464 black screen, diagnostics rom working

[eto]

I got a faulty 464 - the seller sold it together with a 19V power supply, so I guess I have the culprit ;-)

I tried with my normal 5V power supply and, as expected, it was not working. No signal, no beep. I then cleaned the connector and connected a Dandanator. When I first turned it on, the Dandanator worked and showed its normal menu - which was a total surprise. So I turned off and on again while selecting the diagnostics mode. This time, no image was shown, but I heard the beep and a few seconds later further beeps, which indicate a RAM fault.

I am now a bit surprised that it worked one time and then stopped working. I somehow can't imagine that a roasted chip will come back to live for a few seconds just to then die again. And with faulty RAM I should never have gotten an image in the first place. 

Any idea what this could be or where I should start the investigation?

[Bryce]

Sounds like the ROM is damaged.

Bryce.

[eto]

Sounds like the ROM is damaged.

Bryce.

Can that have an impact, when I am using the diagnostics ROM and I hear that it's working?

[Bryce]

Really? I thought the Dandator booted from its own ROM and ignored the internal one?

Bryce.

[eto]

Really? I thought the Dandator booted from its own ROM and ignored the internal one?

Bryce.

The Dandanator has a special mode where it replaces the lower ROM, afaik similar to the X-MEM. When I press a button while turning on the CPC it starts in this mode and I can hear the beeps that indicate that it runs the RAM test.

The Dandanator works flawlessly on another machine and at least once, I got some screen output on the broken machine.

[Bryce]

And what "tells you" that the ROM is working?

Bryce.

[eto]

And what "tells you" that the ROM is working?

Bryce.

Maybe I don't get it:

I don't know if the internal ROM is working but my understanding was that this doesn't matter when I am using the diagnostics ROM.

I know that the diagnostics ROM is working as it produces one beep when it starts and then a second later 3 further beeps, which indicate a RAM error. That's the expected behaviour, just no screen output. 

As diagnostics ROM takes over, I would have expected, that the internal ROM is disabled and should not have impact on the boot process. Is that assumption wrong?

[eto]

Update:

I don't have an oscilloscope but I could check with my multimeter if all chips get the right clock signals. The 6845 does not produce a proper HSYNC and VSYNC signal. VSYNC is 0, HSYNC is 8Hz. other pins like M0 show the expected frequency (500Khz for M0). Would that be an indication that it's just not properly initialized? Or can it still be fried?

Could it also be IC 110 which sets CE for the 6845? 

[Bryce]

The internal ROM is disabled via it's CE pin, but all of its data and address lines are still connected to the associated buses. So if any of these pins are shorted to ground or VCC then it's holding an address or data pin high or low even though the ROM is disabled.

Check if any of these pins are stuck before you start looking at IC110.

Bryce.

[eto]

The internal ROM is disabled via it's CE pin, but all of its data and address lines are still connected to the associated buses. So if any of these pins are shorted to ground or VCC then it's holding an address or data pin high or low even though the ROM is disabled.

Check if any of these pins are stuck before you start looking at IC110.

Bryce.

Great catch. I measured 116 Ohms between D2 and VCC. The others seem to be fine. If the ROM chip can be faulty in this way, I guess also the RAM chip (IC118) would be a likely candidate to cause this issue?

Update:
It's the ROM indeed. I cut D2 of the ROM and I still get 116Ohms between D2 and Vcc but at the other chips, there is no longer a connection.

So next step: burn an EPROM.

Thanks a lot!

[eto]

Finally it's working again. I took the chance to add the 664 firmware with BASIC 1.1 and add a button to switch between Basic 1.0 and 1.1. Works like a charm. 

Thanks a lot Bryce for pointing me into the right direction regarding the ROM.

Really amazing that only a single chip was fried by the 19V. 

[Bryce]

Yeah, you got lucky. If any chip shorts extremely fast and creates a very low resistance short, this will overload the PSU and the voltage on the PCB will drop significantly. In your case, enough so that the other chips didn't get damaged.

Have you got your numbers in for Saturdays Lotto? 

Bryce.

[wed]

I had a similar problem with a faulty 464. It was drawing (way) too much current, no display and way off sync signals, and three of the RAM chips were really hot. 

I changed the RAM chips, got lost for a bit changing the inverters on the crystal (since this Rev D board doesn't use NAND gates like in the schematic for squaring and dividing the clock). And still nothing. The CPC was still drawing more current than I was happy to see. 

After probing signals, the data bus lines were all inactive and several chips were warm, including the ROM. Confirmed the socketed ULA and Z80 were good by checking in another 464, the only other chips that seemed to assert the data bus looked like a 74LS373 and the ROM. SInce the 373 wasn't warm I took out the ROM and saw better signals on the data bus. 

Had an old 27C256 EPROM pulled from an old industrial machine which I managed to erase with a hand COVID sanitizer lamp. Wrote a program to flash it with the 40009-R2 ROM using an ATtiny3216 and a couple of 74HC595 chips, and now my 464 works again with BASIC v1.1!

It was easy to suspect the CRTC because VSYNC and HSYNC were completely wrong, but avoided that trap of desoldering it thanks to one of Noel's YouTube videos where he said there is no video with no ROM since the CRTC needs to be initialised by the ROM before it will create the right signals. 

So there are two traps in the widely available CPC464 schematic to watch for (at least in my Rev D board): the ULA chip pinout is for the de-populated socket below the revised chip, so I was probing all the wrong lines for a while, and the crystal clock circuit doesn't use the same components to divide and square the clock signal, using a 7404 inverter chip and not a 7400 NAND chip. Does anyone have the correct schematic for a Rev D board??

I think the ROM chip failed by its output enable jamming on, causing the data lines to be stuck and the computer was drawing way too much power as other chips on the board were simultaneously trying to assert the data bus. 

Hope this information is useful for anyone with similar problems. Perhaps the 40009 has reliability issues and I'm not the only one who's found this problem. 

[Bryce]

The production methods and technology that were used to create masked ROMs like the 40009 are inherently weaker than the technology used to produce standard logic ICs. So there is always a higher chance that the ROM will fail before other ICs when there is an over-voltage condition.

Bryce. 

[eto]

I have another 464 where RAM chip 5 is shown as malfunctioning. I have replaced the chip (I tried several chips) and no difference. There seems to be no broken trace. It probably also was provided with 19V as the other CPC. 

Would that also be a candidate to remove the ROM chip? This time there is nothing outstanding (like the 116Ohms between D2 and VCC). Anything I can check to "know" if that's the problem?

[Bryce]

Check whether the buffers (244 / 373) are passing the data through first.

Bryce.

[eto]

I replaced the 244 and the 373, still no change. 

I do get a signal on the pins related to D5, so it's not stuck to GND or VCC. Not measured with an oscilloscope, just with my multimeter. 

[eto]

I have now replaced the ROM and everything is working again. 

[WacKEDmaN]

good stuff eto!

hmm... i wonder if the rom maybe causing my gray screen issue...
ill drop the original 464 rom in later n try it!..or atleast pull the chip and try the diagnostics from lower rom board again...