Amiga 600 black screen, no bus activity

[Eduardo L M]

Please, I would like to ask for help on fixing an Amiga 600. I'm in the point of starting to replace SMD chips, but I would like to avoid that by doing a better troubleshooting. I already checked the usual: clocks, voltages, RST, HLT, BERR pins. All is fine. The machine gives me a black screen, so the boot process stops before the amiga starts flashing white, gray or other colors on screen.

When powered on, the machine shows bus activity in data and address lines. However, the activity suddenly ceases, about 1 second after RST goes high. RST, HLT and BERR all goes high correctly. That is the situation with ROM 2.x.

When using diagrom 1.3, I get no bus activity at all, when powering up. RST, HLT and BERR are all high also. So I didn't even bother to check for serial output.

By the way, the 68000 is a chip I already replaced, but had no improvements. The older 68000 in the machine was fine. The kickstart rom was replaced too, no changes also.

I also checked every data and address line pcb traces between ROM, 68000, Agnus and Gayle, all are fine. Checked for broken solder joints on the these chips, reflowed all, checked for shorts, all fine.

I have CSYNC and a valid composite signal, but black.

I will be thankful for any insight on this. And I also have some questions:

- Would bad RAM cause a black screen? I mean, would it prevent the amiga from booting so early?

- I know that OVL signal mirrors ROM into the start of address space, and then, when the 68000 jumps to higher memory, OVL is disabled. Is that possible that the computer hangs right when OVL gets disabled? If so, which chip / part should I check?

- Is there anything special happening after about 1 second of code execution on ROM 2.x, that explains a crash? Maybe access to a different memory area, or some kind of trigger for chipset state change? Or perhaps some TTL / glue logic chip?

- Since ROM 2.x shows 1 second of bus activity, and Diagrom 1.3 shows none, I wonder what are the main differences between them, at early boot stages.

- What is the minimum circuitry that needs to work for the amiga to at least gets to the point of generating some video, or at least a color coded error screen?

Thank you all!

Eduardo

[stephen_usher]

If with the DiagROM you get nothing then that suggests that the CPU isn’t seeing anything valid when it’s trying to read the first four words. Have you verified the ROM image?

Have you monitors the chip select on the ROM to see if it’s even being accessed?

On an A500 I would have at the back of my mind an issue with Gary but it would be too early to point the finger.

P.S. I've managed to look at the schematic now. Check U21 and U22 to make sure that they're working OK. Inputs should follow the outputs when pin 19 is low.

[Eduardo L M]

Thanks for the reply, Stephen!

About my diagrom, it's fine, it works ok on other A600s I have.

About ROM CS, I'll probe it and report back. Same on U21 and U22 when pin 29 is low.

My main difficulty here is trying to delimit the minimum hardware to have bus activity (I don't know if I need some chip ram for diagrom to run, for example), and then, some diagrom serial output. Then I'll have clues on what to do next.

Thanks!

Eduardo

[stephen_usher]

For DiagROM to run you need the Gayle which is doing the address decode for the ROM to work to allow the ROM to be selected. For it to do anything useful will need Gayle to be working along with the two bus transceivers and for the serial lines Paula. For video you'll be Denise and the encoder chip.

So, to get serial output: 68000, ROM, Gayle, bus transceivers and Paula.

[CCRider]

I'm having a similar behavior on my A600 (on a JuneBug Junior motherboard), but I believe I botched a trace under the ROM on removing/reinserting.

Could such a thing cause this indeed?

[supaduper]

I'm having a similar behavior on my A600 (on a JuneBug Junior motherboard), but I believe I botched a trace under the ROM on removing/reinserting.

Could such a thing cause this indeed?

Yes I have just repaired an A600 where someone had broke a trace under the Rom socket when using a screwdriver to remove it so get a magnifier and have a good look



And Eduardo L M, yes bad ram can cause black screen

[alexh]

And Eduardo L M, yes bad ram can cause black screen

Is that true? I thought that the boot ROM code changed the colour of the screen without using any RAM?

[supaduper]

And Eduardo L M, yes bad ram can cause black screen

Is that true? I thought that the boot ROM code changed the colour of the screen without using any RAM?

Yes I know normally you get a colour error. normally green for Ram , but one occasion I had a Faulty ram chip and the screen just went to Black, so cannot explain that one .

[stephen_usher]

If a RAM chip holds that data line low then nothing on that side of the machine can get the correct data value so the system won't be able to initialise the screen.

[Eduardo L M]

Thank you all for the useful info so far! I'll dedicate some time this weekend to make more tests and report back here.

I'm very curious to check pin 19 on bus transceivers, which are driven by /OEL and /OEB lines from Gayle. But this line also has a 470R pull up resistor that must be checked, I bet it'll cause lots of bus problems if it fails.

I'll also check RAM chips, and eventually desolder them. I'm most worried about the Amiga custom chips and I'll desolder them only if needed, but RAM I can easily replace if damaged.

Eduardo