If anyone has anymore ideas on how to test this.. please..
I have now substituted all the socketed chips .. all chips from the bad machine work great in a working machine.
PhilC? suggestions?
If you feel scope is next.. what values am I looking for where? Thanks
James
Wizztonics TOS206 upgrade issue - SOLVED
Re: Anyone have it? know it?
Hi James,
Just had a look at the brief explanation of T4 in the service manual and it says that the MMU is not generating the correct address for the display.
Never had this one before but for starters I think I would try testing the address and mad lines for continuity between the cpu and the ram.
Its also worth trying to see if Diagrom will boot as you should be able to check what memory is present and test it for errors.
I've found that every Mega I have owned has suffered from oxidation or corrosion, hence metering out the address and mad lines.
Also I'd a bank of ram is having problems you will usually see ras and cash go high ( if I remember cirrectly) whereas you should usually see continuous pulsing on these lines.
But so far my money is on corrosion or bad sockets. So please make sure all pins in the plcc sockets are fine and that they are clean.
Just had a look at the brief explanation of T4 in the service manual and it says that the MMU is not generating the correct address for the display.
Never had this one before but for starters I think I would try testing the address and mad lines for continuity between the cpu and the ram.
Its also worth trying to see if Diagrom will boot as you should be able to check what memory is present and test it for errors.
I've found that every Mega I have owned has suffered from oxidation or corrosion, hence metering out the address and mad lines.
Also I'd a bank of ram is having problems you will usually see ras and cash go high ( if I remember cirrectly) whereas you should usually see continuous pulsing on these lines.
But so far my money is on corrosion or bad sockets. So please make sure all pins in the plcc sockets are fine and that they are clean.
If it ain't broke, test it to Destruction.
Re: Anyone have it? know it?
PhilC wrote: Sun Nov 17, 2019 8:10 am Hi James,
Just had a look at the brief explanation of T4 in the service manual and it says that the MMU is not generating the correct address for the display.
Never had this one before but for starters I think I would try testing the address and mad lines for continuity between the cpu and the ram.
Its also worth trying to see if Diagrom will boot as you should be able to check what memory is present and test it for errors.
Not sure.. Diagrom - you mean a diagnostic cart?? Thats how I found the T4 error, but it locks up.. so nothing further on the rom can be tested.
I've found that every Mega I have owned has suffered from oxidation or corrosion, hence metering out the address and mad lines. I will take a look at this..
Also I'd a bank of ram is having problems you will usually see ras and cash go high ( if I remember cirrectly) whereas you should usually see continuous pulsing on these lines.
This machine is 2mb soldered to the board - All the PLCC sockets looked great.. all chips have been subed out.. and work great.. I have not flipped the board over to see if something is bad on the back.
Should I attempt to remove the Wizztonics TOS switcher? The Diagcart does recognize the different TOS.. so the switcher is working.
But so far my money is on corrosion or bad sockets. So please make sure all pins in the plcc sockets are fine and that they are clean.
Re: Anyone have it? know it?
Diagrom is different to a diag cart. Its included on exxos's diag cart if I remember correctly but please check.
Diagrom works in a different way to the atari diag cart.
When you boot into the Diagrom it will tell you how much ram is present and allows you to test it, without having to boot to Tos etc.
But I think you may need to use a scope to diagnose the errors you have and flipping the board over is the next good idea. I have had daft stuff like two legs of a capacitor and resistor touching underneath. You'll likely also have dry joints and corrosion as said before. So flip it over and see what condition it's in. It might be an easy fix if you are lucky.
Diagrom works in a different way to the atari diag cart.
When you boot into the Diagrom it will tell you how much ram is present and allows you to test it, without having to boot to Tos etc.
But I think you may need to use a scope to diagnose the errors you have and flipping the board over is the next good idea. I have had daft stuff like two legs of a capacitor and resistor touching underneath. You'll likely also have dry joints and corrosion as said before. So flip it over and see what condition it's in. It might be an easy fix if you are lucky.
