CPC 664 Repair

[lazapap]

Hello, this is my first post here. I am trying to repair a 664. When first powered on, a blank screen appeared. Soon I discovered a faulty CPU and one RAM IC, which I replaced. Then when powered on, the following pattern appears on screen.
Things done so far with no change:
1) New Z80 installed.
2) One new RAM chip installed and all others put in sockets and tested on other machine.
3) Tested BIOS ROM on other machine and put socket.
4) Tested AMSDOS ROM on other machine.
5) Tested Gate Array on other machine.
6) Replaced CRTC and put socket.
7) Tested IC115 on other machine and put socket.
8.) Tested crystal on other machine.
Any suggestions for the next step?
Thanks
ps. I tried to post photos as attachments but they were rejected for some security reason

[robcfg]

Welcome to the CPCWiki!

Looking at your pictures, I've noticed that the MC0005B board we have scanned has a resistor above IC105 (680 Ω) that yours is lacking. The service manual confirms this.

Maybe you need to put it to get your machine working.


Other than that, the service manual points to faulty ULA, Z80A, 6845, Crystal or RAM.


Good luck!

[lazapap]

Hi. This is weird, but in the scanned mother board there is no label for that resistor. I don't think it is correct

[lazapap]

Maybe this is an undocumented m/b version

[Bryce]

From the picture you're getting, I'd suspect one of the 74LS153 chips has failed.

Bryce.

[lazapap]

Adding a 680Ohm resistor in the proposed position did not make any difference

[lazapap]

From the picture you're getting, I'd suspect one of the 74LS153 chips has failed. Bryce.

Thanks i'll try this next

[robcfg]

Hi. This is weird, but in the scanned mother board there is no label for that resistor. I don't think it is correct

According to the service manual, it's R160. Also, the service manual has a picture of it on a MC0005B board, so it should be installed.

[lazapap]

From the picture you're getting, I'd suspect one of the 74LS153 chips has failed.

Bryce.

Four 74ls153 chips were removed and tested on 6128. All working. Any other suggestion?

[Bryce]

Hmmm, running out of ideas at this point.

Bryce.

[lazapap]

Replaced the IC107 (D8255AC-2). No change. 

[dragon]

Change a crystal don't mean clock work...  Or it arrive to all ics.


You need a oscyloscope to check it right.


Also yo need check if  the comunication between ics works. Probably with one logic analizer to check ic by ic if signal arrive in and go out...  And if you view some anormally, you can check the path..


But if you don't have any of these... What I do in your case first. Was made a little code that try check if anything work.


For example put a infinite sound, change borde colour continually, change mode continually. That's things in a eeprom to view if there is some type of reaction.


Also check if ics voltages entry are o.k.  And  check it outside the  green monitor.


But not change ics only to try if it work by miracle.

[Chinnery]

Probably just the photo, but Pin7 on IC210 doesn't look soldered. Probably fine on the bottom side of the board tho.

[ajcasado]

Hi,
Just in case you haven't already done, have you tested the monitor with another machine?
Also check the PCB tracks, sometimes one is broken and is not obvious to the naked eye. I got my 664 repaired with just one little piece of wire.

EDIT
Sorry, I've read the thread again (better) an my advices were done before by Dragon.

[lazapap]

I finally managed to find the fault.    It can also be seen on the photo of the motherboard with a closer look.
In total, the following were found faulty:
Z-80 CPU
One RAM IC
One Capacitor wrong polarity