Weird CPC664 Fault

[Audronic]

Hi all 

I am Having problems with a CPC664 Motherboard repair.
I was wondering if you may have struck this before.
Can you Help

This is using Geralds WIP Tester

Thanks Keep safe 

Ray

03 9 397 7695 In Williamstown Victoria

[Audronic]

A small update.

Boot up screen is Unwell
The top line should say "AWA" not Isp ?

Any thoughts ?

Thanks

Kep safe

Ray

[GUNHED]

A bug in the firmeware ROM?

[eto]

Maybe a malfunctioning CRTC or broken traces from CRTC to the multiplexers. 

How did you start Geralds RAM test? From ROM? Can you also try LLopis' Amstrad Diagnostics?

[Audronic]

A bug in the firmeware ROM?

Hi Gunhed

Ok Thanks

I have changed the 40015 rom and this has helped
About an Hour later only a Blank screen Grrrrrrr

I am about to change the Gate Array and see what happens.

Thanks

keep Safe

Ray

[Audronic]

Maybe a malfunctioning CRTC or broken traces from CRTC to the multiplexers.

How did you start Geralds RAM test? From ROM? Can you also try LLopis' Amstrad Diagnostics?

Hi Eto

I Have a lower rom board with Geralds and Noels Diags - Switchable..

Thanks I will buzz out the lines from the CRTC  to the Mux's

Keep Safe

Ray

[Brocky]

i think one of the lines on the mux's has failed...
i dont think a buzz out will help... better off lookin at the lines on a scope if ya have one...the physical connection could be fine but one of the chips is outputting bad signal on a line

im not sure about geralds diag... but on noels, if you get a memory error you should be able to track down which ram chip thats 'failing' is connected to which mux chip
(i know ya can with geralds too but.. i not sure what numbers correspond to which ram chip)

[Audronic]

i think one of the lines on the mux's has failed...
i dont think a buzz out will help... better off lookin at the lines on a scope if ya have one...the physical connection could be fine but one of the chips is outputting bad signal on a line

im not sure about geralds diag... but on noels, if you get a memory error you should be able to track down which ram chip thats 'failing' is connected to which mux chip
(i know ya can with geralds too but.. i not sure what numbers correspond to which ram chip)

HI Brocky

Ok I am half way thru testing the continuity, I will keep going to Pin 40 from the CPU to all pins connected to where ever.
Yes I have a scope will do that next.

Ok Noels Diag gives Me 1 Beep (HI) goes thru the Memory test, screen changes to Blue and 3 LOW Beeps and that's it ??

Thanks

Keep Safe

Ray

[Audronic]

Hi Brocky

Have tested all the pins from the Z80 to what it is connected to all Test OK

I will look at the Mux chips with the scope

Thanks

Here is a copy of the Test points for a CPC664
From the Z80 to wherever.

Keep Safe

Ray

[eto]

I have changed the 40015 rom and this has helped
About an Hour later only a Blank screen Grrrrrrr

Did it fully work after changing the ROM? 

Is it blank grey screen with black border now or just a black screen?

[Brocky]

noels diag seems to be passing which is a little odd.. should show a screen with green and red bars if the ram is bad....

&7C  = 0111 1100
&78  = 0111 1000

...could bits 3 be bad? maybe bit 1 and 2 aswell? could still be linked to mux or even crtc... im speculating!! i cant find any info on gerard... the version here shows a different tester...
check the lines with scope around the lower bits ram and muxs and .. n see if theres anything you can see

theres a good bit on it at 8:40 mark in noels vid...

[GUNHED]

A bug in the firmeware ROM?

Hi Gunhed

Ok Thanks

I have changed the 40015 rom and this has helped
About an Hour later only a Blank screen Grrrrrrr

I am about to change the Gate Array and see what happens.

Thanks

keep Safe

Ray

Oh. Good you got one step forward. But I'm sorry to hear about the black screen. Could be some kind of interruption of power or a signal due to heat, but the chances for that are not the best.

Really HOPE it will all work out for you and your nice CPC664. 

[Bryce]

Hi Audronic,
      if you haven't tried a different CPU, then do that first. Then check the timing of the reset circuit. Then check for cracks / dry joints on the address lines around the CPU area.

Bryce.

[Brocky]

edit: oops sorry .. i didnt mean to post!

but yeah dont listen to me! ... bryce knows what hes doing!

[Bryce]

edit: oops sorry .. i didnt mean to post!

but yeah dont listen to me! ... bryce knows what hes doing!

