Troubleshooting black display?

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M68008
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Troubleshooting black display?

Postby M68008 » Fri Feb 23, 2018 10:15 pm

I got a QL and I'm trying to connect it to my TV through a "QL Scart" cable and a SCART->HDMI converter, but nothing appears on my TV. The TV does detect a HDMI signal from the converter, but displays nothing (black screen). Apart from the video, the QL seems to work fine: MDV1 turns on after pressing F1/F2 and I can type BEEP commands and hear the sound.

Is there anything I can do to troubleshoot the issue if I don't have a scope? Any other ways to check whether the QL video output shows signs of life?

Thanks for any suggestions!


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Cristian
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Re: Troubleshooting black display?

Postby Cristian » Fri Feb 23, 2018 11:15 pm

Try to connect directly the cable to a TV which has the SCART port. Connect always to SCART 1 (AV1 signal).
Also you may try to connect to an old TV via the antenna cable.
Is your QL-SCART cable homemade? Which HDMI converter model are you using?


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tofro
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Re: Troubleshooting black display?

Postby tofro » Fri Feb 23, 2018 11:26 pm

If it's completely working otherwise, it is very likely that the computer is actually producing a video signal, but the HDMI converter cannot make heads or tails of it - Or the TV doesn't know what to do with a somewhat distorted HDMI signal.

I have recently tested my HDMI-converted QL (similar setup as yours) on a number of HDMI monitors. On my main QL monitor (cheap Chinese TV), it works perfectly, also on an older Sony. A new Sony doesn't display a proper picture (some characters missing on the right), and an old Iyama wasn't able to properly sync.

Apparently, what the HDMI converters make from a QL picture cannot be understood by every HDMI monitor.

Tobias


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Re: Troubleshooting black display?

Postby 1024MAK » Sat Feb 24, 2018 2:11 pm

Yeah, keep in mind that the video signals from the QL do not fully meet the broadcast TV standards.

So what, if anything that you get on a TV (or monitor) very much depends on the flexibility of software in the SCART to HDMI converter AND the software in the TV (or monitor).

The other variable, is the SCART control switching signals. Most SCART to HDMI converters, just like modern TVs, need the RGB mode to be selected.

Mark


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M68008
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Re: Troubleshooting black display?

Postby M68008 » Sat Feb 24, 2018 8:10 pm

Unfortunately I don't have an old TV anymore (SCART or otherwise). I've now tried my only other monitor which is my PC's monitor, with similar results: the monitor detects the HDMI being plugged in but it doesn't see a signal on it ("entering power save mode").

SCART cable I'm using: https://www.ebay.com/itm/Sinclair-QL-Hi ... Sw~4taeLLK
HDMI converter: https://www.ebay.com/itm/SCART-HDMI-to- ... NU7nYwcIBQ
The only control on the converter is a 720p/1080p switch. The TV detects the difference, but displays nothing in both cases.

This is a UK QL and I'm running it in the US, but both the converter and TV should be able to cope with both US and European standards.


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Re: Troubleshooting black display?

Postby martyn_hill » Sat Feb 24, 2018 10:56 pm

Hi there!

Along with the comments already made, a couple of thoughts:

a) That RGB/SCART cable is probably the best one you'll find for the QL (I use one myself, as do many others here), but I believe that it is designed for RGB as opposed to CVBS. I.e. I don't know whether or not the CVBS signal is passed through.
b) That particular SCART to HDMI converter is designed only for CVBS. There is at least one other upscaler generally available for around £20 on eBay, that specifically processes the RGB signal (typically better than CVBS).
c) A failed MC1377 IC (PAL encoder) in the QL could also be the culprit - everything other than video would work as expected. That said, I don't think the RGB and sync-signals needed to drive RGB SCART are generated by the MC1377, so seems less likely the problem. Replacement ICs are readily available and its socketed on the QL for ease of swapping.

Good luck - I suspect you are very close to a working unit :-)


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Re: Troubleshooting black display?

Postby martyn_hill » Sun Feb 25, 2018 4:12 pm

Hi again

martyn_hill wrote:a) That RGB/SCART cable is probably the best one you'll find for the QL (I use one myself, as do many others here), but I believe that it is designed for RGB as opposed to CVBS. I.e. I don't know whether or not the CVBS signal is passed through.


