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The famous grey screen black frame issue.

Started by oyshals, 12:52, 13 February 23

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Bryce

The FDC (IC201) will get a little hot if the boot sequence gets stuck somewhere because the chip gets trapped in some sort of initialisation loop. I've had this a few times and more than once it was ROM 7 that had failed.

Bryce.

oyshals

I socketed and replaced both the 40015 and 40025 roms tonight, sadly no difference. The new roms however has not been tested with a working computer though. Waiting for a diagnostics rom which @Rabs was kind enough to send me now.

oyshals

#152
So I started thinking that I have a shortcirquit, or almost anyways.

With board disconnected I measured between +5v and gnd on random chips, and found it to be 1,4k ohm, on some chips as low as 1,2. Seems a bit low. Can anyone confirm?

If this is the case, are there any usual suspects, or would I have to desolder chips one by one measuring?

Rabs

Quote from: oyshals on 17:53, 02 May 23So I started thinking that I have a shortcirquit, or almost anyways.

With board disconnected I measured between +5v and gnd on random chips, and found it to be 1,4k ohm, on some chips as low as 1,2. Seems a bit low. Can anyone confirm?

If this is the case, are there any usual suspects, or would I have to desolder chips one by one measuring?
I think you are just measuring the parallel resistance of all devices on the board.

oyshals

Thanks! I think I'll just wait for the diag rom now 😊 Thanks again for that!

Btw; Should I try to remove the three capacitators I found to be cracked? (the orange ones by the ram banks)

Rabs

Sorry, I don't have any experience of failed ceramic capacitors on the CPC but they are needed to smooth the power to the IC. I would wait until you get Diagnostics ROM.

oyshals

Quote from: Rabs on 19:41, 03 May 23Sorry, I don't have any experience of failed ceramic capacitors on the CPC but they are needed to smooth the power to the IC. I would wait until you get Diagnostics ROM.
Will do that, just been reflowing some solderings today, can't seem to leave it alone 😅

oyshals

Thank you @Rabs for the diagnostic rom!

Sadly I haven't been able to get it to work. I actually seem to get the same symptoms as when I push the diagnostic button on the Dandanator, the screen turns black, no grey square 🤔 Almost like something is blocking any diagnostic roms from initializing.

I've fiddled alot with the board now, so I think I will start retracing my steps.

Rabs

Quote from: oyshals on 22:26, 20 May 23Thank you @Rabs for the diagnostic rom!

Sadly I haven't been able to get it to work. I actually seem to get the same symptoms as when I push the diagnostic button on the Dandanator, the screen turns black, no grey square 🤔 Almost like something is blocking any diagnostic roms from initializing.

I've fiddled alot with the board now, so I think I will start retracing my steps.
I guess that this is not unexpected and proves the Dandanator is working. Can you check VSYNC again and make sure this is 50Hz. If it's not, then why is it correct when the original ROM is used? I guess the only thing you have not changed is the CRTC.

oyshals

Quote from: Rabs on 22:49, 20 May 23
Quote from: oyshals on 22:26, 20 May 23Thank you @Rabs for the diagnostic rom!

Sadly I haven't been able to get it to work. I actually seem to get the same symptoms as when I push the diagnostic button on the Dandanator, the screen turns black, no grey square 🤔 Almost like something is blocking any diagnostic roms from initializing.

I've fiddled alot with the board now, so I think I will start retracing my steps.
I guess that this is not unexpected and proves the Dandanator is working. Can you check VSYNC again and make sure this is 50Hz. If it's not, then why is it correct when the original ROM is used? I guess the only thing you have not changed is the CRTC.
Will compare all signals with the original rom and with the diag rom, will also re-visit all my solderings. I actually have a new CRTC and a socket for it, so might try that one as well.

Rabs

I agree, it would be a good idea to retrace your steps and check each stage. Only yesterday, I was tracking down a problem with a new board I had built for my rack based z80. I concentrated on the board itself only to find a ZIF socket on the ROM board was causing ramdom program failures. So I know one bad connection can cause havoc on the bus.

My guess is something is corrupting the bus and there are a number of devices on the bus that can do this, including RAM, AMSDOS ROM, Disk Controller and PIO.

My assumption is that if you get a VSYNC of 50Hz, that the Lower Diagnostics ROM has initialised the CRTC, which is one of the first things it does and is the distinguishing difference between a grey screen and a black screen. What I don't understand is why you get a grey screen with the standard Lower ROM and a black screen with the Diagnostics ROM.

Early on the Lower Diagnostics ROM also programs the AY sound chip to emit a beep. I guess you have never heard this?

I am also assuming that the Z80 and Gate Array are good because you have replaced these at some stage but it would be better to confirm this with a known working CPC. I am also assuming the CRTC is working because you see a grey screen and in the past a colour pattern.

I would, check the simpler 74 series logic ICs, just because this is simple to do, and make sure the logic outputs match the inputs, just incase one of these is incorrectly asserting a device on the bus.

After that I would, start with a minimal system running the Lower Diagnostic ROM and hence remove the chance of bus corruption as much as possible. So no AMSDOS ROM, no RAM, no Disk Controller and even maybe no PIO.

oyshals

Maybe it just wants to die 😅 (I won't let it)

No, haven't had any beeps during my process.

I'm actually itching to try to replace the disc controller as I got a pretty strong reaction just by touching it. And because the malfunctioning disc drive was the reason it was put away 25 years ago. But I'll retrace first, as it has become a bit unstable. I think I didn't clean properly before installing sockets.

oyshals

@Rabs I've got something to show you....

Rabs


oyshals

Ok, this is embarrassing 😅 I've forgotten how to upload pictures to the forum 🙈

However, I've got news. After removing the disc controller, the diagnostic's started! First sign of life!!! However, on my screen, I've got a big, black square on the top left which I would like to post an image of here when I find out how to post it again 😅

Rabs

Go to preview and add attachments 


Rabs

Quote from: oyshals on 16:52, 03 June 23Thanks!
Oh that's an odd one, what do you have installed at the moment?

oyshals

I think it's related to my bad soldering skills, think this is the time to re-visit my old work...You cannot view this attachment.

Rabs

Quote from: oyshals on 17:05, 03 June 23I think it's related to my bad soldering skills, think this is the time to re-visit my old work...You cannot view this attachment.
Well done getting somewhere now  ;D

oyshals

I'm having some issues with the display, suspect this might be an issue with the connector. The display issue comes and goes.

The MUX chips and poor socketing (my recent work) are also one of my suspects. Both for the display issue and the RAM issue. I've replaced the RAM chips, but the fault doesn't change. 1 and 3 working, and the rest faulty. Will address this issue when I get the time.You cannot view this attachment.

Rabs

Quote from: oyshals on 20:42, 03 June 23I'm having some issues with the display, suspect this might be an issue with the connector. The display issue comes and goes.

The MUX chips and poor socketing (my recent work) are also one of my suspects. Both for the display issue and the RAM issue. I've replaced the RAM chips, but the fault doesn't change. 1 and 3 working, and the rest faulty. Will address this issue when I get the time.IMG_20230603_180629.jpg
Time to check all the tracks, its easy to lift a pad when desoldering. But at least you can see activity now.

oyshals

@Rabs Does your Diganostics rom test ram bank 0 or 1 when it boots? Maybe I have to do the fal-trick?

Rabs

Only bank 0 to start with. Only if this passes do you get to the main screen to test upper banks of RAM.

There is a github repository on Noel's diagnostic ROM for info.

Rabs

In case you need it, the github page is Amstrad Diagnostics.

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