Sinclair ql black screen

[Traindriver69]

Hi all
I have just recently brought a ql as untested and unfortunately it does not work.
When plugged in it's just a black screen.

But if you press the reset it sometimes shows this screen.

I have purchased a minerva rom hopeing the ram test would pick out faulty ram chips but it fails to boot with the minerva rom as well.
Any help would be great.

[Popopo]

Hi,
There are some basis things to test first at all when getting a new old computer.
1. Power voltages from power supply.
2. Shorts in main board between power lines and GND
3. Measure of the power regulators inside the computer, in this case in QL the main one is the +5v from the linear regulator.

Please to do that firstly, before continue.
Also would be very very very useful to take some photos of your main board where we could see version, how it is, capacitors state... whatever.

I would pay attention to the membrane, probably it will need replacement, but not to buy it before to get working main board.

[stephen_usher]

If you have the ability to write an EPROM then swapping the Minerva ROM on its board with a copy of my diagnostic ROM may give you more information about the fault.

[Derek_Stewart]

Hi,

The above is all good advice, but lack of display, in most cases is a faulty 8301

I would check the operation of CPU, 8301, 8302, 8049.

[Traindriver69]

Hi
What do I need to look for to test these chips I had read that if on pressing F1 you don't hear the microdrive boot which it doesn't then it is unlikely to be the 8301 or is this rubbish.
Many thanks

[stephen_usher]

Hi,

The above is all good advice, but lack of display, in most cases is a faulty 8301

I would check the operation of CPU, 8301, 8302, 8049.

The second image shows a display, so the video output is fine.

It's probably a lower memory bank memory error, which is bad enough that Minerva can't start up.

However, the basic 68xxx CPU checks are important, but to do them you need an oscilloscope or at the very least a logic probe and multimeter.

The golden set of tests for all CPUs: Power (is it getting the correct 5V?), Clock (is there a good clock frequency going into the chip?), Reset (Is it staying low for a while before going high when power is applied?)

On the 680x0 chips it's good to check that the /HALT line is high.

[stephen_usher]

Hi
What do I need to look for to test these chips I had read that if on pressing F1 you don't hear the microdrive boot which it doesn't then it is unlikely to be the 8301 or is this rubbish.
Many thanks

The system isn't getting far enough to look at the function keys. It's not getting far enough to run the Minerva ROM in any real sense.

[Traindriver69]

Hi
Tested the cpu the voltage is 4.91v the clock signal is showing this.

With the reset it is showing this.
https://youtube.com/shorts/ag-cjybbV9M? ... 4JgIvzISPY

[Popopo]

Hi
Tested the cpu the voltage is 4.91v the clock signal is showing this. IMG_20250710_100416_edit_41840307230913.jpg
With the reset it is showing this.
https://youtube.com/shorts/ag-cjybbV9M? ... 4JgIvzISPY

Beautiful oscilloscope, I love it.
So the main rail of +5v is working fine & not shorts between +5v and GND.

Next... use the same oscilloscope and check each addrs line related with Memory and also Data out of each RAM IC.
If we are lucky, there will be one that has not Data Out activity.

Consider also the 2 multiplexers bound to the RAM, if one of them fails, you could experiment something similar to what you are experiencing now (too).

After you check that, we can continue with other ICs (ZX8301).

Please to note and share each ADDR line and DATA line that works fine, Since many ICs are connected to the same lines, it helps to keep in mind those others.

Note: Share photos. You can use https://postimages.org/

Note2: it may happen that after some seconds (about 60 sec) the system boots fine but you don't see anything. Since probably your membrane (keyboard) is faulty, perhaps you could manually simulate a key press, if the system is working properly (but the video module) you may hear in the speaker the beep key pressed. That is also a really easy test to do that discard a lot of suspects.

[Traindriver69]

Hi
I have replaced the membrane with a brand new one so the keyboard should be working.