UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Computers

Notices

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

Closed Thread
 
Thread Tools
Old 27th Sep 2021, 12:23 am   #141
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: Sharp MZ-80K help needed on repair

Regarding post #134

Quote:
I redid the eprom and now all I get is this
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.

Last edited by SiriusHardware; 27th Sep 2021 at 12:32 am.
SiriusHardware is online now  
Old 27th Sep 2021, 9:17 am   #142
pudwink
Pentode
 
Join Date: Jan 2011
Location: Nottingham, Notts. UK.
Posts: 207
Default 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.
pudwink is offline  
Old 27th Sep 2021, 12:35 pm   #143
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default 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.
SiriusHardware is online now  
Old 27th Sep 2021, 1:59 pm   #144
pudwink
Pentode
 
Join Date: Jan 2011
Location: Nottingham, Notts. UK.
Posts: 207
Default 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.
pudwink is offline  
Old 27th Sep 2021, 6:00 pm   #145
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default 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.
SiriusHardware is online now  
Old 27th Sep 2021, 6:21 pm   #146
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default 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.
SiriusHardware is online now  
Old 27th Sep 2021, 6:31 pm   #147
Mark1960
Octode
 
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 1,264
Default 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.
Mark1960 is online now  
Old 27th Sep 2021, 6:36 pm   #148
Mark1960
Octode
 
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 1,264
Default Re: Sharp MZ-80K help needed on repair

Quote:
Originally Posted by SiriusHardware View Post
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.
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.
Mark1960 is online now  
Old 27th Sep 2021, 8:04 pm   #149
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default 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.
SiriusHardware is online now  
Old 27th Sep 2021, 11:30 pm   #150
pudwink
Pentode
 
Join Date: Jan 2011
Location: Nottingham, Notts. UK.
Posts: 207
Default 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.
pudwink is offline  
Old 29th Sep 2021, 2:20 pm   #151
ScottishColin
Octode
 
Join Date: May 2012
Location: Perth, Scotland
Posts: 1,762
Default Re: Sharp MZ-80K help needed on repair

Same thing happened to me with my Commodore PET. I cut a leg off a potentiometer and soldered it back onto what was left of the leg on the chip - it's still there in my (now working) PET.

Colin.


Quote:
Originally Posted by pudwink View Post
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.
Attached Thumbnails
Click image for larger version

Name:	IMG_20210104_191551.jpg
Views:	44
Size:	27.6 KB
ID:	242371  
ScottishColin is offline  
Old 29th Sep 2021, 7:39 pm   #152
pudwink
Pentode
 
Join Date: Jan 2011
Location: Nottingham, Notts. UK.
Posts: 207
Default Re: Sharp MZ-80K help needed on repair

Hi all
I was hopeing that the 8255 would have come but the seller has only sent it out today so I have bodged up the old one to take a look at the reset side.Ic31 pin 4 acted like it should pin 1 stays high,Ic24 pin 6 stays low,Ic24 pin 10 stays low,Ic46 pin 26 stays high Ic 4 pin 9 stays high and Ic4 pin 8 stays low.if I keep pressing the reset I do sometimes get the correct ram tester screen.As you see from the picture the switch is just simple.
Attached Thumbnails
Click image for larger version

Name:	IMG_20210929_193023[1].jpg
Views:	51
Size:	102.6 KB
ID:	242384  

Last edited by pudwink; 29th Sep 2021 at 7:41 pm. Reason: forgot picture
pudwink is offline  
Old 29th Sep 2021, 8:58 pm   #153
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: Sharp MZ-80K help needed on repair

A non working or intermittent reset circuit certainly will not help our case so this is a good area to focus on for a moment.

You didn't say whether you used the scope or the logic probe to look at what is happening from IC31 pin 1 onwards. I would guess the scope.

If you only used the scope then as Mark1960 said, it is possible that the pulses after IC31 are so short that they are difficult to see on a scope. It's also possible that your scope is telling you the truth, and there really are no reset pulses from IC31 pin 1 onwards. So let's find out.

Your logic probe may have a pulse on / off slide switch, if it does then turn it to ON. If it has a CMOS / TTL switch then I would suggest setting that to TTL.

As you probably already know you have to run the probe on the same main rail voltage as the ICs you are probing, so for 74 series ICs and microprocessor ICs like the ones in this computer, the black supply clip for the probe should go to 0V / GND of course, and the red supply clip should go to +5V because that's the voltage which the ICs you are probing run on.