Don't underrate your own suggestions, even the most experienced experts forget or miss the simplest things sometimes. Every opinion should be heard.

Bryce. 

[Audronic]

Hi Audronic,
      if you haven't tried a different CPU, then do that first. Then check the timing of the reset circuit. Then check for cracks / dry joints on the address lines around the CPU area.

Bryce.

Hi Bryce

I have done all of those things EXCEPT the RESET area

The Machine  has now completely changed is activity it won't wake up, if it does it shows various screens.

Thanks for all your help

Keep Safe

Ray

[Audronic]

Hi Bryce

You are correct The Bl---dy reset circuit is unwell it gives a double plus during "INSTANT" reset.
I have compared it to an other 664 that I have been using for comparison

CPC664
Do you know What the Diode 101 " 1S2472-HS " is used for ??
Its in Parallel with a 3.3Meg Ohm Resistor ??

The Manual for it basically says a HIGH Speed Switching Diode.

Do you know of a working replacement - Please

Keep Safe

Ray



 

[Audronic]

Hi All

I have changed the diode for a 1N4148 (seems to be an equivalent).
The chip comes out next.

CPC664
This is what a FAULTY RESET Pulse looks like


Keep Safe

Ray

[eto]

if you haven't tried a different CPU, then do that first. Then check the timing of the reset circuit.

Nice catch. And I am curious (and eager to learn)...

I would never have thought about those areas based on the symptoms.

What of the symptoms lead to your conclusion?

[GUNHED]

Well, I would bite into a 8" disc if the CPU would be the problem   But nevertheless the Z80 is into a socket, so swapping it will not take much time. Give it a try! 

[Audronic]

if you haven't tried a different CPU, then do that first. Then check the timing of the reset circuit.

Nice catch. And I am curious (and eager to learn)...

I would never have thought about those areas based on the symptoms.

What of the symptoms lead to your conclusion?

Hi Eto

The reason I was led to the Reset area was by Bryce and the intermittent nature of the machine trying to start ??
I have noels diag (External) and it would start " Most times" 
Have had a few Z80 Marched thru the machine and roms  etc etc.

I have replaced the Diode and the IC and the results are the same --POOR Reset signal
I will have to find a .1MF capacitor tomorrow

The resistor values are OK and they are connected to the IC Socket  ---Phew this is a monster Machine

Keep Safe

Ray

[Bryce]

if you haven't tried a different CPU, then do that first. Then check the timing of the reset circuit.

Nice catch. And I am curious (and eager to learn)...

I would never have thought about those areas based on the symptoms.

What of the symptoms lead to your conclusion?

The clue was the incorrect Manufacturer. The 8255 reads the manufacturer from the links on its port B, but it needs slightly longer than the Z80 to initialise on startup. So when the CPU requested the info from the 8255 it wasn't ready to answer.

@Audronic : A 1N4148 should be fine. Check for dry joints and that the resistors R101, 102, 145 haven't drifted or gone open circuit and that the capacitor is still the correct value. If all the passives are good, IC110 is probably the issue. IC125 could be the issue, but it's also used to define the clock signal, so the clock would probably be dodgy too if it was the problem. If you do need to replace IC125 it has to be the HCU version, because it doesn't have a buffer like all the other 74xx04 chips have. The buffer would stop the clock from oscillating properly.

Bryce.

[Audronic]

@Audronic : A 1N4148 should be fine. Check for dry joints and that the resistors R101, 102, 145 haven't drifted or gone open circuit and that the capacitor is still the correct value. If all the passives are good, IC110 is probably the issue. IC125 could be the issue, but it's also used to define the clock signal, so the clock would probably be dodgy too if it was the problem. If you do need to replace IC125 it has to be the HCU version, because it doesn't have a buffer like all the other 74xx04 chips have. The buffer would stop the clock from oscillating properly.

Bryce.

Hi Bryce

Thanks

Yes I have replaced it with the same type 74HCU04.

My Measurements were done on Pin 14 on the 74HCU04 before the 74HC132 IC110

I will pull the chip out of the socket tomorrow (to reduce loading on the resistors) and confirm the Resistors.

I think by the "Bounce" in the reset line that the Capacitor is probably Unwell

Thanks

Keep safe

Ray

[Bryce]

Isn't the capacitor a ceramic disk cap? They rarely fail other than when they are physically damaged. If the two IC's have been swapped and the resistors are good, it's probably a dry joint or a broken track you should be looking for.

Bryce.