CPC 664 repair ***SOLVED***

[repetto74]

Hi Guys,


I picked a CPC 664 untested for a few bucks some days ago and first time I powered it up the led was off and black screen. I firstly suspected the infamous power switch but a test with the multimeter confirmed it was ok. The PCB was covered with a thick layer of dirt and dust so I decided to give it a good cleaning before continuing the troubleshooting. I removed all socketed IC's, cleaned with water and soap and let it dry under the sun. I sprayed some contact cleaner on sockets, power connector and video connector.
Second power up made things moving as the power led came to life but still a black screen, then I power cycle again and after a few seconds the prompt screen pop up like magic and looked fine. Then after another power cycle I got a black screen again and again. I though the problem may be bad caps so I replaced all of them. On first cold power up the prompt  blue screen was there again then another power cycle and black screen again  . I am trying to guess where the problem is coming from. I think that the machine is powering up correctly but something is preventing the video to be displayed once one or more components are energized (as I said on a cold power up it will turn on then if the CPC is switched off and on again it will just give a black screen). I have tried to wiggle the video connector and applied fresh solder but no change.
Any suggestion on where to look at? I have also a CPC 464 in case some IC's need to be swapped for testing.
Thanks,
Rick


PS : I have good skills in soldering and desoldering station for easy IC removal in case.

[dragon]

If it boot, you can check it pressing del, then it should made a piriiiiiiii from the speaker. Or run"disc, if its booting it try read the fdd. If it sound, that mean he have read the rom and boot, so you can discard  the rom the z80 the ppi and the ay at least.

Also you can check first if all ics are reciving the 5v when it show black screen.

[Bryce]

It sounds like a problem with the reset circuitry, which is unusual, that happens extremely rarely. But it may just be a dry joint somewhere in the system, check all the solder joints.

Bryce.

[repetto74]

It sounds like a problem with the reset circuitry, which is unusual, that happens extremely rarely. But it may just be a dry joint somewhere in the system, check all the solder joints.

Bryce.

Any particular area where to look at and apply fresh solder?


All caps where replaced except for one Non Polarized caps of 1uF (I don't have this NP in my stock and it is one of the caps sitting top left area close to the DIN9 connector) but I really doubt this can cause the issue. I will try to make it boot again and check with the keyboard connected as suggested by Dragon then look for areas where to apply some fresh solder.

[Bryce]

I've marked the chips here that look after the reset. Look closely at the clock circuitry (directly above the right red circle) too as an unstable clock could also be an issue. Also check that the voltage isn't too close or just 4.75V as this can also cause something like this. It may be a problem with resistance in the 5V socket or power switch.

Bryce.

[repetto74]

I've marked the chips here that look after the reset. Look closely at the clock circuitry (directly above the right red circle) too as an unstable clock could also be an issue. Also check that the voltage isn't too close or just 4.75V as this can also cause something like this. It may be a problem with resistance in the 5V socket or power switch.

Bryce.

So cold start now will give just a black screen . I have measured at the power connector a steady 5.08V. Is that too much? The power switch seems not to be the problem as when I take it out and shorten the two power pins on the pcb with a screwdriver I have the same black screen of death. Need deeper investigations              
I checked the resistors line and the diode close to the crystal and they look fine. I am close to the resistance values for each but for a proper test I should normally desolder them and test out of circuit. The diode is ok, resistance on one side and infinite the other way around.
Could be the 74LS 132 or any chance the Z80 will not reset properly?

[Bryce]

Try shorting pin 26 of the Z80 to GND just for a second. This will give a soft reset. If the computer boots you have a problem with the reset circuitry, if it still has a black screen, then you most likely have a RAM or ROM issue.

5.08V is perfect. Less than 4.75V or more than 5.25V is a problem.

Bryce.

[repetto74]

Try shorting pin 26 of the Z80 to GND just for a second. This will give a soft reset. If the computer boots you have a problem with the reset circuitry, if it still has a black screen, then you most likely have a RAM or ROM issue.

5.08V is perfect. Less than 4.75V or more than 5.25V is a problem.

Bryce.

Thanks Brice, I'll do that and report back.

[repetto74]

I made a short with a wire between pins 26 and 29 on the Z80 but still silent. The ROM is the one socketed marked 40015 right? Can I program an Eprom 27C256 and test it? Where can I find the bin file to load? Alternatively can I use a 32k bib with both Basic and OS of the 6128? Just need to find a 32k compiled binary ready to be programmed.


PS : The Basic+Rom should be the 40022 as the 40015 is the AMSDOS rom as far as I understood.

[dragon]

They are in the wiki, but splitter.


http://www.cpcwiki.eu/index.php/ROM_List

[repetto74]

They are in the wiki, but splitter.


http://www.cpcwiki.eu/index.php/ROM_List

Found a 32k bin for the CPC664 coming from the MESS romset thanks.

[repetto74]

They are in the wiki, but splitter.


http://www.cpcwiki.eu/index.php/ROM_List

Just to know for future manipulations. When merging the two 16k BIOS and BASIC into one single 32k what do you put first?

