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

pudwink 15th Sep 2021 6:16 pm

Re: Sharp MZ-80K help needed on repair
 
I have today ordered a second hand oscilloscope so if you could tell me what I need to do with it when it arrives it would give me some practice on learning how to use it.

Timbucus 15th Sep 2021 6:24 pm

Re: Sharp MZ-80K help needed on repair
 
Of course it is quite easy once you get the hang of it - and a real fascinating tool to examine things with.

Mark1960 15th Sep 2021 7:22 pm

Re: Sharp MZ-80K help needed on repair
 
What kind is it? Does it include scope probes?

ScottishColin 15th Sep 2021 8:40 pm

Re: Sharp MZ-80K help needed on repair
 
I have nothing to contribute (sorry) but I'm loving reading this thread. It'll get fixed with the help of people on here. It's not possible to get more helpful and knowledgeable people.

Colin.

SiriusHardware 15th Sep 2021 11:36 pm

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

ic43 might have one bad channel, instead of writing a space character to every screen position its writing the code for a vertical line.
The results when manually forcing different character values, plus the clean but random video character output with the CPU removed, does suggest that the video circuit is displaying OK so therefore what is being placed in the video RAM for it to display (vertical lines) must be wrong.

Pud sounds confident and capable of changing IC43 which is a common, relatively low cost IC, so I agree, remove IC43 in one piece and test it and if the tester shows it to be faulty, fit a socket and a new replacement IC43.

Note to pudwink: We are assuming you have plenty of experience at removing ICs in one piece from double sided PCBs. If not, it is better to cut all the legs off the IC high up next to the IC body and desolder and remove the pins from the board individually. If you do this you deny yourself the opportunity to check / test the original IC but you are much less likely to damage the PCB while removing the IC.

Don't be too despondent if replacing this IC does not in itself fix all of your problems - for one thing you need a full working set of RAMs as well, but there may be more problems waiting to be found. If so, we'll try to chase them down one by one.

pudwink 16th Sep 2021 3:05 am

Re: Sharp MZ-80K help needed on repair
 
I have brought a Hitachi V525 50Mhz Twin channel oscilloscope with probes.
With the IC I do have two adjustable soldering irons one with hot air and a decent solder sucker but I will play it by ear I do not want to cause any damage to the board.

Mark1960 16th Sep 2021 7:31 am

Re: Sharp MZ-80K help needed on repair
 
That looks like quite a capable analog scope, it should be fine for repetitive signals, but can be a bit more tricky to set up.

Are the probes x1, x10 or switchable?

SiriusHardware 16th Sep 2021 7:36 am

Re: Sharp MZ-80K help needed on repair
 
Your 50MHz Hitachi scope would have been considered quite high end in its day, most general purpose scopes at the time would have been 20MHz. If it is coming by post or courier I hope the seller has made every possible effort to protect it from impact or drop damage.

I have a V212 which was my only scope for many years.

As far as removing an IC intact from double sided PCB goes, the one really important thing to remember is, when you've removed 98% of the solder from the IC pins, never, never try to lever the IC up out of the PCB by inserting something under one end and trying to crowbar it up out of the board. If you do that the IC will come out, yes, but there is a good chance that one or more of the top side PCB pads or tracks will be torn off the board.

Once you have removed as much of the solder as you possibly can the IC will still be attached by thin solder bonds where the wider upper part of the IC pins rest on the top side solder pads. What you have to do then is use a blunt wooden 'pusher' like the square end of the wooden handle of a wire brush to push the whole IC from side to side, left, right, left, right until the remaining solder bonds crack and you can just lift the IC out without any force at all.

For ICs which are cheap and easily available, we normally recommend just cutting the pins off the IC, removing the IC body and then desoldering and removing the individual pins as that is much less difficult to do and less likely to harm the PCB.

pudwink 16th Sep 2021 1:11 pm

Re: Sharp MZ-80K help needed on repair
 
All I know about the probes is they are Hitachi AT-10AK and yes I hope the seller does pack it up well but I have had some very poor packing off sellers recently.
Well today I have cut the chip out rather than cause any damage to the board and put a socket in I could not get a SN74LS245N but got a DM74LS245N I am hopeing they are the same thing I turned on the machine but still the same lines on the screen.

Mark1960 16th Sep 2021 5:05 pm

Re: Sharp MZ-80K help needed on repair
 
Yes the DM74LS245N is the same thing, just a different manufacturer. Bad news that didn’t work. We’ll give some directions for further debug with the scope.