I've since checked the wiring of that QL to SCART cable (I use the same item) and can confirm that the QL Monitor output Pin 1 (Composite PAL - CVBS) is NOT connected to the SCART Pin 20 (which is used either as CVBS or else as just the Composite SYNC, when running in RGB mode.)

Instead, Pin 1 (QL Monitor - CVBS) is connected to SCART Pin 16 - nominally 'Blanking', but depending upon the actual voltage applied, will also trigger either CVBS or RGB SCART mode of the destination TV/Monitor. Its quite a clever idea - the QL's Composite PAL/CVBS output appears to be just sufficient - even though oscillating - to trigger RGB mode by applying the necessary >0.4V (should ideally fall between 1 and 3V) to the SCART Blanking input.

Thus, your particular SCART to HDMI upscaler is ignoring the RGB signals from the QL as its designed solely for CVBS, and only receiving the QL's Composite SYNC signal.

This is why you get a steady/valid but otherwise blank display - the upscaler is dilligently and correctly converting what is effectively a blank display signal(only the SYNC)!

If that is the case, you have a couple of options:
a) Re-wire your SCART cable (in the SCART plug itself) thus:

i. Unscrew the captive fastener/strain-relief and delicately prise-apart the SCART shell.
ii. Disconnect the resistor/wire from Pin 20 (Purple wire in my SCART cable.) Tape it up or fold-away - it's not needed in CVBS mode.
iii. Disconnect the wire from Pin 16 (Orange in my SCART cable.)
iv. Re-solder the wire from Pin 16 instead to Pin 20.
v. You can leave Pin 16 unconnected, as SCART is designed to default to CVBS mode if the Blanking pin is left floating. In any case, your HDMI/SCART upscaler will probably ignore this signal pin.
vi. Close the SCART plug, careful not to crimp any of the wires and screw-back the fastener/strain relief.

or (my personal preference):

b) Leave your SCART cable intact and purchase a true RGB upscaler (as opposed to only CVBS type.)

I can recommend this one (I've no connection with the seller or eBay!) - it gives almost the full width of the QL display in RGB mode on my HDMI TV (slightly more than SCART RGB and certainly more than SCART CVBS), with only the last 8 pixels or so on the very RHS missing (this is a phenomenon of how the QL Video display cct was designed - 'overscanning') and probably the most stable and pleasing picture I get from a real QL (the new Q68 on VGA is even better!):

https://www.ebay.co.uk/itm/SCART-HDMI-t ... 2749.l2649

There are other sellers of this item, but I recently purchased a couple of these units for £21 ea. form this seller and they arrived in less than a week (in the UK.)

Let us know how you get on!


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tofro
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Re: Troubleshooting black display?

Postby tofro » Sun Feb 25, 2018 4:20 pm

martyn_hill wrote:Leave your SCART cable intact and purchase a true RGB upscaler (as opposed to only CVBS type.)

I can recommend this one (I've no connection with the seller or eBay!) - it gives almost the full width of the QL display in RGB mode on my HDMI TV (slightly more than SCART RGB and certainly more than SCART CVBS), with only the last 8 pixels or so on the very RHS missing (this is a phenomenon of how the QL Video display cct was designed - 'overscanning') and probably the most stable and pleasing picture I get from a real QL (the new Q68 on VGA is even better!):

https://www.ebay.co.uk/itm/SCART-HDMI-t ... 2749.l2649


I have the same upscaler (at least it looks the same from the outside), and one of by TVs is actually able to display 100% of the QL monitor mode pixels with it. As written above, this, however, varies based on the used TV.

Tobias


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Re: Troubleshooting black display?

Postby martyn_hill » Sun Feb 25, 2018 5:03 pm

Hi Tobias

Without wishing to go too-far off-topic (sorry, M68008 - I'll keep it short!)

tofro wrote:I have the same upscaler (at least it looks the same from the outside), and one of by TVs is actually able to display 100% of the QL monitor mode pixels with it. As written above, this, however, varies based on the used TV.Tobias


Yeah - so I've purchased three of these over time - my first one popped, but it DID render the full 512 QL pixels. I recently purchased a couple of replacements - look more or less identical, though a very slightly different marking on the lid - and these replacement units drop the last few pixels (I see only one pixel plotted when I POKE_W 131072 + 124, 255) when tested on the very same HDMI telly...

I've emailed the distributor to see if there is any adjustment possible, mentioning that the earlier model rendered a more full image. Haven't heard back (and don't expect to... Pity, as its a lovely unit otherwise.)



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