With the computer on, probe connected to the supply, put the probe tip on IC31 pin 1 and watch the 'pulse' LED on the probe as you push the reset switch. Does it flash every time you press the switch?
SiriusHardware is online now  
Old 29th Sep 2021, 9:26 pm   #154
pudwink
Pentode
 
Join Date: Jan 2011
Location: Nottingham, Notts. UK.
Posts: 207
Default Re: Sharp MZ-80K help needed on repair

Just tried pin 1 of Ic31 with the logic probe when connected the red light shows so it is high then when I press the reset button the red light stays lit and the pulse light comes on every time it is pressed.
pudwink is offline  
Old 29th Sep 2021, 9:38 pm   #155
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: Sharp MZ-80K help needed on repair

OK, so proceed to the reset pin (pin 26) of the Z80, what happens with the logic probe LEDs when you look there with the logic probe and press the button?
SiriusHardware is online now  
Old 29th Sep 2021, 9:47 pm   #156
pudwink
Pentode
 
Join Date: Jan 2011
Location: Nottingham, Notts. UK.
Posts: 207
Default Re: Sharp MZ-80K help needed on repair

Get Same result as Ic 31 pin 1.
pudwink is offline  
Old 29th Sep 2021, 10:47 pm   #157
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: Sharp MZ-80K help needed on repair

That suggests that there is a reset pulse happening, it is just very very short.

The circuit formed by C31 converts the signal from the reset switch into a momentary pulse output on pin 1, the length of which is determined by the values of R37 and C22.

From the data for this IC it looks like the pulse length is given by the formula

0.69 x R x C

C = = 0.01uF = 0.00000001 (Farads)
R = 15K = 15000 (Ohms)

0.69 x 15000 x 0.00000001 = 0.0001035 Seconds = 103.5 uS (microseconds). Somebody please check my maths, (my worst subject).

If it's right this is close enough to 100uS that I would suggest 100uS was what they were aiming for with these values.

You should be able to see that pulse on your V525 scope but it is hard to catch because it happens only once each time you press the reset switch.

Try with the following settings:

Make sure the small CAL knobs in the middle of the Volts / Div controls, plus the SWP / VAR knob are all rotated fully clockwise to their click stop positions so that the UNCAL LEDs next to them are all unlit.

Set Trigger 'LEVEL' rotary control to about midway and pull the 'Level' knob out to select falling edge trigger.

MODE switch to 'CH1'. INT TRIG switch below that to 'CH1'. 'Source' switch set to INT.

CH1 AC/DC/GND switch set to DC
CH1 Volts / Div control set to '1' (not 0.1 or .1).
Scope probe x1 / x10 switch set to x1

TIME / DIV control - set to 20uS. Note the first half of the dial is marked in mS (millisecond) values and the second half is marked in uS (microsecond) values. Choosing 20uS (per square) means that the trace takes 100uS to travel half way across the screen and 200uS to travel all the way across. If there is a 100uS pulse happening, it should look about half a screen wide with these settings.

Use the CH1 Vertical Position control to place the trace on the lowest horizontal line on the scope graticule.

Put the tip of the scope probe on a known +5V supply point. The line should jump up five squares.

Now move the probe to the Z80 reset pin (26) the line should again jump up, perhaps not as high as five squares. Keep the probe on the pin and keep rapidly pressing and releasing the reset switch. Can you see the line bouncing momentarily downwards every time you press the switch?
SiriusHardware is online now  
Old 30th Sep 2021, 1:13 am   #158
Mark1960
Octode
 
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 1,264
Default Re: Sharp MZ-80K help needed on repair

I just noticed there are two Mode controls, the knob in the middle and the lever next to trigger level.

If you set the Mode lever to Normal, the trace should stop and only appear each time you press the reset switch. Then it should start low after the falling edge trigger and then be high after a short delay. This might let you see the reset pulses easier if you repeat press the button as you should get some persistence of vision.
Mark1960 is online now  
Old 30th Sep 2021, 11:10 am   #159
pudwink
Pentode
 
Join Date: Jan 2011
Location: Nottingham, Notts. UK.
Posts: 207
Default Re: Sharp MZ-80K help needed on repair

Set up the scope as SiriusHardware sugested get the 5v but nothing moving when reset switch pressed I also tried norm mode but no trigger when reset pressed.
pudwink is offline  
Old 30th Sep 2021, 12:54 pm   #160
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: Sharp MZ-80K help needed on repair

It might still just be a question of timing, looking at short-duration one-off events is one of the harder things to do with a pure analogue scope, and the reason storage scopes were invented.

Can I ask you to revisit this measurement with the logic probe :-

Quote:
Just tried pin 1 of Ic31 with the logic probe when connected the red light shows so it is high then when I press the reset button the red light stays lit and the pulse light comes on every time it is pressed.
In particular, can you confirm that when the reset button is not pressed the normal state of IC31 pin 1 is high (red, on the logic probe) as you have said above? Or is it low? (green).

Edit: While doing that could you please make the same observation on IC31 pin 6?

Last edited by SiriusHardware; 30th Sep 2021 at 1:08 pm.
SiriusHardware is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 5:45 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.