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-   -   Sharp MZ-80K help needed on repair (https://www.vintage-radio.net/forum/showthread.php?t=183455)

Mark1960 10th Sep 2021 7:46 pm

Re: Sharp MZ-80K help needed on repair
 
ScottishColin bought a Hantek 6022Bl which he used for quite a lengthy debug of his PET 3016, and this seemed to provide quite good results. The MZ80K looks to be similar technology level so it might be worth looking at one of those. Note these rely on a PC to provide the display and controls, probably needs windows. The difference between the 6022BE and 6022BL is that the BL includes a 16 channel logic analyser, but I don’t remember if Colin used that feature.

Timbucus 10th Sep 2021 8:07 pm

Re: Sharp MZ-80K help needed on repair
 
Sirius persuaded me to buy a standalone digital scope a while back and it was the best investment I made. The fact it is not tied to a computer and is light and easy to move makes it very versatile. I was really lucky with an offer on Amazon at the time to get the 100Mhz version by Hanmatek the DS1102 and now they are only around 40 dearer.

https://www.amazon.co.uk/DOS1102-osc.../dp/B0833X3RFK

While just looking for it I notice the OEM OWON (who make a lot of these clones and is the origin of the badged Hanmatek list the slower model - fast enough for older machines like the Sharp) if you want slightly cheaper:

https://www.amazon.co.uk/SDS1022-Osc.../dp/B07RD6FMM7

One thing I have missed on mine is the external trigger which my 20MHz Hanmeg features. No idea on the brand but, these are probably all much of a muchness I spotted this one which has the 100Mhz and an external trigger - probably well worth the extra 30 and a great foundation for developing your electronics skills...

https://www.amazon.co.uk/UNI-T-UTD21.../dp/B092ZB7678

SiriusHardware 11th Sep 2021 7:49 am

Re: Sharp MZ-80K help needed on repair
 
All of the above is good advice but unless pudwink has deep pockets, why not wait and see if fixing what appears to be a definite RAM fault fixes the machine? As he says, he has already laid out quite a bit on replacement RAM, plus a 2716 Eprom which he did not necessarily need.

Pudwink, when the RAM arrives I suggest you test every one of the new chips in your tester before putting them in the machine. All of us here have had experiences recently of finding out that one out of x chips in a batch of new RAM was dud - hopefully yours will all be OK but it's better to check them independently before putting them in.

Are you planning to replace the sockets that the corroded RAMs were in, or do you think they look OK?

Timbucus 11th Sep 2021 2:43 pm

Re: Sharp MZ-80K help needed on repair
 
Agreed - we are not at a stage where we need to bring a scope to bear - it is a luxury investment as he has a logic probe and meter.

I recently found another set of new RAM chips that had 1 of the 8 faulty - as it was the last bit of a 16K I had been chasing an addressing issue for the second bank for ages as the onboard test failed at 7FFF - not 8000 which would have been in the next bank... I just didn't think...

pudwink 11th Sep 2021 6:13 pm

Re: Sharp MZ-80K help needed on repair
 
Dont worry every chip will go through the tester first I dont fancy replacing all the sockets so I will proberly fit the new chips first see what result I get then if the same look at all the sockets then if still the same get some tissue to wipe the tears of frustration and come back to you guys for some expert help.

Mark1960 11th Sep 2021 8:07 pm

Re: Sharp MZ-80K help needed on repair
 
Maybe after fitting the chips perform a continuity check to the pins, at least for the supply and ground pins. If one of the three voltages is missing it could possibly damage the new chip.

SiriusHardware 11th Sep 2021 8:57 pm

Re: Sharp MZ-80K help needed on repair
 
Also, with all of the dud chips still out, power on and measure on the pins of the sockets to make sure that you have all of the expected supply voltages on the power supply pins of all of the sockets.

pudwink 12th Sep 2021 12:36 pm

Re: Sharp MZ-80K help needed on repair
 
I didn't want to but looking at the socket pins under a magnifying glass there is some corrosion on some of them so I surpose I am going to have to replace all 24 sockets for peace of mind oh joy.

Timbucus 12th Sep 2021 1:52 pm

Re: Sharp MZ-80K help needed on repair
 
You could try some spray contact cleaner first - I have rarely had to replace a socket unless the pins had no grip left or were actually black.

pudwink 13th Sep 2021 5:18 pm

Re: Sharp MZ-80K help needed on repair
 
The ram has turned up today out of the 20 chips 8 passed on my tester 1 failed totally 2 showed only half good and 9 passed the high speed test but failed the retension test so I dont know if they are good or not.
I have put them in anyway and still the screen is the same no different.

