Mega ST4 - no video output at all

[asapreta]

Well, that means that MMU isn’t seeing the CPU ask for address zero and enabling the ROM.

When the CPU comes out of reset it does four 16 bit reads starting at address 0x00000000. The MMU should see this and set the ROM0 signal low.

What does the /AS line do at the MMU, on the chip “pin”, not the socket as we need to see what the chip is seeing. /AS (address select) should go low, the MMU should then enable /ROM0 and pull /DTACK (Data ACKnowledge) low to tell the CPU that the data is ready on the bus, then the CPU reads it and raises /AS the. MMU raises /DTACK. That’s what’s supposed to happen anyway.

/AS line is HIGH, on MMU, CPU and GLUE

[asapreta]

But /OE is not low on ROMs, both are HIGH.

Never? Not even some pulses after reset? You have to test this after reset, and before the CPU is halted. Same as you did with the CAS signals. Once the CPU is halted ROM will not be accessed anymore.

I was only able to capture this faint pulsing lines when reset was pressed in /AS

[ijor]

Never? Not even some pulses after reset? You have to test this after reset, and before the CPU is halted. Same as you did with the CAS signals. Once the CPU is halted ROM will not be accessed anymore.

I was only able to capture this faint pulsing lines when reset was pressed in /AS

I forgot that most analog scopes can't trigger on simple pulses. I never had an analog scope, but I assume this is faint because there is no proper digital triggering. Anyway, I'm pretty sure that AS must be working, because otherwise I don't see how the CPU would get halted.

You didn't say if you could see pulses on the ROM chip select pins after reset. Probably they are working. But just in case, please confirm you can see some kind of activity on the ROM chip select pins after reset.

Once you do that, I recommend again to concentrate on the MMU pins and socket. And I ask again, how do you remove the PLLC chips? Please be very careful because those sockets are quite fragile.

[asapreta]

I was only able to capture this faint pulsing lines when reset was pressed in /AS

I forgot that most analog scopes can't trigger on simple pulses. I never had an analog scope, but I assume this is faint because there is no proper digital triggering. Anyway, I'm pretty sure that AS must be working, because otherwise I don't see how the CPU would get halted.

You didn't say if you could see pulses on the ROM chip select pins after reset. Probably they are working. But just in case, please confirm you can see some kind of activity on the ROM chip select pins after reset.

You mean /CE signal?

Once you do that, I recommend again to concentrate on the MMU pins and socket. And I ask again, how do you remove the PLLC chips? Please be very careful because those sockets are quite fragile.

I have just finished probing all MMU pins and they have connection.

I use one of these to extract the MMU/GLUE

extrator-plcc.jpg

I was willing to buy new PLCC sockets, like these:

plcc-1.jpg

plcc-2.jpg

[asapreta]

If you are not getting a CAS signal then there's absolutely no point looking at the RAM, full stop. Forget the RAM!

Tests, with CPU in socket and /HALT not lifted:

Put oscilloscope probe on Pin 6 (CAS1L) of MMU socket and hit reset: Note if there's any signal.
Put oscilloscope probe on Pin 7 (CAS1H) of MMU socket and hit reset: Note if there's any signal.
Put oscilloscope probe on Pin 8 (RAS1) of MMU socket and hit reset: Note of there is any signal.

*IF* there are signals there then the MMU has a connection to the socket.

Go to chip U59.

Put scope probe on Pin 17 (CAS1L) and note if signal after reset.
Put scope probe on Pin 15 (CAS1H) and note if signal after reset.
Put scope probe on Pin 4 (RAS1) and note if signal after reset.

If the same signals appear on those pins as on the MMU sockets you have verified that the traces between the MMU and the buffer chip are correct.

Check the power and ground pint on U59. Check pins 1 and 19 are also grounded (0V).

Put scope probe on pin 5 (CAS1L output) and note if signal.
Put scope probe on pin 3 (CAS1H output) and note if signal.
Put scope probe on pin 16 (RAS1 output) and note if signal.

Because the 74LS244 is always enabled the outputs should always follow the inputs. If they don't then the 74LS244 is probably bad. You may be able to tast this with piggybacking a known good chip if the outputs are always high and the inputs are toggling.

