6128 doesn't output video (no sync, nothing)

Started by issalig, 14:33, 16 July 21

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Hi all, I recently got a "for spares" 6128 board but before salvaging components I want to try to resurrect it (and/or learn a lot).
ALL RAM chips were superhot, I removed them and check with a DRAM tester they were all faulty.
Appart from this 765 controller was also hot, so I removed it also and I will leave it without it for now 'cause I do not need the disk drive.
As there is no video (no hsync, novsync, nothing) I decided to check 6845 CRT controller and I have realised that EN (pin23) is always "0"
I went back through the lines, and checked NAND IC110 pins (8,9) which are /IORD and /IOWR and were all "1", thus "0" for NAND is ok.
After that I check OR gate IC112 which outputs /IOWR and /IORD from Z80 and it happens that /RD and /WR seem ok (5V pulses) BUT /IORQ is always 5v "1" causing OR gate to be always "1"

I checked the schematics and it results that /IORQ is pulled-up via resistor R154
Then I thought about a faulty Z80 that cannot set /IORQ to "0" but I tested it ok in another computer.

Gate Array is also ok because I tested it in other computer.HAL16L8 is also working ok .

Any idea?

I also checked the data lines, and while some look nice (0v-5v pulses) others present strange pulses (see images below with 2v or 3v maximum)
At this point I am also supecting about latch registers IC114 and IC115, 74LS373 and 74LS244 respectively, so I am planning to remove them and check them with TL866ii

Any ideas?


Is there any signal on pins 39 and 40 of the CRTC?



No, HSYNC (39) VSYNC (40) are 0 volts. Looks like a fried CRT controller :(


It sounds like the system has had an over-voltage incident.



Yes, it looks like it was powered with more than 5V (maybe 9V)
I have checked latches and the 2nd image is the ouput of a 0v signal with some little spikes similar to the little ones it presents. But i do not understand this 2v, It is clear that this is not normal.
I also checked 74LS373 and 74LS244 and they work ok, so the bad signal should originate in other place, maybe time to check the capacitors of those data lines.
Keep on investigating and hope not to break more things :) , but this is the funny side of learning to repair :)


I changed all the RAM chips, and tried with a fresh ROM and finallyt I could see something: a white screen with black border. Thus, CRT controller is sending HSYNC and VSYNC (i.e. not fried but now I have 5 CRT controllers on the desk :) )

Then I checked 74LS32 that drives IORQ because it was stuck alwys to 5V but logic chip was ok.Then I noticed some data lines were around 2V. It looked like the repair was getting too hard.

At this point I decided to take an extra W27C512 and burn Amstrad Diagnostics ROM from @llopis and Eureka!!!!!!!

I have checked all the "new salvaged" RAM chips with and arduino tester from (https://forum.defence-force.org/viewtopic.php?t=1699) and they were ok, but Diagnostics ROM spotted #0 as a problematic one, I changed it and another 6128 came to life.
Arduino powers the chip with 5V and my 6128 with 4.5V maybe this chip was in the limit and this is the reason.

Hope my adventure can help others and thanks to everybody that helped me.

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