SiriusHardware 13th Sep 2021 5:27 pm

Re: Sharp MZ-80K help needed on repair
 
I don't quite follow the numbers, only 8 passed, but 9 passed the high speed test?

Regardless, that sounds like quite a high failure rate. Is there any chance you can return the 'bad' ones? As I said, I have had some occasional bad ones but I have never had any problem getting the bad devices exchanged.

One thing which may be worth trying is just cleaning up the edges, not the flat sides, of the pins of one or two of the 'bad' devices to see if they then pass. If your chip tester has a ZIF socket as I imagine it does then it makes contact with the thin edges of the chip pins rather than the flat sides of the pins, so the pin edges need to be clean and bright.

Am I right in thinking that your chip tester passes all of the original fitted SHARP branded RAM chips?

SiriusHardware 13th Sep 2021 5:38 pm

Re: Sharp MZ-80K help needed on repair
 
Also, can you try the check suggested in #13, #14 by Mark and Tim to determine whether the video display circuit is independently working? If you need more detail, don't hesitate to ask.

pudwink 13th Sep 2021 6:38 pm

Re: Sharp MZ-80K help needed on repair
 
Hi
I purchased 20 4116 chips and only 8 passed I have contacted him and I believe he is going to send some more out and all the sharp ram passed with the video test If you could explain it in simple terms I would appreciate it as I do not want to cause any damage to something.
I did remove the CG-rom and did get a white screen but with shorting the pins I take it you mean the ground pin 16 to the data pins is that 5,6,7 10,11,12 and 13 of the CG-rom socket.
Is this correct.

Mark1960 13th Sep 2021 6:59 pm

Re: Sharp MZ-80K help needed on repair
 
Ground pin of CGROM is pin 12, you can also use pin 18 or pin 20 as they are also grounded.

With CGROM removed.

As Tim said earlier use a small diameter wire to avoid widening the socket pins. Wire wrap wire is probably good for this, but if you don’t have any then use a single strand from a 7x0.2 flex cable.

While the white display is shown, connect pin 12 to each of the data output pins, 9, 10, 11, 13, 14, 15, 16, 17. You should see a dark vertical band in different positions depending on which pin you ground.

This should verify the output stage of the video circuit.

Next try connecting pin 13 (thirteen) to each of pins 6, 7 ,8. Instead of a solid dark vertical band it should be a dashed line. With long, medium and short dashes. This will confirm the vertical row generated to the character generator.

pudwink 13th Sep 2021 7:20 pm

Re: Sharp MZ-80K help needed on repair
 
Many thanks sorry was looking at the wrong pinout I Will try this

pudwink 14th Sep 2021 2:17 pm

Re: Sharp MZ-80K help needed on repair
 
4 Attachment(s)
Took CG-Rom out and as picture get white screen if I then short pins 9, 10, 11, 13, 14, 15, 16, 17 I get a white screen with lines down as second picture and if I short pins 13 to 6,7,8 I get a couple of diferent pictures last two pics.

Mark1960 14th Sep 2021 4:25 pm

Re: Sharp MZ-80K help needed on repair
 
I think this means the shift register on the output of CGROM is working and so is the character row generator.

Either the video ram is being written with the pattern that displays vertical white lines or there is a problem with the video ram. I’ll take another look at the schematic and see if there is anything we can do to safely stop the z80 running so we know we see an uninitialised display.

Mark1960 15th Sep 2021 4:39 pm

Re: Sharp MZ-80K help needed on repair
 
Try with the Z80 removed and the CGROM fitted. Then the video ram should have random content and the screen should show random characters.

If it still shows the same vertical lines then I think this points to faulty video ram ic41/ic42, the ls245 buffer ic43, or possibly the control logic. We can try and narrow it down further after you try the above test.

pudwink 15th Sep 2021 4:56 pm

Re: Sharp MZ-80K help needed on repair
 
1 Attachment(s)
With the Z80 removed and CG-rom installed we get this.

Mark1960 15th Sep 2021 6:03 pm

Re: Sharp MZ-80K help needed on repair
 
I think thats good news, probably the video ram is ok as the z80 was clearing that random data to show the vertical bars, though it should be clearing it to blank.

I’m going to make a guess that ic43 might have one bad channel, instead of writing a space character to every screen position its writing the code for a vertical line. If you have a character set listing for the mz80k we might verify that is possible.

It would probably need a scope to try and verify the outputs of the buffer on the video ram side.

Unless anyone else can offer a better suggestion, I think it would be worth removing ic43, test it on a chip tester if you have one, and fit a socket and a new 74ls245.


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