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pudwink 20th Sep 2021 12:08 pm

Re: Sharp MZ-80K help needed on repair
Hi all
Oscilloscope has turned up today I have turned it on and set it to get two lines on the screen connected the probes set them at 10x but get nothing when I connect the tip to the .5v cal I assume the ground wire of the probe needs to be connected to the ground point on the front of the oscilloscope but tried it with and without but just two straight lines Am I doing something wrong.

SiriusHardware 20th Sep 2021 2:34 pm

Re: Sharp MZ-80K help needed on repair
Is there an AC/DC/GND switch on each channel? Try setting both to AC or both to DC.

pudwink 20th Sep 2021 3:02 pm

Re: Sharp MZ-80K help needed on repair
Yes it has and that worked getting a reading now.and answer to earlier question got a eprom eraser all the way from China turned up Saturday.

SiriusHardware 20th Sep 2021 3:36 pm

Re: Sharp MZ-80K help needed on repair
Sounds like you will be better equipped than we are for this sort of work by the end of the week. Have you managed to find the MROM code? I haven't looked but I assume the same site which had the CGROM code also has that code as well.

Edit: Yes, I see it under the same area as the CGROM code.

Mark1960 20th Sep 2021 4:05 pm

Re: Sharp MZ-80K help needed on repair
When you calibrate the scope probes use the dc setting for both channels.

pudwink 20th Sep 2021 4:18 pm

Re: Sharp MZ-80K help needed on repair
Problem is If I get this working I will have all the equipment and nothing to use it on just waiting on the EPROMs coming and see what happens next.

SiriusHardware 20th Sep 2021 4:36 pm

Re: Sharp MZ-80K help needed on repair
Bear in mind that even if you replace the apparently dud MROM the system still may not do anything sensible until it has a full set of known good RAM.

pudwink 20th Sep 2021 7:41 pm

Re: Sharp MZ-80K help needed on repair
I am hoping With the two lots of ram the seller sent me the replacement lot only 6 out of the 14 passed fully and he said I am the first buyer out of selling 300 of these drams to say there was an issue so I don't know,and I have found a batch of 7 I brought six years ago I did not know I had so should be just enough.

Timbucus 20th Sep 2021 8:01 pm

Re: Sharp MZ-80K help needed on repair
I have to ask do you have an antistatic strap and mat as the RAM's are very sensitive to handling?

SiriusHardware 20th Sep 2021 8:54 pm

Re: Sharp MZ-80K help needed on repair
I believe (but maybe pud can confirm again) that the original SHARP branded RAMs always test OK and that's in spite of being handled just as the other non-Sharp parts are.

Pudwink, do either of your programmers also have a chip test \ RAM test mode? Alternatively, do you have another computer, like a Spectrum, which you could test the non-Sharp RAMs in? Also did you try what I suggested, cleaning up the edges of the pins on one RAM device which always fails and then trying it again?

pudwink 21st Sep 2021 8:00 am

Re: Sharp MZ-80K help needed on repair
With the new memory they are not showing failed completely they are passing the high speed test but failing the retention test which the instructions state that they could still work.i did try cleaning the legs on a couple but still the same result programmer does not do memory test and yes I do have an anti static strap I have no other computers that use these drams.

SiriusHardware 21st Sep 2021 8:42 am

Re: Sharp MZ-80K help needed on repair
I would like to hope they are actually OK. Fingers crossed, let's hope a replacement MROM moves you forwards.

pudwink 23rd Sep 2021 10:52 am

Re: Sharp MZ-80K help needed on repair
1 Attachment(s)
Hi All
The eproms finaly turned up today I have programmed one up and the good news is no more lines bad news is what is shown in the picture

SiriusHardware 23rd Sep 2021 5:09 pm

Re: Sharp MZ-80K help needed on repair
This is an interesting contrast between what you had before with the apparently blank MROM in and what you have now - essentially what you had when you didn't have the Z80 fitted (your post #59) so it looks like:-

-The video circuit is working as it is independently displaying the content of the video RAMs

-The Z80 is not executing code, or it is, but not getting as far as the code which clears the screen, or it is, but the screen clear is failing due to some hardware problem.