[repetto74]

Try shorting pin 26 of the Z80 to GND just for a second. This will give a soft reset. If the computer boots you have a problem with the reset circuitry, if it still has a black screen, then you most likely have a RAM or ROM issue.

5.08V is perfect. Less than 4.75V or more than 5.25V is a problem.

Bryce.

I have swapped the Z80 with another one and same issue. I have programmed a 27c256 with a 32k BIOS+Basic for the CPC664 and will swap the ROM. If that does not fix then I will start with RAM. Is there any ram diagnostic I can use with an Eprom?

[dragon]

Just to know for future manipulations. When merging the two 16k BIOS and BASIC into one single 32k what do you put first?

Lower rom+basic.

[repetto74]

Lower rom+basic.

Thanks 

[gerald]

I have swapped the Z80 with another one and same issue. I have programmed a 27c256 with a 32k BIOS+Basic for the CPC664 and will swap the ROM. If that does not fix then I will start with RAM. Is there any ram diagnostic I can use with an Eprom?

Ram test : http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/quick-and-dirty-ram-test-for-cpc

[repetto74]

Back with some more testing reports.


Replacing the ROM with a programmed Eprom does not fix this  . Next step will be ram I suppose? Chances that the video chip is gone?

[Bryce]

It's unlikely, but they do on rare occassions fail. The quickest way to test it is to stick a scope on pin 39 and/or pin 40. These are the outputs for VSync and HSync and they usually stop outputting anything when the CRTC fails.

Bryce.

[repetto74]

It's unlikely, but they do on rare occassions fail. The quickest way to test it is to stick a scope on pin 39 and/or pin 40. These are the outputs for VSync and HSync and they usually stop outputting anything when the CRTC fails.

Bryce.

Does not have a scope unfortunately. I do have a replacement for this chip I will keep this option later. Changing ram IC's is the easiest now. Suspecting also the crystal to not working properly at this stage.

[repetto74]

Ram test : http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/quick-and-dirty-ram-test-for-cpc

How does this work exactly. There is a bin file of 485 bytes. Can I program this into an eprom and swap the original rom?

[gerald]

How does this work exactly. There is a bin file of 485 bytes. Can I program this into an eprom and swap the original rom?

Yes, no more, no less !
Just program the .bin file at the beginning of a 32k eprom and put it in place of the system ROM.
It will test part of the RAM at boot and report result on screen. Anything red is not good.

And a black screen mean that there is something that prevent the test to be run.

[repetto74]

Yes, no more, no less !
Just program the .bin file at the beginning of a 32k eprom and put it in place of the system ROM.
It will test part of the RAM at boot and report result on screen. Anything red is not good.

And a black screen mean that there is something that prevent the test to be run.

Ok thanks I will try that also :-)


Unfortunately today I removed all rams, put sockets and replaced with a fresh new set of TMS4164 but still silent 


Actually this CPC has booted a couple of time randomly and then definitely died.
What has been done :
All caps replaced except the Non Polarized one of 1uF which I do not have
Z80 swap and soft reset
ROM swap with a 27C256
All rams removed and replaced
still no signs of life.


I will try to run the ram test tomorrow. I am just hoping that cleaning this pcb with water and soap has not killed it but the only signs of life I had was after this good cleaning.
I am start to think that if the video popped up suddenly this may be that the machine boots up normally but I cannot see it cause one or more component of the video circuitry are dead. There are a couple of transistor close to the video din socket and I may have to test those.
Other issue may just be the CRTC dead or the crystal not working. Any suggestion would be appreciated at this stage.


What is the purpose of this black box?

[dragon]

Its the cassette relay. The thing that molested me, as in the begging of all  you don't have power led. Thats because i tell you check the voltage in all ics, even when you have led later.


You don,t have another cpc 464 or 6128 with a 40010?. You can try your gate array in another cpc to discard is socketed in all cpcs.

[repetto74]

Its the cassette relay. The thing that molested me, as in the begging of all  you don't have power led. Thats because i tell you check the voltage in all ics, even when you have led later.


You don,t have another cpc 464 or 6128 with a 40010?. You can try your gate array in another cpc to discard is socketed in all cpcs.

I dragon,


Ok so this relay cannot prevent the machine from booting. Yes I do have a CPC 464. Good idea I can swap at least ROM, Z80 and the Gate Array 40010 to confirm they are working on the 464 :-). By the way the 664 has also an AMSDOS Rom 40015. Could it be that a bad 40015 can cause a black screen? And what about the I/O 8255? Can this also prevent from booting?


PS : the disk drive is disconnected (but I had a couple of succesfull boot before this eternal BSOD)

[dragon]

Well, probably it cause more a bluescreen that black screen. Because the fdd is initalise later that the screen in the rom i think remember.


The ppi control the tape part, keyboard, and the sound and is atached to the crtc vsync.


Probably your next option when the gatearray was tested. should be if the clock works and crtc works.


You have ear a cpc sometime?. The cpc make a little noise when is working a trutruturrutrutuu. That can indicate if the clock work. Totaly crazy test. Try if you ear it in the 464 when boot and compare with the 664.


But if you can check the clock with normal equipament better .