ST Rev B troubleshoot no video output

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stephen_usher
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Re: STM Rev B troubleshoot no video output

Post by stephen_usher »

Steve wrote: Wed Jun 16, 2021 8:54 pm
stephen_usher wrote: Wed Jun 16, 2021 8:21 pm A) Do you have diag cartridge and something to monitor the serial port output?
B) Do you have an oscilloscope so that you can watch the address and data bus (and possibly ROM select line from the MMU) when you switch on?

Without those items you might as well read tea leaves.

A mostly dead machine like that could be almost anything which will stop the CPU reading the ROM and initialising the shifter. MMU, RAM with stuck high output, bad ROM, bad traces, anything!
a) Yes I do I'll definitely use it for this
That would be the first thing I would try. It's non-invasive and if the CPU can at least read the first few instructions in the ROMs it should do something.

Remember, in fault diagnosis do only one change a a time however tempting to do a "but I can just do this too" as otherwise you can start chasing your own tail. Also do the simplest and non-destructive (and which don't involve a soldering iron) first.
B) Yes I've got all the tools these days, but it'll be the first time I use them for proper diagnosis so please don't assume I know how to do certain things yet =)
Again, another non-invasive thing to do... Set your oscilloscope to a 800nS or 400nS range, connect the fly lead to ground and hold the probe on one of the data lines on a ROM, any will do. Switch on and see if you see any squarish pulses between 0v and 5v. If you do at least something's going over the data bus. You can do the same on the address lines. Do each in turn. If any look really wonky, such as never going to 0v, then you may have found the first clue.

If you're seeing stuff on the address bus but not the data bus check the chip enable pin on each of the ROMs. If it's not going low then the MMU isn't enabling the ROMs so the CPU will get nowhere. This would suggest an MMU fault (or dead CPU).

If any of the data lines is constantly high or low when the others are going up and down this would suggest something is forcing that line to high or low. This would probably be a ROM or RAM chip but could be a passive component. Removing the ROM and getting the same (if constantly low) would rule out the ROM and probably point to a RAM chip. If the line is neither 0v nor 5v then it could be a bad pull-up or broken track.

At least these simple things should at least start you in the right direction.

You've said that the STM doesn't have a modulator so it sounds as if you've been hacking this board already. Check what you've disturbed first, though merely manhandling the board to remove the modulator could have broken tracks etc.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
Steve
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Re: ST Rev B troubleshoot no video output

Post by Steve »

By the way this board didn't come with a modulator, I didn't remove it or anything.

Also, here is the VGA led I was talking about, I'm thinking this is the root of the problem. Here is a photo of my good ST, as you can see the VGA LED is on shining orange even though the machine is turned off, the power plug is just in. On the bad ST there is no standby light. I'm guessing if I trace that power back I'll find my problem?

PXL_20210618_122726311.jpg
PXL_20210618_122726311.jpg (359.82 KiB) Viewed 1556 times
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PhilC
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Re: ST Rev B troubleshoot no video output

Post by PhilC »

@Steve it's probably something to do with the 12v, my rev c has no 12v going to the monitor socket so the vga adaptor does not work as it should.
If it ain't broke, test it to Destruction.
Steve
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Re: ST Rev B troubleshoot no video output

Post by Steve »

@PhilC Ahh okay so that would be normal for a Rev B, no worries thx. I'll be going through everything else mentioned by @stephen_usher next :)
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