CPC 464 Flickery Green & Cyan bands

[AMS]

Hi,

I have a CPC 464 that isn't working. The board is a Z70100 with no further identifying numbers (one of the earlier ones I think?).

I did some basic checks and voltages and clocks look OK. When I checked the Z80 M1 looked a bit quite so I replaced it. This alone made no real difference and it mostly starts to a black screen. I wrote a new OS and Basic ROM to a 27C256 and inserted it. Now if I cycle the power button a few times I get a cyan and green screen that flashes and rolls, sometimes accompanied by the tape relay clicking like slow morse code. Sometimes the screen amongst the mess looks like it's hiding the proper blue and yellow start screen but maybe that's just wishful thinking? If I press the delete key I don't get any sound.

I tried writing the diag rom to a 27C256 by concatenating the AmstradDiagUpper.Rom then the AmstradDiagLower.Rom files but it doesn't do anything when inserted to the IC103 position, but perhaps this isn't correct?

Any ideas what the issue may be? I have a scope and a multimeter so can check anything specific as needed.

Cheers

H

[AMS]

OK seems I should have written AmstradDiagLower.Rom to the IC to put in place of the OS rom.

Have done this and now I get different random screen output and a much slower more intermittent clicking of the tape relay, so it seems that the OS rom (or rom in its place) is not being read correctly perhaps?

[AMS]

Further checks would reveal that vsync looks bad and is jumping around between several different frequencies, is this because it's not being initialised correctly because it doesn't get read from the rom as it should or something else perhaps?

[AMS]

I socketed the 6845 and tested it, it's fine, even tried a different 6845 no difference. I lifted the v-sync leg of the 6845 outside the socket and checked it and it looks OK (I think), but the vsync signal on the GA does not look good. I think the issue may be the GA

[McArti0]

you can still check if the old sockets have all connections between the IC and the socket functional.

Look here... https://bread80.com/2021/06/03/understanding-the-amstrad-cpc-video-ram-and-gate-array-subsystem/

[AMS]

Checked the connections between the CRTC the ROM, the Gate array and the ram buffer, all looks good. There is connectivity between the GA and the CRTS hsync and vsync too.

How should pin 2 look from the 40007 GA the READY signal, mine looks like a 1Mhz clock is this correct?

Thanks

[McArti0]

How many chanells have you in scope.

[AMS]

2 channels

[McArti0]

1MHz Ready is good.

Swith off ROM by permanent +5v to ROMDIS

Then z80 will be in a non-chaotic sequence read FF from addr 38H from ROM, write to ram 39h, write to ram 00. check M1.

Ps. Have you 50 hz vsync and 15625Hz hsync on crtc? Pin40,39?

[AMS]

Not sure how to do this bit " read FF from addr 38H from ROM, write to ram 39h"

Hsync is ~4Khz on CRTC Vsync is jumping around all over the place or nothing

[McArti0]

I wrote what the z80 processor will do in circles. If you connect two channels to RD and WR pin 21,22 z80 you will see one read two writes.

[AMS]

Seems 21 + 22 are both just high in this state

[McArti0]

Z80 not work? 

CLK pin6 - 4MHz

pin 16,17,25,26 Hi

pin 19,27,28 freq signal.

[AMS]

Z80 is a replacement should be working fine

Pin 6 - 4Mhz clock signal ✔

16 - a few hundred mv changing
17 - high  ✔
25 - high  ✔
26 - high  ✔

19 -  square wave variable freq  ✔
27 -  square wave variable freq  ✔
28 -  square wave variable freq  ✔

[McArti0]

Z80 somethink doing

Seems 21 + 22 are both just high in this state

What's going on here?

[AMS]

This is only with ROMDIS held high - seems it doesn't like this?

What about this?

16 - a few hundred mv changing (so low)

This is without ROMDIS held high

[McArti0]

pin21,22  when z80 work, always  request read or write by low pulse on this pins.

ROMDIS has no effect to pin21,22

pin16 its interrupt request from GA

[AMS]

With pin 22 (ROMDIS) held high on IC103 this is when pin 21 and 22 on the Z80 are just high

If I don't hold ROMDIS high then I see pulsing on both pin 21 and pin 22 on the Z80

In your previous post you suggested that INT should be high, but it is low?

Thanks

[McArti0]

With pin 22 (ROMDIS) held high on IC103 this is when pin 21 and 22 on the Z80 are just high

It is not good. Check what happens then on lines D0-D7 z80.

In your previous post you suggested that INT should be high, but it is low?

INT is low when GA send short Lo pulse to z80.
Normally this happens at 300hz.