Sinclair ql black screen

[Traindriver69]

I have tested the ram chips DOut and the address lines A0 to A6 all showing activity but checked the VCC of each chip and it's showing 2.68v is this correct.

[Popopo]

I have tested the ram chips DOut and the address lines A0 to A6 all showing activity but checked the VCC of each chip and it's showing 2.68v is this correct.

As far as I know, no.
That is not correct.
Those chips must be feed by +5v

I share a schema of one QL version in case it may guide to to check where part of this power is draining.
Since I don't (as I suggested to share) your board version, here is the iss6 schema. Pretty similar to others in general means.

QL Issue 6 revised.pdf

(72.86 KiB) Downloaded 1 time

What surprises me is that you have Dout activity when the RAM ICs are not powered enough (may response in some cases, I don't remember the power range but I think this is pretty low).
Also, this share the same power line (+5v) of the CPU, so ... if it is 2.68v for RAM ICs, why do you get +5v in CPU?
Did I understand you right?

Are you checking Data in or Data out? D is data in (pin#2), Q is Data out (pin#14).
Edit: My mistake, I didn't remember QL has both Datas lines conected. So you may be reading Data In activity and not Data out.
So the CPU is trying to write and that is what you see.

The most suspect is a RAM IC or a Multiplexer.
Check now power levels for ZX8301, ZX8302.
IC29 and IC20 : Check power and outputs. If they are fine (before resistors) 2 things less to worry about.

I think you have one or more RAM faulty ICs. Since you have reset, clock, ADDR & Data Activity.

Do you notice any RAM IC getting specially hot?

[Traindriver69]

Hi
Forget what I said about the VCC voltage as the pinout I was following although it said it was the right chip was wrong so now have rechecked and voltage is fine and DOut and address lines showing activity.

[Popopo]

Hi
Forget what I said about the VCC voltage as the pinout I was following although it said it was the right chip was wrong so now have rechecked and voltage is fine and DOut and address lines showing activity. IMG_20250710_123608_edit_44377249836977.jpg

If I am not wrong (my eyes are problematic today),
Your board is an ISS5.
I see you are using the original ROMs,
Didn't you place the Minerva ROM replacement?
That helps a lot to see which RAM IC is faulty (if so).
Yet, the 2 resistors over Z8302 please to take the wire farther from PINs, I am worried they could short them.

You have Dout activity because you have Din activity.
So something is using them, I would focus in RAM ICs firstly. Dout you cannot realize it is working because Din and Dout are linked, as you can see in the schema. So at first sight you cannot tell if what you see is Din activity or Dout.

You can see that RAM Datas are DBn, and they are connected to ZX8301 and ZX8302, very important ICs. They have activity.
In the other side, the CPU use bus D, and in the middle is IC21 that bus transceiver. So now, please to check data signals in both sides of it.

Have you check the temp (with your bare hands) of the RAM ICs looking for any hotter than the rest?

[Traindriver69]

I have tried the minerva and it makes no difference but I have noticed with the original roms pressing the reset will bring up a garbled screen but with the minerva rom pressing the reset does nothing to the screen.

[Pr0f]

X axis looks ok for a 7.5MHz, given that signal is repeating roughly 2/3 of the scale interval

[Popopo]

Re-reading the thread but didn't find this...
have you remove the ICs from the sockets, cleaned the sockets and also the pins from each IC and re-seat them?

That is also basic & easy to do, and could produce what you are suffering.
Since you have activity, it may be related with bad IC connections.
Of course, without discard a faulty IC.

I prefer to check every stupid thing before to advice to buy anything.
Even if it is tedious, it is just a matter of time and not wasting money.

[stephen_usher]

Indeed, reseating the chips would be the first thing to do. i.e. do all non-destructive things first.

However, if this doesn't work then I would suspect the RAM. Minerva can't start under some RAM fault conditions, which is why I would suggest getting an EPROM written with my diagnostic ROM, which can run even if no RAM is present. You may need to connect a serial terminal to ser2 if there's no working RAM at all though.

[Traindriver69]

Just checked IC21 on the A side I am getting this.
https://youtube.com/shorts/Jo_yYMqE690? ... wfYpTOwIbr
And on the B side I am getting this
https://youtube.com/shorts/1tZc5g3p2wQ? ... Wu6W-mENJz

[Derek_Stewart]

HI,

Here is my unfinshed update to the QL Service manual, which details Fault Finding in Section 2 and I have added Tonty Firshman's Fault Finding process in Section 6.

I must state I have a lot of work to do to complete the drawings of the QL circuit board.

QLSM04.pdf

QL Service Manual Update 4

(747.77 KiB) Downloaded 9 times

The details of the Service Manual is taken from the original manual, only the Schematic, section, board layouts have been improved.

This might help.