CPC 664 repair ***SOLVED***

[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.

Bryce the hsync and vsynch were measured with the scope not set up properly so better if I redo this.

[Bryce]

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?.

Technically yes, but you would have to also connect the ground of both computers together and keep the wires as short as possible, you may have to even use shielded wires.

Bryce.

[repetto74]

Here are the result with measures taken again with the replaced 74HCU04. I had to tweak a lot to try to lock the waving signals it is very sensitive.
Please note that I have used the 2V scale but for the IRQ Z80 pin 16 and the Vsinc on the CRTC I had to go down to 50mV scale to properly display a wave.
The first measures on cold start where exactly the same as the one posted before in this topic but strangely after letting the CPC run for a while I started having the signals here below. 

PS : I forgot to align the two beams on the scope to the centre but I think that does not alter the results.

Three first pix are for the 16/4 and 1 mhz signals


pix 4 and 5 for the Hsync and Vsync(50 mV scale)


Pix 6 the IRQ signal (50 mV scale also)

[dragon]

A clock is a clock it not should change over time c130, c131, c132?

[repetto74]

A clock is a clock it not should change over time c130, c131, c132?

Of course I still have a black screen. I am just keeping it turned on for a while to see if any changes. How to test the three ceramics? Can they fail? Is the Vsync signal on the CRTC correct and not weak then?


I have this 16mhz crystal in case...

[gerald]

Clock look fine for me, but are you sure you are on the 2V scale for the 3 first picture. I would consider the gate array to be OK.
The 16MHz is lower than the other, but that's expected. The OX710 only have a 15MHz bandwidth, that mean a 15MHz sine would be attenuated by 3dB ie divided by 2.


Other picture taken on 50mV range just mean you are capturing noise, not signals.

BTW, if you are only using one channel, disable the second one (ie only YA key pressed)

So if the Z80 and ROM are proven good, the CRTC is probably dead.

[dragon]

But the clock is diferent when it power up gerald thats normal in a clock?.(crtc apart)..

Of course I still have a black screen. I

If you force a reset signal when the clock is estable?.

[repetto74]

Clock look fine for me, but are you sure you are on the 2V scale for the 3 first picture. I would consider the gate array to be OK.
The 16MHz is lower than the other, but that's expected. The OX710 only have a 15MHz bandwidth, that mean a 15MHz sine would be attenuated by 3dB ie divided by 2.


Other picture taken on 50mV range just mean you are capturing noise, not signals.

BTW, if you are only using one channel, disable the second one (ie only YA key pressed)

So if the Z80 and ROM are proven good, the CRTC is probably dead.

Yes I was on the 2V scale as suggested by Bryce for the scope set up. The 40010 has been tested good on the CPC464. The Z80 has been also tested good on the 464 and the ROM was replaced with a programmed 27C256 but that did not fix so I assume the ROM is ok also. All 8 ram chips are replaced with fresh new TMS4164 also.
The Vsync signal on the CRTC is completely flat on the 2V scale.

[repetto74]

But the clock is diferent when it power up gerald thats normal in a clock?.(crtc apart)..



If you force a reset signal when the clock is estable?.

you mean a soft reset by shorting pins 26 and 29 of the Z80? tried already but no luck.

[gerald]

But the clock is diferent when it power up gerald thats normal in a clock?.(crtc apart)..



If you force a reset signal when the clock is estable?.

Well, the clock should start "instantly".
If the issue is only a clock startup issue, yes, a reset could do something.

[dragon]

But better hardware reset: http://www.cpcwiki.eu/index.php/Reset_Button

[repetto74]

But better hardware reset: http://www.cpcwiki.eu/index.php/Reset_Button

The CPC664 pcb is different but it may not be a good idea to add a circuit and more uncertainty. If the clock signal appear to be ok and the gate array is fine then the CRTC may be the problem. It may have worked a couple of time when I had a correct boot then failed permanently. Or could be also one of the TTL logic on the pcb.

[dragon]

I refer more to made the same that with soft reset made a shortcut in the expansion connector.


But if you prefer try the crtc perfect.

[repetto74]

I refer more to made the same that with soft reset made a shortcut in the expansion connector.


But if you prefer try the crtc perfect.

I can try shorting Pin 40 of the expansion port to gnd. Is is the long connector at the back?

[repetto74]

But better hardware reset: http://www.cpcwiki.eu/index.php/Reset_Button

No reset after shorting pin40 of expansion port to GND.

[dragon]

. Then try the crtc, the good news is if he broken in another time you have all in socket. 

[repetto74]

Guys I have desoldered both 6845 from the two CPC, socketed and swapped. Believe it or not but the CPC464 will boot up normally so the CRTC is fine also  . I do not know anymore where to look at.
There is the I/O chip 8255 but apparently this is not prompt to fail (does he stop the booting if it fails?).
It can be anything at this stage. I have no clue where to look at. Too bad this thing came to life for a brief period of time
Can I test the CPC664 Rom with the CPC464? Rom version is 40022 where the CPC464 is 40009.
****EDIT**** I swapped the 40022 ROM in the CPC464 and normal boot. With this test I can declare at least that all main IC's are fine (missing sound chip and 8255).
Could be also the sound chip? Does it give a black screen when faulty?


If I let this machine run for a while the only two IC's getting warm are the Z80 and the CRTC, the Gate Array and Rams are cold.
As already suggested by Dragon the CPC emits a buzz from the crystal when powered on right? In my case there is a complete silence. Is it possible that the crystal is not oscillating correctly?

[dragon]

You can try what i suggested early, use the cpc 464 as clock, at least to not unsolder the crystal the three caps and the resistance for nothing. And discard it 100%


The next step for me was take the polimeter, take the service manual, and check continuity between ics resistance valors etc etc. But as bryce tell the 8255 can be a remote posibility.


Or take logic analyzer and take a look at the bus to view what is happen here between the ics.

[repetto74]

Hi Dragon and Bryce


SUCCESS!!!!! 


In a last desperate attempt I tried to put some fresh solder around the crystal circuit area on all transistors, caps and resistors. Believe it or not it is alive again!!! 
Another CPC664 saved from the trash!
Thanks a lot to all of you for your help and guidance. Job is done!


"IT'S A SMALL STEP FOR MAN BUT A GIANT LEAP FOR CPC!" 

[dragon]

One question You have  a buzz now?.

[repetto74]

One question You have  a buzz now?.

No buzz. I am 45 I think my ears are ok but maybe the frequency range is out of my reach 

[dragon]

Jeje,well thats oscyloscope deformed signal nerver sound well to me. Maybe you can check the 16mhz now   to view how it should be really for future reparations.

[repetto74]

I have tested the keyboard, no sound from the speaker with the DEL key and some keys are not working such has "ENTER", "F7","F4" and a few others. The keyboard needs a good cleaning I hope to fix this.

[dragon]

Membrane. And in the 3,5mm socket?  You have sound?

[repetto74]

Membrane. And in the 3,5mm socket?  You have sound?

Not tested yet. I will fix the membrane before and then reassemble everything.