CPC 664 repair ***SOLVED***

[Bryce]

If the AMSDOS ROM is just not answering, then the CPC will freeze with half of the welcome screen displayed just before the Ready prompt appears. If the ROM has a data bit stuck high or low this could cause a black screen. Almost the same can be said for the 8255 - general failure would allow the CPC to boot, but you would see problems with things like sound, keyboard and cassette functions, but it's also connected to the databus, so a stuck bit could theoretically cause a black screen. However, the 8255 is an extremely robust chip and other than serious over-voltage or reverse polarity cases, I don't think I've ever seen an 8255 fail at all.

Bryce.

[repetto74]

If the AMSDOS ROM is just not answering, then the CPC will freeze with half of the welcome screen displayed just before the Ready prompt appears. If the ROM has a data bit stuck high or low this could cause a black screen. Almost the same can be said for the 8255 - general failure would allow the CPC to boot, but you would see problems with things like sound, keyboard and cassette functions, but it's also connected to the databus, so a stuck bit could theoretically cause a black screen. However, the 8255 is an extremely robust chip and other than serious over-voltage or reverse polarity cases, I don't think I've ever seen an 8255 fail at all.

Bryce.

Hi Brice. Today I have tested again the Z80 and the Gate Array 40010 into a CPC 464 and they do work properly. The ROM was also replaced and did not fix so I assume that Z80, 40010 and ROM are fine. The 40015 cannot prevent from displaying something on screen so probably this is fine also.
Now as suggested by Dragon I powered on and put my ear close to the PCB in a silent environment, well there is absolute silent, no buzzing coming from the crystal or whatsoever. I measured voltage at the two crystal pins and read 0V (do not know if this is correct). I have an old scope here at my parent's place I can try to use but maybe if someone can let me know how to set it up? The probe on X or Y? Which pin on the Z80 to measure the clock signal?

[repetto74]

Ok so putting the probe on X now gives some activity


First pix is what I have on one of the crystal pin (for ground I am using the ground wire of the floppy drive).
Pix 2 is at pin 39 of the CRTC and Pix 3 at pin 40 (no activity).

[Bryce]

Hi,     you can't measure a crystal by connecting a meter to it, you will just cause the signal to collapse and it will always look like it's not working.

Now to the scope (nice retro scope by the way): There are several points you can measure. The base clock (16MHz), the CPU clock (4MHz) and the sound clock (1MHz). They can all be measured at the 40010: 16MHz = Pin 24, 4MHz = Pin 19, 1MHz = Pin 14. The ground pin of the scope probe needs to be connected to the CPC GND (that wire with eyelet that goes to the disk drive).

Now, setting up the scope: Using channel X. Turn up H and adjust F until you have a horizontal line across the centre of the screen. Now re-adjust H until the line is as sharp as possible. Set G to the middle choice (where it is in the picture). Turn off C1 to 5 and D1 to 5 (ie: all buttons out). Turn A all the way to the left. Turn B to the smallest µs possible. You may have to re-centre the trace with I after doing this. Set E to 2V and you should be ready to go.Stick the probe on any of the pins mentioned above. The ITT 710 is only a 15Mhz scope, so don't expect a crisp squarewave to appear. 1MHz will be a rounded off square wave, but 4MHz will look something like a sinewave drawn after drinking 15 beers and 16MHz will just be a blurred area across the middle of the screen.

Bryce.

[repetto74]

Hi Brice,


Thanks a lot for your tutorial and your precious time!   I think I have set this as you asked (just confirm to me that for C1 and D1 you are asking each of the most right button to be pressed?). This is the best I can have in return from the measures.
pix 1 : 16mhz
pix 2 : 4mhz
pix 3 : 1mhz


1 mhz measure is silent while for the two other I can barely see a wave but there is one. Maybe some more tweakings to do?

[Bryce]

No, all of those 10 buttons should be un-pressed. Or whatever you had chosen for your tests above, which should be fine too.

Bryce.

[repetto74]

Ok Brice sorry and G was not set to the middle position.
Now we have something better 16 - 4 and 1 respectively

[Bryce]

Is E set to 2V? If so, the clock really doeas have a problem, it looks like they are peaking at 1V?? A proper clock signal at those points should be showing a wave that covers at least 2 boxes in height.

Bryce.

[repetto74]

Hi Brice


Yes E knob is set to the 2V scale. Consider also the 40015 as tested and working on another CPC. So you think the crystal is dead or something after it? There is a TTL marked TC74HCU04P which gets the clock generation from the crystal I think.

[Bryce]

Yes, I suspect the 74HCU04 might be the issue. Try replacing it. It needs to be the HCU version, a 74LS04 will not work.

Bryce.

[dragon]

Great work guys, i upload a capture of the probably  part broken in the schematic.

