The Sinclair QL Forum

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.

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

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

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.

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.

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

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?

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.

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.

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.