CPC-464 freezes on start-up with black background colour

[Tezz]

On the off chance have you got or tried a spare Z80?

Yes I've swapped the Z80 previously with the known working one in the long board 

[Tezz]

Check wire LK1, LK2 - 4

Please check this!

LK1, LK2, LK3, LK4. or 8255 pin 19,20,21,22 to ground and turn on CPC. changing Brand on screen

Test Does this computer read any IO!!!

Ok, will take a look at that, thanks 

[McArti0]

Still not sure because if the IORD was at fault, would the CPU just not read random data and not not crash or hang?

Rabs. What can be read in CPC? "Never" read CRTC, 8255 PA, 8255 PB. thats all.
PB at start and as Tape, PA as keyboard.

[Rabs]

Actually links on PORT B of the PIO is not a bad shout. These drive the branding on the start screen and looking at earlier when you had the OS and BASIC ROM in it looked ok. So reading that ok, maybe. I am stuck at this point.

[McArti0]

IC 112 pin6 voltage nearest 5V - sound ~50Hz.

[Rabs]

Oh hang on @McArti0 may be correct about the 60Hz thing.

So you don't have any link in LK4, so should be 50Hz (I think). You are using a modern TV (I assume with a SCART connection?) and you get black and white. Quick google and there is some inference that if at 60Hz you get black and white (maybe someone else can confirm this happens). What if the CRTC is a bit dodgy, somehow?

Your PCB below
You cannot view this attachment.

By the way I socketed IC112 and tied pin 6 high to simulate a fail on IORD and it does not give your symptoms.

[Tezz]

Oh hang on @McArti0 may be correct about the 60Hz thing.

So you don't have any link in LK4, so should be 50Hz (I think). You are using a modern TV (I assume with a SCART connection?) and you get black and white. Quick google and there is some inference that if at 60Hz you get black and white (maybe someone else can confirm this happens). What if the CRTC is a bit dodgy, somehow?

Your PCB below
You cannot view this attachment.

By the way I socketed IC112 and tied pin 6 high to simulate a fail on IORD and it does not give your symptoms.

Hi, yes I'd kept an LCD TV from the time they still included analogue connections along with digital. My other systems have various TVs and monitors. I do also still have a 100hz CRT TV in my conservatory that I use when working on repairs that I can use. The CPC is connected via a SCART which is the same one I used before the fault occurred. It works fine with the long board btw on this TV.

Good to know that you've looked into IC112. Thanks for doing that. Will eventually discover the issue by process of elimination!

[Rabs]

If you do decide to swap the CRTC, try to get a type 0.

Type 0 (Hitachi HD6845S, UMC UM6845)
Type 1 (UMC UM6845R)
Type 2 (Motorola MC6845)

[McArti0]

"socketed IC112 and tied pin 6 high to simulate a fail on IORD and it does not give your symptoms."

Nonsens. permanet  +5v on pin 6 going to crash output LS132 and never read keyboard.

Two tests to true.

Check IC 112 pin6 voltage nearest 5V - sound ~50Hz.

Wired for ex. Lk2 and turn on cpc.

Two test at 60 sec!!! Not 4 days.

[Rabs]

"socketed IC112 and tied pin 6 high to simulate a fail on IORD and it does not give your symptoms."

Nonsens. permanet  +5v on pin 6 going to crash output LS132 and never read keyboard.

Two tests to true.

Check IC 112 pin6 voltage nearest 5V - sound ~50Hz.

Wired for ex. Lk2 and turn on cpc.

You are correct sorry, I just left pin 6 floating, not connected.

[Tezz]

If you do decide to swap the CRTC, try to get a type 0.

Type 0 (Hitachi HD6845S, UMC UM6845)
Type 1 (UMC UM6845R)
Type 2 (Motorola MC6845)

Thanks Rob, I was reading up about the types a few days back. I'll certainly look to get the HD6845S. I read that a lot of sellers pass off the other types listing them as HD6845S so I'll have to make sure to check with the seller. The diagnostics procedure should verify it once installed too incase it's been re-screen printed in China. I'll get the original CRTC socketed today so that's it's ready for the replacement.

[McArti0]

But why!!! Check that IORD pin6 ic112 first and PB

[McArti0]

Your CRTC is not detected because Soft not ending this loop:

    ld     b,#f5            ; wait Vbl
@vbLoop1
    in    a,(c)
    rra
    jr    c,@vbLoop1

This loop is reading PB 8255 and dont read Bit 0 because all IN instruction read &FF in your CPC.

You understand?

[Rabs]

Hi, I know what you mean, but what confuses me is that the Diag ROM is able to read the link status connected to port B.

It is even reporting the refresh of 50Hz from link 4.

You cannot view this attachment.

Then it hangs at the point it attempts to read the CRTC register.

And then why is it in black and white.

I must admit I am very confused.

By the way I did close link 4 on my CPC to select 60Hz and on my TV it was still in colour.

[McArti0]

confuses me is that the Diag ROM is able to read the link status connected to port B.

Not able to read 

Look. If You have bad read FF that is mean like LK1-4  Hi , Printer Busy Hi, Vsync always Hi. Tape data read Hi. Exp is Hi
 All is "good"

[Tezz]

Ok, catching up this morning. This would suggest either the PIO or IC112 (SN74LS32N). 

Could you explain clearer the test needed at pin 6 of IC112.

I could remove it and test in the TL866II+ if necessary. They're only 50p or so if it needs to be replaced 

[McArti0]

Pin6.  Check by multimeter - voltage ~4,9V
           Pin6 wired to sound line pin1 expansion port. Hear sound 50hz. If ic112 is seek then no sound.

[McArti0]

Second. Lk2 wired and turn on cpc. On screen should be brand Orion not Amstrad

[McArti0]

How sound work You can test on pin3 ic112 IOWR because work good.

[Tezz]

Pin6.  Check by multimeter - voltage ~4,9V
          Pin6 wired to sound line pin1 expansion port. Hear sound 50hz. If ic112 is seek then no sound.

Thanks! I'll do those tests this afternoon.

[Tezz]

Ok, voltage at pin 6 of IC112 is 3.91v. pin 6 of IC112 to expansion port pin one produces a quiet buzz with line noise (This is with the diagnostics ROM installed). shorting LK2 with a wire link, diagnostics ROM still reports Amstrad.

[McArti0]

do You check pin3 to compare.?  This is LOUD Buzz.

[McArti0]

shorting LK2 with a wire link, diagnostics ROM still reports Amstrad.

Crash ic112. Insert original Rom and repeat test.

[Tezz]

I've retested with the standard ROM installed, IC112 pin 6 voltage is 3.8v, pin 6 of IC112 to expansion pin 1 gives the same output as previously with the diagnostics ROM installed, a quiet line noise. Pin 3 of IC112 to expansion pin 1 produces clear loud buzz. Shorting LK2 still shows Amstrad.

[McArti0]

Last test ic112 before desolder.
Pin 37 expansion port BUSRQ to ground, pin 4,5 ic112 to ground and You should have 0V on Pin6 but you have not.

Pin 37 expansion port BUSRQ to ground, pin 1,2 ic112 to ground and You should have 0V on Pin3 and you have.