[repetto74]

Better pix of waves by tweaking the knob right side of A


PS : half an hour I am trying to upload three pix but keeping saying that there is already an attachment with same name 

[repetto74]

Hi Bryce


here are link to pictures as I cannot post pictures anymore



https://ibb.co/qdxKWTx
https://ibb.co/9yfdHXP
https://ibb.co/swpKcZk


16, 4 and 1 mhz waves

[dragon]

To me sound defective, the 16mhz wave is distorted. But bryce tell better.

[repetto74]

To me sound defective, the 16mhz wave is distorted. But bryce tell better.

Ok let's wait for Bryce's conclusions on this 

[repetto74]

Today I replaced the 74HCU04 with a 74HCU04N and.....back to square one unfortunately . Still a black screen.


Do not know where to look at except the crystal and the CRTC (I forgot to measure with the scope the signal on pin 39 and 40, have to go back to where the scope is).
In case the CRTC is bad does it prevent the machine from booting or can I hear the speaker beep when pressing the DEL key?

[dragon]

Repeat the test with the osciloscope first with the new 74hc.


And try power on the cpc change it  to other positions, vertical, it maybe can help if its a bad solder.


Maybe some track can be cut. Or loss the solder connection.


You don't know the status of the speaker. So if you test del, make a doble chek with headphones and the speaker.

[repetto74]

Repeat the test with the osciloscope first with the new 74hc.

Yes I'll do that later and report then.

[gerald]

In case the CRTC is bad does it prevent the machine from booting or can I hear the speaker beep when pressing the DEL key?

Dead CRTC = no sound !
The CRTC is configured for proper display, then the GA uses the signal generated from the CRTC to generate the video signal and 300Hz interrupt.
That interrupt is used for keyboard scan and sound queue processing.
So if you do not have any HSYNC/VSYNC, there is no chance of having any sound, at least from original FW.

BTW, can you check the vsync (pin 40) / hsync (pin39) on CRTC :
hsync should look like a serie of pluse at 15.625KHz (64us period)
vsync should look like a serie of pulse at 50Hz (20ms period)

as well as IRQ on the Z80 (pin 16). Here you should have low pulse at 300Hz.
If the pin stays low, this mean the Z80 is not accepting IRQ and something is wrong (ROM/RAM ....)
If the pin stays high, this mean no hsync is being seen by the gate array.

[Bryce]

The strange shape of the clock isn't that important. The fact that it is less than 2V Peak to peak is a problem, maybe the crystal really is dead?

@Gerald: He tested the Syncs here and they seem to be giving relatively normal looking signals (considering the scope he has available): http://www.cpcwiki.eu/forum/hardware-related/cpc-664-repair-***work-in-progress***/msg176337/#msg176337

Bryce.

[dragon]

Anyway if clock continue bad chek all around the 74hc resistance etc etc.

[repetto74]

Repeat the test with the osciloscope first with the new 74hc.


And try power on the cpc change it  to other positions, vertical, it maybe can help if its a bad solder.


Maybe some track can be cut. Or loss the solder connection.


You don't know the status of the speaker. So if you test del, make a doble chek with headphones and the speaker.

I think speakers are ok  , when they are connected and I power cycle the machine I can hear a clicking sound coming out from it.
I tested again with speakers and keyboard connected but no beep from the speakers with the Del depressed. A clicking is heard when powering off.

[repetto74]

The strange shape of the clock isn't that important. The fact that it is less than 2V Peak to peak is a problem, maybe the crystal really is dead?

@Gerald: He tested the Syncs here and they seem to be giving relatively normal looking signals (considering the scope he has available): http://www.cpcwiki.eu/forum/hardware-related/cpc-664-repair-***work-in-progress***/msg176337/#msg176337

Bryce.

Hi Bryce,


Going back to the place where the scope is and will take again the measures of 16/4 and 1mhz signals with the new 74HUC04 to check if any difference. In case I have a 16mhz crystal brand new with me.

[repetto74]

Dead CRTC = no sound !
The CRTC is configured for proper display, then the GA uses the signal generated from the CRTC to generate the video signal and 300Hz interrupt.
That interrupt is used for keyboard scan and sound queue processing.
So if you do not have any HSYNC/VSYNC, there is no chance of having any sound, at least from original FW.

BTW, can you check the vsync (pin 40) / hsync (pin39) on CRTC :
hsync should look like a serie of pluse at 15.625KHz (64us period)
vsync should look like a serie of pulse at 50Hz (20ms period)

as well as IRQ on the Z80 (pin 16). Here you should have low pulse at 300Hz.
If the pin stays low, this mean the Z80 is not accepting IRQ and something is wrong (ROM/RAM ....)
If the pin stays high, this mean no hsync is being seen by the gate array.

Hi Gerald,


Will check again pin 39 and 40 of the CRTC and the Z80 IRQ with the scope and report back.

[dragon]

One question, if he use the 464 has clock?.


I mean take of the gate array in 464, put a wire in the clock socket pin.


Then in 664 take of the 74hc, and connect the wire to the pin socket in the 74hc that go to the gate array.


Then he can test 100% if with all changed it boot.


Is tecnically possible?.