If there's no signal then the lines should either drift high or float at some undefinted voltage. If they are low then something is pulling them down and you may have a short. If AND ONLY IF pins 3, 5 or 16 are low when the inputs are toggling then you could possibly have a bad RAM chip which has failed in a way I've NEVER ever seen before or a short between the line and ground.

While you're at it check for shorts between CAS1L/CAS1H and ground (and each other).

@stephen_usher

Do this looks a CAS signal?
I had do do some tweaking reading this but I think I have amplified a floating signal:

WhatsApp Image 2024-10-21 at 22.27.57.jpeg

[ijor]

Do this looks a CAS signal?
I had do do some tweaking reading this but I think I have amplified a floating signal:

So it seems you have CAS working after all and we are back to square one. I suggest you wait until getting proper diagnostic tools, or may be get some help from a local technician, because otherwise we are all wasting a lot of time based on wrong information.

[asapreta]

Do this looks a CAS signal?
I had do do some tweaking reading this but I think I have amplified a floating signal:

So it seems you have CAS working after all and we are back to square one. I suggest you wait until getting proper diagnostic tools, or may be get some help from a local technician, because otherwise we are all wasting a lot of time based on wrong information.

My doubt is if this was really a CAS signal or it is just amplified garbage as I had to tweak a lot (used different ajustments in oscilloscope then the ones I used to read RAS, data and so on in the DRAMs). If so, this signal was there all the time.

As I said, it is the first time I service a ST. I am used to fix all my retro computers, mostly are 8bit. Despite I have fixed my two Amiga 500, with memory issues.

The only new diagnostic tool I will have in a few days will be the diagroms. I ordered empty eeproms yesterday and hopefully they will be delivered by the weekend.
I do have a 520 ST with I already swapped GLUE, MMU, TOS and all are working fine. CPU I swapped with one of the Amigas which were handy.
I would like to know if I can also test the Shifter, DMA and 68901 in the 520 ST. Can I?

Cheers!

[stephen_usher]

The Mega ST is essentially the same as the normal ST, just in a different form factor, so the other chips are testable in the other machine.

The PLCC sockets you showed will not fit in the ST as it uses the early staggered pin types rather than the later straight layout ones.

The "CAS" trace you showed was just noise. If it doesn't go from ground and higher than 3V then it won't work.

[asapreta]

The Mega ST is essentially the same as the normal ST, just in a different form factor, so the other chips are testable in the other machine.

The PLCC sockets you showed will not fit in the ST as it uses the early staggered pin types rather than the later straight layout ones.

The "CAS" trace you showed was just noise. If it doesn't go from ground and higher than 3V then it won't work.

I was looking for the PLCC and noticed the pins are like in "zig-zag" in ST.
I will buy, when possible, some adapters from exxos.

As the equipment are the same, probably in the weekend I will swap these ICs. I think DMA and 68901 are soldered in 520ST.
I was very skeptical about those CAS signals, they really look like noise as you said.

Do a bad RAM (or more than one) can "lock" the CAS signal high?

Thank you.

[stephen_usher]

As the equipment are the same, probably in the weekend I will swap these ICs. I think DMA and 68901 are soldered in 520ST.
I was very skeptical about those CAS signals, they really look like noise as you said.

Do a bad RAM (or more than one) can "lock" the CAS signal high?

Thank you.

Definitely not at the MMU end as there's a 74LS244 between the MMU and the RAM. Have you tested/replaced that 244. It's highly unlikely that it's that which is the problem as chip inputs hardly ever go short but it's something you can look at. 244s do go bad but usually they just don't output anything on one or more pins when the input changes. Having said that, with the line high it's unlikely to be a TTL chip as they have very weak pull-up ability and the MMU would have no trouble overpowering that.

With a multimeter could you check what the resistance is between the CAS pin ON THE CHIP, not the socket, and the CAS input pin on the 244. It should be tiny if the MMU socket is OK.

Checking the resistance between the CAS line and VCC would be useful as well, in case there's a short somewhere.