UK Vintage Radio Repair and Restoration Discussion Forum

UK Vintage Radio Repair and Restoration Discussion Forum (https://www.vintage-radio.net/forum/index.php)
-   Vintage Computers (https://www.vintage-radio.net/forum/forumdisplay.php?f=16)
-   -   Sharp MZ-80K help needed on repair (https://www.vintage-radio.net/forum/showthread.php?t=183455)

SiriusHardware 23rd Sep 2021 10:26 pm

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.

pudwink 24th Sep 2021 8:43 am

Re: Sharp MZ-80K help needed on repair
 
4 Attachment(s)
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.

SiriusHardware 24th Sep 2021 11:10 am

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.

pudwink 24th Sep 2021 11:16 am

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.

pudwink 24th Sep 2021 11:53 am

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.

SiriusHardware 24th Sep 2021 11:58 am

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'.

SiriusHardware 24th Sep 2021 11:59 am

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.

Mark1960 24th Sep 2021 3:13 pm

Re: Sharp MZ-80K help needed on repair
 
Quote:

Originally Posted by pudwink (Post 1409120)
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.

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?

pudwink 24th Sep 2021 3:16 pm

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.

Mark1960 24th Sep 2021 3:54 pm

Re: Sharp MZ-80K help needed on repair
 
Quote:

Originally Posted by pudwink (Post 1409207)
No sometimes just get blank screen other times random for a shortime then blank screen also get some popping from the speaker.

But you donít 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.

SiriusHardware 24th Sep 2021 3:59 pm

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.

pudwink 24th Sep 2021 4:06 pm

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.

SiriusHardware 24th Sep 2021 4:51 pm

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.

SiriusHardware 24th Sep 2021 5:22 pm

Re: Sharp MZ-80K help needed on repair
 
1 Attachment(s)
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.

pudwink 24th Sep 2021 7:56 pm

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.

Mark1960 24th Sep 2021 8:20 pm

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?

pudwink 24th Sep 2021 10:14 pm

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.

Mark1960 24th Sep 2021 11:51 pm

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.

SiriusHardware 25th Sep 2021 7:19 am

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?

SiriusHardware 25th Sep 2021 7:39 am

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.


All times are GMT. The time now is 5:34 am.

Powered by vBulletin®
Copyright ©2000 - 2022, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.