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Jameson
23rd May 2017, 02:33
I have a a VGC QL here which gives me a black screen no matter what I do - I have changed every socket-ted chip and no change.

So, I guess a motherboard transplant is in order. Chasing a working QL motherboard, any revision, any Rom type.

Thx.

DjCook
23rd May 2017, 17:54
Hi,
have good experience with Sincliar computer rapairs, also have all rare chips for QL here, both ULA's and Motorola PAL/NTSC IC included..
Black screen is quite often a symptom of dead PSU, or AC voltage missing.. So grab a multimeter, and check it first!
If no one offers You a working board - i can repair it for sure, however, postage is quite expensive from Australia to Germany..

regards,
Viktor

Jameson
24th May 2017, 01:19
Hi Viktor,

thanks fr your help buddy - this QL is in very good condition with a brand new keyboard membrane so it would be shame to let it go to waste.

The PSU is good as I have three QLs and the other two run perfectly on the PSU.

I have replaced all of the socket-ted chips and no change. I tested the voltage regulator (under the large heat-sink behind the microdrives) and it seems to be fine.... I am stumped. Any suggestions? The only thing left (unless I am overlooking something which is always a possibility) are the logic ICs which are soldered to the motherboard and the passive components - diodes, capacitors, resistors etc.

I would welcome any suggestions to track down the problem.

Thanks,

Steven.

DjCook
25th May 2017, 14:19
Hi Jameson,
the good point to start is to check the power connector - these are fragile on the QL boards and have dry joints quite frequently. Also check Motorola PAL/NTSC chip - it's responsible for the video output.
regards,

Viktor

Jameson
25th May 2017, 21:26
Hi Viktor,

thanks for your help. I have already replaced the video encoder chip, I have not checked the power input connector - I will tonight - however I doubt it is the issue as I am getting the correct voltages on the motherboard.

Any other ideas?

DjCook
25th May 2017, 22:37
Hi Jameson,
almost whole unit works from DC line, however, Microdrives, RS232 port and also Video output also needs AC voltage that is converted to DC inside QL.. I don't know wich board revision do You have, You should be able to find correct schematics online, the differences between revisions are not so big, the ROM part is different, as You can use own burned Eproms instread of Prom IC's on revision 5 motherboard..
regards,

Viktor

Jameson
26th May 2017, 04:02
Hi Viktor,

thank you again for the help. I will check the AC voltage at the input connector and report back. ;)

Jameson
13th June 2017, 11:56
Bump! Still looking for a working QL motherboard guys.... either that or a QL guru could point me in the right direction to fixing the one I have....