If it ain't broke, test it to Destruction.
Re: Anyone have it? know it?
Diagrom isn't on my cart, it needs its own cart. Though I don't know what the licencing side of things would be with it since Chucky developed it, TF patched it.PhilC wrote: Sun Nov 17, 2019 9:41 pm Diagrom is different to a diag cart. Its included on exxos's diag cart if I remember correctly but please check.
If the machine is booting TOS or trying to, then the Atari diagnostic cart (the one I make) should be fine for such issues. You can also drive it via the serial port for diagnostics.
By the sounds of it, he has a duff MMU. He needs to change it and see before looking at more serious diagnostics.
Re: Anyone have it? know it?
That's what I was thinking too exxos, or the glue but he has swapped out those chips. Hence thinking corrosion or dry joints.
I'm presuming the diag cart starts and the screen turns red with the T4 error shown.
I'm presuming the diag cart starts and the screen turns red with the T4 error shown.
If it ain't broke, test it to Destruction.
Re: Anyone have it? know it?
There could be bad joints on the CPU or MMU or anywhere really. I even had a MEGA ST which had a faulty /AS line from CPU to MMU. The track was broken somewhere but wasn't visibly broken anywhere. The CPU lines which go to the MMU & GLUE should be metered out first of all.
Re: Anyone have it? know it?
So there ya go @Bikerbob , grab your multimeter and a copy of the schematics for your mega 
@exxos out of all of my classic computers, the Megas have been the worst for corroding underneath the lacquer on the pcb.
@exxos out of all of my classic computers, the Megas have been the worst for corroding underneath the lacquer on the pcb.
If it ain't broke, test it to Destruction.
Re: Anyone have it? know it?
I have replaced both the Shifter and the MMU - both those chips work fine in KNOWN working Mega.. so thats not it. ( I in fact 1 at a time replace all socketed chips)
BUT - after the suggestion of corrosion or bad solder joints. I removed the backplane and the lower plate and looked at the bottom. Although very clean.. well maintained.. I can see joints that have been reflowed? maybe a part or two replaced? also arrow marks on solder points that I guess were suspected to poor. My point.. this board has been through this before.
On examination I do not see any BAD joints.. but I guess I could reflow the joints of the MMU and the SHifter?? As a first step? Most seem to believe that this T4 error has to do with these two.
Attached are a bunch of pictures, of the board front and back where you will see what I mention above.
It was suggested that I replace the two 74LS244 and two 74LS373 are found left hand where the RAM is placed. Can they be tested? or do you just RE-RE and thats the best way?
James
BUT - after the suggestion of corrosion or bad solder joints. I removed the backplane and the lower plate and looked at the bottom. Although very clean.. well maintained.. I can see joints that have been reflowed? maybe a part or two replaced? also arrow marks on solder points that I guess were suspected to poor. My point.. this board has been through this before.
On examination I do not see any BAD joints.. but I guess I could reflow the joints of the MMU and the SHifter?? As a first step? Most seem to believe that this T4 error has to do with these two.
Attached are a bunch of pictures, of the board front and back where you will see what I mention above.
It was suggested that I replace the two 74LS244 and two 74LS373 are found left hand where the RAM is placed. Can they be tested? or do you just RE-RE and thats the best way?
James
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Re: Anyone have it? know it?
yes Exxos, this would seem to be what has happened to this board before, there are reflows and arrow marks at bad joints. Seems like this one will not be going for sale SOON.exxos wrote: Sun Nov 17, 2019 10:32 pm There could be bad joints on the CPU or MMU or anywhere really. I even had a MEGA ST which had a faulty /AS line from CPU to MMU. The track was broken somewhere but wasn't visibly broken anywhere. The CPU lines which go to the MMU & GLUE should be metered out first of all.
If I remove the Tos Switcher to get the CPU back to stock, I would put a socket in. Then I need that blitter patch board? I have seen a pic of a board in the motherboard revision section that has a Ribbon cable seeming to come out of the CPU area.. what the heck is that?
James
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