.