When the scope arrives check for any physical damage first and power it up. With nothing connected you should be able to get two flat lines, one for each channel. Try initial settings as 1ms/div on the horizontal timebase, 0.5v/ div on each channel. Trigger select switch at the top right set to auto. Turn the brightness to about mid setting. You probably need to adjust the vertical position of each channel and possibly even the horizontal position. After you have the two lines visible, turn the brightness down and adjust the focus to get a nice clear line.

If the probes have a switch, set them to x10. If they are fixed x10 they should have a small trimmer in a hole on the body of the probe, or possibly on the connector that goes to the scope to compensate for frequency response.

Connect the scope probes to the scope, and check the calibration output of the scope at the bottom left of the controls, you should see a square wave at this point. Adjust the vertical and horizontal settings of the scope so the square wave is about five divisions height and width.

If the probes are x10, adjust the trimmer on the probe, this should change the shape of the rising edge of the square wave. You want to adjust this for as square a shape as possible, flat at the top with no overshoot. Each probe should be calibrated like this for the channel its going to be used on, then avoid swapping them, or calibrate again after swapping.

Mark1960 16th Sep 2021 6:40 pm

Re: Sharp MZ-80K help needed on repair
 
Did you read the contents of MROM to verify it contains the correct code?

If its blank and reads FF, this would be a RST 38 instruction, pushes program counter to the stack then starts executing from 0038, if thats also FF this writes every memory location with 0039 and might look like its clearing the screen to a fixed pattern.

pudwink 16th Sep 2021 7:46 pm

Re: Sharp MZ-80K help needed on repair
 
I do not have a setting for a 2332 chip so used 2732 settings to read and it is showing blank with these settings but can you tell me if a different chip setting is required to read it please let me know.

Mark1960 16th Sep 2021 9:02 pm

Re: Sharp MZ-80K help needed on repair
 
I think reading a 2332 as if it were 2732 should show FF from 000 to 7FF, but the contents from 000 to 7FF of the 2332 should be seen at 800 to FFF.

Reading a 2332 as a TMS2532 should work, if you have that option.

pudwink 17th Sep 2021 6:47 am

Re: Sharp MZ-80K help needed on repair
 
XGPRO does not have TMS2532 either reading it as a 2732 shows FF all the way

SiriusHardware 17th Sep 2021 8:24 am

Re: Sharp MZ-80K help needed on repair
 
If you want to be sure about the MROM and you don't mind parting with it for a few days I can read it for you and say for sure whether it is faulty. PM me if you want to go that way. (I wouldn't ask or expect anything for doing that).

Alternatively, it is possible to make an adaptor which will allow you to read the 2332 as though it is a 2732 by rearranging the wiring of several of the pins.

If you already feel that the original MROM device is dud there is a forum member here (1980s_john) who was recently able to supply programmed TMS2532 EPROMs programmed with any code of your choice, hopefully he still can provide that service. Try PMing him for details of availability and cost. You would need to be able to email the correct code to him or point to the relevant file on the internet somewhere.

https://www.vintage-radio.net/forum/member.php?u=1784

pudwink 17th Sep 2021 8:51 am

Re: Sharp MZ-80K help needed on repair
 
Many thanks for your kind offer but as the rom itself is not in the best shape corroded legs and one broke off so had to replace it.
I think if I could get a replacement somehow even if it's not the fault it might be better so I have sent a message to 1980s_john and see what he says.
It is a shame I can't burn one myself as I enjoy creating things myself if I can.

SiriusHardware 17th Sep 2021 9:15 am

Re: Sharp MZ-80K help needed on repair
 
There is another way, look in the usual places and you will find adaptor PCBs which are designed to allow a 2732 to be plugged into a socket originally meant for a 2332/2532. You program the code into a 2732, plug it into the adaptor and then plug the 2732+adaptor into the socket meant for the original ROM. I don't recall the adaptors being especially cheap, but maybe you could look at the commercially made ones and knock one up yourself. Either way, that would be a way for you to progress without having to rely on third-party help. I completely understand when you say you would prefer to do as much of the actual hands-on work as you can yourself.

If you wanted to make it look neater later you could always get a 2532 programmed at that point.

pudwink 17th Sep 2021 4:11 pm

Re: Sharp MZ-80K help needed on repair
 
right I have gone for plan b I have another programmer coming tommorow that will do 2532 eproms.

pudwink 18th Sep 2021 5:58 pm

Re: Sharp MZ-80K help needed on repair
 
Got the new programmer and monitor rom still shows as blank so I am hopeful this could be the cause.
Have some 2532 EPROMs on the way and will see if I can program one up.

SiriusHardware 20th Sep 2021 11:07 am

Re: Sharp MZ-80K help needed on repair
 
Probably not a good time to ask you this, but do you have an EPROM eraser? Any second hand devices you buy may not necessarily be blank (so do a 'blank check' before you try to programme them).


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