Sinclair QL - blank screen.... HeLp!

[abraXXious]

Hey guys,

I am fairly new to the QL scene. I have two QLs, one in perfect working order, the other not. It is the second I need help with.

The second unit was purchased supposedly in fully working condition and is cosmetically in mint condition, however it will not display anything except a black blank screen.

I have tried both the monitor output and the UHF RF output - both give a solid stable black image and nothing else.

I am using a known good PSU and it has been tested on the other working QL to confirm it is fine (plus I checked the voltages).

I have substituted every socketted chip from the working QL to the non-working unit and there is no change, so it is not one of the socketted chips.

I have checked the voltage regulators and they are fine.

Rich from RWAP (lovely fellow) was even kind enough to loan me his Minerva ROM to check if it was a faulty RAM IC - even with the Minerva installed I still get the same solid, stable black screen.

The unit looks like new and even has a brand new keyboard membrane installed so it would be a shame to relegate it to the 'lost cause' cupboard. I am fairly handy with a multimeter and soldering iron and am willing to try anything - any advice or help a more knowledgeable fellow could give would be greatly appreciated.

Thanks guys.

[dilwyn]

I don't know whether there is anything in the QL service manual which might help
http://www.dilwyn.me.uk/docs/ebooks/index.html

[tofro]

Hi,

First: If you press F1 on your blank screen, does mdv1_ start to whirr? If yes, it truly is a video problem, if not, we need to look somewhere else.

Nearly all video failures normally indicate a faulty 8301, this is a socketed chip, so I would assume you already checked that.

When you're saying "Monitor", are you referring to the Composite or RGB video output? In case you checked on a composite monitor - can you do the same using an RGB one?

RGB is directly produced by the 8301, so should work with a working ULA.

Composite is produced by the MC1377 (a chip still produced, to my knowledge, IC28 in issue 5). Either the chip itself is toast or the +12V generation is not working properly.

Composite

[abraXXious]

Hey guys,

firstly, thank you for the help.

I should have mentioned before, I downloaded the QL manual and stumbled through the troubleshooting to no avail.

Also, I tried pressing F1 and there is absolutely no activity from the Microdrives etc.

[abraXXious]

Oh, and I tried the composite. I can try RGB but it will be a bit of a struggle as most TVs sold in Australia do not have scart.

I have replaced the video encoder chip (you are right - they still make them and they are dirt cheap - I purchased a dozen for my spare parts stores )

Where is a good/easy place to check the +12v?

Thanks again.

[tofro]

In case the QL microdrive doesn't show activity when you press F1, the fault is most probably not with the video circuitry. The QL should just ignore a dead 1377 and go on with its startup, so no microdrive at F1 is a bad sign.

+12V and -12V are generated by IC37 (7812) and IC36 (7912) - I assume you have an issue 5 QL. The voltage regulators sit on the top right end of the QL's PCB, the 7812 is somewhat buried under the large heat sink for the 7805 and a large inductivity coil. That's a bit hard to reach on a working QL. I would recommend you check pin 30a (+12V) and 31a(-12V) against 29a (0V) on the expansion port. The +/- 12 V lines are also somewhat easier to access on IC25 pins 14 and 1 against pin 7 (GND). IC25 is the serial port's line driver and can be found - oh wonder - close to the serial ports.

Tobias

[abraXXious]

Hi Tobias,

thanks for the help.

I have checked the voltages on IC25 (SN75188N). Pins 14 and 1 show 12.54v and -12.48v respectively.

Strange though, I removed the large heatsink that the 7805 was attached to find the other voltage regulators.... but there are none that I can see other than the two on the Microdrive side.

Not sure which revision QL this is as the silk screening simple says "Sinclair QL 1984".

Any ideas?

[abraXXious]

Just to add. On one occasion I managed to get a screen full of corruption (coloured lines and blocks) to appear, not sure how, but every other time I still get a sold stable black screen and no activity when pressing F1.

[Derek_Stewart]

Hi,

If you are getting a black screen, the QL is not getting to the ram test, failure on Ram Tests is usually a white screen.

If you have tested all the plug-in chips then you may have a fault on one or more logic chips.

I had a similar fault many years ago, it took me a 2 hours with a logic probe to find the faulty chip. In my case it was the HAL chip that had developed a fault.

[abraXXious]

Hi Derek,

unfortunately I do not have access to a logic probe so I am going to have to try and test/replace the logic chips one at a time.

Which IC is the "HAL" chip (none of mine seem to be marked "hal"), also, can you suggest the order in which I should test/replace the logic chips?

Thanks again,

Steven.