MegaST - something recient.

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PhilC
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Re: MegaST - something recient.

Post by PhilC »

@Bikerbob that's great then.

As for learning how to use it I'd start with your working board and see what sort of signals you are getting on the address bus and databus lines and use than knowledge in working out what is different on the bad board.

Respectfully, if you've not used a scope before then this isn't a good scenario to learn how to use it.
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Icky
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Re: MegaST - something recient.

Post by Icky »

Our very own @rubber_jonnie has done a lot of guides and one of them for using an oscilloscope - https://www.exxosforum.co.uk/forum/viewt ... =91&t=3142
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Re: MegaST - something recient.

Post by Bikerbob »

Icky wrote: Thu Nov 04, 2021 5:41 pm Our very own @rubber_jonnie has done a lot of guides and one of them for using an oscilloscope - https://www.exxosforum.co.uk/forum/viewt ... =91&t=3142
Thanks Icky I will take a look.

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Re: MegaST - something recient.

Post by Bikerbob »

PhilC wrote: Thu Nov 04, 2021 5:37 pm @Bikerbob that's great then.

As for learning how to use it I'd start with your working board and see what sort of signals you are getting on the address bus and databus lines and use than knowledge in working out what is different on the bad board.

Respectfully, if you've not used a scope before then this isn't a good scenario to learn how to use it.
Well unfortunately where I am there are not a ton of scope users local to me to learn from. So I will do the best I can. Have to learn sometime right?

James
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Re: MegaST - something recient.

Post by PhilC »

Well good luck then @Bikerbob i'm sure you will work it out eventually.
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Re: MegaST - something recient.

Post by exxos »

Just connect to the clock and press the auto button, that's how I roll.
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Re: MegaST - something recient.

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PhilC wrote: Thu Nov 04, 2021 6:00 pm Well good luck then @Bikerbob im sure you will work it out eventually.
Yeah,, I have used it for measuring clock signals, and just seeing if a specific pin is hi or lo, but I have no troubleshooting skills, where to start, what to look for.

I guess start by seeing what waves and how they work on a good machine, and see what changes on the bad one.

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Re: MegaST - something recient.

Post by PhilC »

@Bikerbob the short answer is yes, learn what you should have find what's different, then work out why it's different.
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Re: MegaST - something recient.

Post by rubber_jonnie »

@Bikerbob
Ok, so let's just wind things back a moment. I know this may all have gone before, but a re-cap is always useful.

It's my understanding that the problem you have is as follows:

1. After a short period of running the Mega starts to show signs of failing and diagnostics show errors and don't accurately report memory.
2. The PSU works just fine on another machine when left for a long period of time (2 hrs).
3. The problem still occurs with a new Delta PSU.
4. You've replaced the bus resistor packs as per the mandatory fixes.

It's probably unlikely to be the PSU or bus resistors.

The next steps are basics. You may have already done some of these. You need to do these prior to breaking out the oscilloscope. A working machine is also very useful to allow for direct comparisons with the bad machine.

I know you have checked for bad chips with freezer spray, but also if you can touch them to see if any are running hot once it fails. This is all major chips plus the RAM.

Have you used a magnifying glass or microscope to look for bad solder joints on the main chips?

Scope basics are here if you haven't already looked: https://www.exxosforum.co.uk/forum/viewt ... =91&t=3142 and include videos that you may find helpful.

If you haven't already done so, remove the GLUE and MMU and follow this guide: https://www.exxosforum.co.uk/forum/viewt ... hilit=plcc to reset the socket pins. Blitter too if fitted.

Before reinserting the chips clean the pins with IPA and give the sockets a spray with DeOxit, Electrolube or Servisol contact cleaner. If there are any other socketed chips, such as the Blitter, do the same for them, and in the case of any DIP chips such as the ROMs, the sockets should be fine, but clean the chip legs with IPA and give the sockets a spray as above.

Check ALL of the chips have good power and ground connections. To check this you will need to have the schematics to hand as they will show you which pins on which chip do what. If you can get datasheets and/or pinouts for the custom ICs then all the better for later.

For the 5v tests you can use a multimeter or scope. A scope is a fancy multimeter, so using it to check voltages is a good way to get started with the scope. Connect the small crocodile clip on the scope probe to ground (GND) which can be the negative side of any capacitor and use the probe to touch the 5v (VCC) pin of each chip in turn to verify 5V is present. There may be a 1x/10x switch on the scope probe, make sure you're set to 1x and on the scope screen you should see 5v on screen as 5 divisions on the y axis. The x axis represents time, so for voltage measurement we don't care about that right now, just that 5v is registered. I'd suggest you test this with a good 5v supply so you can see the scope behaviour.

To test ground (GND), do it with power off and simply use a multimeter in continuity mode (Beep test) to verify each GND connection to each chip beeps out, signifying a connection to GND.

With power off, and using a multimeter in continuity mode check all of the address and data lines between the main chips, so CPU, MMU, GLUE etc. If you can, run the same check immediately after it has just failed with power off.

I'd also check with a multimeter at this point that the CPU /RESET line, pin 18 and the /HALT line, pin 17 are at 5v when the machine is running. Pressing and holding the reset button should make /RESET drop to 0v. The /HALT line only generally comes into play on an error, and that can either come from the CPU if the executed code is garbage, or from an external source reporting a /HALT worthy issue.

It's worthwhile checking the clocks around the board with the scope. Again make sure the scope probe is set to X1, connect the flylead to GND and check the following by touching the probe on the relevant pin:

1. CPU - Pin 15 8Mhz
2. MMU - Pin 5 16Mhz, pin 19 4mhz, pin 20 8Mhz
3. GLUE - Pin 34 8Mhz, pin 43 500Hz, pin 54 2Mhz
4. Shifter - Pin 2 32Mhz (This may not be a countable frequency on your scope, but it should still give you a trace), pin 39 16Mhz (32Mhz will show as twice as many peaks and troughs as this)
5. DMA - Pin 39 8Mhz IIRC, but I can't find out what it should be from my book for some reason.

The scope 'should' to a degree automatically figure out what you're measuring and preset it 'On Screen'. Sometimes you may need to adjust the timebase or set a trigger to capture what you need, but each scope is different so it is a case of RTFM.

It's important to rule out the basics prior to diving in with a scope, as if for example some chips have a bad GND or don't get 5v, then using a scope won't help, as you need to sort out the fundamentals.

My money would be on one of the following:
1. A chip failing when it's gets warm. Cooling it may not help as the failure has occurred and may be unrecoverable, which is why I look for hot chips. You can cool spray chips and hit reset to see if it helps too.
2. A socket may be failing when warm, but is subject to the same issue stated in point 1, so check for hotspots, or use the cool then reset method.
3. A joint may be failing when warm. This is much harder to find, but possible nonetheless. You can press on a socket and hit reset to see if it recovers.

Overall, do as many basic checks as possible before you pull out the scope.
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Icky
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Re: MegaST - something recient.

Post by Icky »

Great walk through @rubber_jonnie.

@Bikerbob following this through should uncover the underlying problem.
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