The Sinclair QL Forum

bwinkel67 wrote:Dave just bought a working US QL, maybe you can work with him to see if he can figure out what's going on.

Thanks for the hint. An analysis of the clock module would certainly be interesting. But to adapt QL-VGA to it I'd probably need access to one of these QLs myself and I'm not sure many exist on this side of the pond. If the board is the problem, that is, but I'm currently out of ideas otherwise.

mk79 wrote:

varmfskii wrote:I am reasonably handy with a soldering iron. I'll try that. What does the 74HC mod do?

Well, I've yet to meet somebody who knows, nobody has ever documented it. If it is what I think it is (again, I only know the one grainy photo) it's technically redundant, like Bwinkel my guess would be that it was due to some FCC thing. But fact is no QL outside of the US has this modification and they all work fine. And QL-VGA was tested with well over 50 very different QLs without any problems, so it would be interesting to see what disabling it will change. Thanks in advance for trying

It didn't seem to change anything. BTW looking at the plans for the QL, it has what I believe is the main crystal as 15 MHz. Is this accurate. The crystals on mine seem to be different. The main crystal being 15.10489 MHz.

varmfskii wrote: It didn't seem to change anything. BTW looking at the plans for the QL, it has what I believe is the main crystal as 15 MHz. Is this accurate. The crystals on mine seem to be different. The main crystal being 15.10489 MHz.

So the VGA issue was still there after you removed the board?

mk79 wrote:

bwinkel67 wrote:Dave just bought a working US QL, maybe you can work with him to see if he can figure out what's going on.

Thanks for the hint. An analysis of the clock module would certainly be interesting. But to adapt QL-VGA to it I'd probably need access to one of these QLs myself and I'm not sure many exist on this side of the pond. If the board is the problem, that is, but I'm currently out of ideas otherwise.

There was one on eBay in early June that went for $245 (I think Dave bought that one)...but that's the first one I recall seeing. Maybe Urs has one...he seems to have every variety...that he could lend out. I only have the one otherwise I'd offer...it's the one from my youth so I'm attached to it :-/

varmfskii wrote:It didn't seem to change anything. BTW looking at the plans for the QL, it has what I believe is the main crystal as 15 MHz. Is this accurate. The crystals on mine seem to be different. The main crystal being 15.10489 MHz.

Aaaaaah, now we're talking! Yes, that is a straight 15Mhz in the rest of the world. The clock is divided down by 2/3 to form the pixel clock. Nominal QL pixel clock is 10MHz, as I wrote I designed for a clock variance from 9.976 to 10.024 MHz in QL-VGA. But yours would be 10.0699Mhz, halve a percent off In principle it should be easy enough to fix, I just need to enlarge my lookup table for more values, IF that is the only problem. But how to get the fix to you is another question

I see a few options:

• A fitting JTAG adapter can be had for 20-25USD from China, but getting and installing the necessary software can potentially be a huge pain and might need good technical knowledge.
• I ship you a new QL-VGA in exchange for yours. In that case I'd like to test the fix first though, preferably by modifying one of my QLs to 15.10489 MHz. Problem is, while 15Mhz crystals are a dime per dozen, I have yet to find a single source for a 15.1MHz crystal. Maybe I can feed an external clock using a frequency generator... I'm still thinking about this one.
• You try exchanging your crystal with a standard 15MHz one.
• We make some return of QL-VGA or maybe sell it on to somebody else in the US to not ship it around the globe too often and you get some kind of compensation for your troubles from me.

That's what I can come up with on the spot. I'd like to reproduce it on one of my QLs, but not sure how difficult it will be to feed an external clock into them yet. The nearest crystal I can readily buy is 15.36Mhz, which is also way off but if I could even get those to work... might be an option. But it will take a bit of time to do this.

Just had a search through my box of crystals and I have nothing between 15MHz and 16.00Mhz. In theory you should be able to drive just one terminal of the crystal inputs on the 8301 with a signal generator, which would mean you could dial in the frequency of your choice...

bwinkel67 wrote:There was one on eBay in early June that went for $245 (I think Dave bought that one)...but that's the first one I recall seeing. Maybe Urs has one...he seems to have every variety...that he could lend out. I only have the one otherwise I'd offer...it's the one from my youth so I'm attached to it :-/

Yeah, that's too much trouble anyway. I already got 7 QLs + 1 loaner, so I'm not looking for more Other than one in its original packaging, that is I might try to modify one of mine and see if I can reproduce the problem.

bwinkel67 wrote:

varmfskii wrote: It didn't seem to change anything. BTW looking at the plans for the QL, it has what I believe is the main crystal as 15 MHz. Is this accurate. The crystals on mine seem to be different. The main crystal being 15.10489 MHz.

So the VGA issue was still there after you removed the board?

Yes, the problem persists after removing the board.

I'll look into a JTAG for reprogramming the FPGA.

varmfskii wrote:I'll look into a JTAG for reprogramming the FPGA.

That'd be cool, this way we could tweak it with little turnaround time. It must be Spartan 6 compatible. This is the cheapest I could find on AliExpress https://www.aliexpress.com/item/1005002213595772.html but there are also a ton of those on eBay like https://www.ebay.com/itm/391876366607.
The supplied 7-pin duppont cable comes in handy for connecting to QL-VGA.

Cheers.

If you need a x-tal you can not get from any standard product portfolio, this may helpful:

http://www.andyquarz.de/quarze.html

German only, but Euro 21,- for a x-tal you need and you can not get seems quite reasonable.

Rgs,

Stephan