Before we dive in too deep, let's look at a few basics which will also act as a kind of 'trainer' for your scope. From the sort of things you've already done I think we can assume you know how IC pins are numbered. I'm also assuming you have had a bit of a tinker with your scope by now and know roughly how to use it, but if you need any specific help please don't hesitate to ask.

As you may or may not know the ground of the scope needs to be connected to the 0V / ground of the system under investigation. You can do this either by connecting the flying earth lead attached to the scope probe to a ground / 0V point near to where you will be taking the measurement from, or the scope may have a front-panel earth socket intended to accept a 4mm 'banana' plug - if so you can take a lead from there to your system ground / 0V and that will provide the scope's ground connection to the MZ-80.

Set the AC/DC/GND switch for channel 1 to 'GND', then adjust the vertical position of the channel 1 trace so that it is sitting on the lowest line on the scope 'graticule', the 'grid' overlaid on the screen.

If the scope probe has a 'x1 / x10' switch on it set it to x1 for now, and turn the Volts / Div knob for channel 1 to the '1' position, not to be confused with '0.1 or .1' further around the dial.

Rotate the small grey knob in the centre of the larger Volts/Div knob fully clockwise until it goes 'click' at the position marked 'Cal' - and leave it in that position.

Now, set the channel 1 AC/DC/GND switch to 'DC'. Touch the channel 1 scope probe tip to a point which is known to have +5V on it and you should see the line jump five squares upwards, each square representing one Volt. If that's all working, then carry on.

Let's look first at the reset input to the Z80, on IC46 (The Z80) pin 26. You can do this check with a meter, with your logic probe or with your scope, but however you do it check that pin 26 normally has a high / logic 1 voltage on it, but goes low / logic 0 when you press and hold the reset button. When you let go of the reset button it will stay low for a little bit longer, then return high. If this is not happening, report back.

Using only channel 1 on the scope for now, look at the signal on pin 8 of IC7 and also at the signal on pin 8 of IC9. You should see square-waves on those two pins. If necessary, wind the Time / Division knob setting upwards or downwards in order to display several cycles of the waveform across the screen.

The service manual has pictures of what the waveforms on these two pins should look like, they are the first two waveforms shown on PDF page 13 / Manual page 11. Let us know if those two waveforms are present. I think they probably are, but they will be good waveforms for you to look at while you are getting to know your scope.

You may find that the waveforms don't display as a steady 'picture' but instead wander from right to left or left to right across the screen. We'll explain how to sort that out in a bit.

Mark1960 23rd Sep 2021 5:10 pm

Re: Sharp MZ-80K help needed on repair
I think you said the original MROM had some corroded pins, so maybe try checking the connection of each MROM pin through the socket to the pcb pads. Probably also worth doing the same with the z80.

SiriusHardware 23rd Sep 2021 5:25 pm

Re: Sharp MZ-80K help needed on repair
1 Attachment(s)
For anyone following / assisting, here is a found image of the Hitachi V525 scope front panel.

pudwink 23rd Sep 2021 5:49 pm

Re: Sharp MZ-80K help needed on repair
I have replaced the main ic sockets earlier so they should be fine with doing a reset there is no reset button so I take it turning it off and back on will do.

SiriusHardware 23rd Sep 2021 5:57 pm

Re: Sharp MZ-80K help needed on repair
There is a reset switch shown on the circuit diagram, left of IC31 on the circuit diagram on PDF page 28 / Manual page 26 - if it is there it may be a small switch on the mainboard rather than an external / keyboard switch. Unfortunately I don't know the machine well enough to be able to tell you.

SiriusHardware 23rd Sep 2021 6:02 pm

Re: Sharp MZ-80K help needed on repair
1 Attachment(s)
Your machine doesn't have a reset switch in the position marked, (see attached?)

pudwink 23rd Sep 2021 8:17 pm

Re: Sharp MZ-80K help needed on repair
No there is an option to install one but mine has not got one fitted

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