MegaST - something recient.

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exxos
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Re: MegaST - something recient.

Post by exxos »

Same with ROMs in fact as well.
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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Re: MegaST - something recient.

Post by Bikerbob »

This happens on regular boot.. with Roms.. with Cloudy.. all kinds.

I did replace RP1 with a 2.2k RP.. what will that do.. Do I need to replace the 4.7k there?

Same result with the bus resistors replaced.. unless me doing RP1 is screwing something up.


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Re: MegaST - something recient.

Post by exxos »

You need to do the ones I already posted about https://www.exxosforum.co.uk/forum/viewt ... =30#p73486
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Re: MegaST - something recient.

Post by Bikerbob »

exxos wrote: Wed Nov 03, 2021 7:44 pm You need to do the ones I already posted about https://www.exxosforum.co.uk/forum/viewt ... =30#p73486
Yeah, I jumped the gun and did all 6.. I changed RP1 back to 4.7k

All done same issues.

James
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Re: MegaST - something recient.

Post by exxos »

What I tend to do in such situations is put a 10K resistor on a flylead probe and connect to 5V somewhere. Then when things start failing, just go round every pin on the CPU one at a time with the fly probe, and just keep pressing reset and rebooting and see if you can narrow it down to any particular pin.

I also do the same with a 33pF capacitor on a fly probe connected to gnd as close to the CPU as possible.

You will either have a bad connection somewhere or just fallen victim to noise which the mega unfortunately the worst machine for. It's why I stopped officially supporting my upgrades for original machines as they just end up spending more time trying to get the stock machine working even before upgrading them.
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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Re: MegaST - something recient.

Post by Bikerbob »

OK, looking for a ground for the floppy fix.. - figured I would do it since I had a spare 10k sip.

I have 5.15-5.2 coming into the board from the PSU..

But if I measure around the board.. trying to find a good 5v to grab.. its 4.9 sometimes 4.88v - SO maybe its not the psu.. but maybe something is causing resistance??

I will try again tomorrow when things have cooled down and see if the 5v is higher.

Where is the best point to grab 5v for the floppy write fix on a mega? off a cap? or a resistor? or does it matter?

James
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Re: MegaST - something recient.

Post by exxos »

Generally off the capacitors which are close to the chip is the best point.

But of course measuring 5V on the PSU you will get voltage drops right across the board. Likely the voltage will be lowest on the RAM area furthest away from the power supply. This is why need good capacitors on the power supply and motherboard.
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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Re: MegaST - something recient.

Post by Bikerbob »

exxos wrote: Wed Nov 03, 2021 9:07 pm Generally off the capacitors which are close to the chip is the best point.

But of course measuring 5V on the PSU you will get voltage drops right across the board. Likely the voltage will be lowest on the RAM area furthest away from the power supply. This is why need good capacitors on the power supply and motherboard.
No idea if this means anything.. but I hooked up another MegaST - using the PSU I was using on one with issues. It has been running now for 2 hours on the diagnostic cart no issues.. and when I measure Power around the board I am getting between 5.12v and 5.2v down at the memory, etc.. all over.

Now the original one works when really cold.. So could this be some issue with something heating up with the power and then causing Voltage drop? I dont know the power circuit on the board well enough.. but I am surprised one board to the other is accounting for up to a 0.3v difference???

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Re: MegaST - something recient.

Post by PhilC »

@Bikerbob I think you're barking up the wrong tree there with the 0.3V thing.

I think you're going to struggle to see what is going on without the use of a scope. Something is breaking down as it gets warm and without being able to scope out various data lines you won't find it, sorry.
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Re: MegaST - something recient.

Post by Bikerbob »

PhilC wrote: Thu Nov 04, 2021 5:21 pm @Bikerbob I think you're barking up the wrong tree there with the 0.3V thing.

I think you're going to struggle to see whites going on without the use of a scope. Something is breaking down as it gets warm and without being able to scope out various data lines you won't find it, sorry.
Well PhilC, I have a scope.. I unfortunately never learned how to really use it yet. Any good book, or tutorial on youtube or some instruction that would help?

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