CRT syncing issue, any ideas?

A place to discuss general QL issues.
User avatar
Chr$
QL Wafer Drive
Posts: 1304
Joined: Mon May 27, 2019 10:03 am
Location: Sachsen, Germany
Contact:

CRT syncing issue, any ideas?

Post by Chr$ »

A particular QL I have seems to have an issue with RGB video display. The monitor (an old CRT RGB) is absolutely fine with many other QL's. The dodgy QL does this:

https://www.dropbox.com/s/xaq58tpk7iv9l ... 767169.mp4

Ignore the poor image quality as my phone converted it to mp4. The issue is the obvious "losin' it" at about 4 secs and 21 secs (and that pattern repeats itself, never seeming to settle down).

I consulted the schematic, then swapped out the ZX8301 and then the MC1377. Neither made a difference. It produces a perfect picture when connected to the QL-VGA.


https://QXL.WIN
Collector of QL related computers, accessories and QL games/software.
Ask me about felt pads - I can cut them to size and they have proved excellent for mdv data recovery.
martyn_hill
Aurora
Posts: 909
Joined: Sat Oct 25, 2014 9:53 am

Re: CRT syncing issue, any ideas?

Post by martyn_hill »

Hey Chris!

Poor sync is sometimes due to the voltage level on the CSYNC signal being too high for the monitor, though a noisy signal might also cause that sort of behavoiur.

A couple of questions:
a) What sort of cable are you using to connect this QL to the CRT? Does it have any series resistors built-in?
b) Do you use the same cable between this CRT and the other, stable-picture QLs?


User avatar
Chr$
QL Wafer Drive
Posts: 1304
Joined: Mon May 27, 2019 10:03 am
Location: Sachsen, Germany
Contact:

Re: CRT syncing issue, any ideas?

Post by Chr$ »

martyn_hill wrote:Hey Chris!

Poor sync is sometimes due to the voltage level on the CSYNC signal being too high for the monitor, though a noisy signal might also cause that sort of behavoiur.

A couple of questions:
a) What sort of cable are you using to connect this QL to the CRT? Does it have any series resistors built-in?
b) Do you use the same cable between this CRT and the other, stable-picture QLs?
Hello Martyn,

The cable is a straight-through 8 pin DIN to whatever number of pins the monitor has. No resistors. Yes, I always use the same cable and it's been fine with at least 5 other QLs. It's also always in the same position from the same power sockets, so I can rule out that's it's something nearby causing an issue. The problem must be within the problematic QL.


https://QXL.WIN
Collector of QL related computers, accessories and QL games/software.
Ask me about felt pads - I can cut them to size and they have proved excellent for mdv data recovery.
martyn_hill
Aurora
Posts: 909
Joined: Sat Oct 25, 2014 9:53 am

Re: CRT syncing issue, any ideas?

Post by martyn_hill »

Right-o.

As for 'noise', I as thinking more from within the QL itself.

As the Vertical hold seems stable, you could focus just on the CSYNC signal.

Looking at the cct diagram, we see that there are only 4 connections made to the CSYNC signal after it leaves the 8301:
a) Directly to the Monitor socket
b) To the Expansion bus
c) To pin 2 of the 1377 PAL encoder (only used to generate PAL comp-video to the UHF and monitor sockets)
d) To the composite-monochrome summing transistor TR9 via R56

You might want to explore those areas, e.g.:
1. Remove the 1377 altogether - not needed in this scenario
2. Check the Expansion connector for any shorts against pin 13B
3. Perhaps isolate R56 from CSYNC (same side that connects to Pin 2 of the 1377 encoder)
4. Any other possible failures (esp. shorts) across the CSYNC PCB trace

Good luck!


Martin_Head
Aurora
Posts: 847
Joined: Tue Dec 17, 2013 1:17 pm

Re: CRT syncing issue, any ideas?

Post by Martin_Head »

It looks like a Line sync problem. Does the 'tear' move down the whole screen. Or is it just where we see it?

It could be an AC ripple on a supply rail. Or maybe a bad earth return somewhere.

Have you got an oscilloscope that you can look at the video signals. Too see if there is any ripple.


martyn_hill
Aurora
Posts: 909
Joined: Sat Oct 25, 2014 9:53 am

Re: CRT syncing issue, any ideas?

Post by martyn_hill »

...and on a slightly different tack, we could be looking at an off-set 15MHz clock to the 8301.

The 15Mhz crystal oscillator (X1) and associated RC components (R70 and C2) could be failing, causing the PLL inside the monitor to loose sync against the CSYNC being generated by the 8301, based off of its 15Mhz reference.

As per Mr Head's question, it would be interesting to note whether the tearing occurs any further up the display - but you'll only see that after getting a non-black colour higher up the screen - say after pressing F2 to show the red Window #1.

If the tearing still occurs only further down the screen - as we see it in the original mp4 - the progressive loss of sync after a time period following each VSYNC could confirm the PLL sync issue.

Some CRTs have a small adjustment for both Vertical and Horizontal sync, sometimes exposed on the exterior panel, that could be gently adjusted preferably with a small plastic driver, so as not to go too far off to cause your other 5 QLs to loose their image, but sufficient to better align with this QL's clock.

Anyway, keep us posted!


Martin_Head
Aurora
Posts: 847
Joined: Tue Dec 17, 2013 1:17 pm

Re: CRT syncing issue, any ideas?

Post by Martin_Head »

Another thought on the bad ground idea.

Check the connectors in the DIN socket of the QL. Look for bent, or damaged one's. There could also be internal fractured connections in the socket.


User avatar
Chr$
QL Wafer Drive
Posts: 1304
Joined: Mon May 27, 2019 10:03 am
Location: Sachsen, Germany
Contact:

Re: CRT syncing issue, any ideas?

Post by Chr$ »

Thanks for the suggests. I do have a basic oscilloscope. I did already notice that R56's value is about half what it should be - measured in circuit.

The k/b membrane is currently mangled so I can't get past the F1/F2, but during the tweed pattern I do see the problem moving down the whole screen, briefly. And in different places on each reset, so I am quite sure it cycles down the whole screen.

The monitor has no exposed adjustment knobs. No doubt there are some inside, but obviously I'd rather it worked 'just like that' with all QLs. The m/b will come out and I shall experiment/test further.


https://QXL.WIN
Collector of QL related computers, accessories and QL games/software.
Ask me about felt pads - I can cut them to size and they have proved excellent for mdv data recovery.
User avatar
Chr$
QL Wafer Drive
Posts: 1304
Joined: Mon May 27, 2019 10:03 am
Location: Sachsen, Germany
Contact:

Re: CRT syncing issue, any ideas?

Post by Chr$ »

Ok, my observations:

Measuring the CSYNC pin on the RGB connector. The pulsing looks ok, but it's pulsing about 8.4v on the vmax. I checked another QL and it was pulsing at the same speed but at around 5v.

I checked the crystal X1 too and that was oscillating at a perfect 15.001mhz but it was also doing so at over 8v.

That can't be normal, right? Where on earth is the 8v coming from?!

Edit: just checked the voltages elsewhere and there is 7.4v at both ROMs and about the same coming out of the 7805, so that's gotta go for starters!

Another edit: A new 7805 fixed the display problem! Multiple power cycles to check/measure things now seems to have fried a RAM chip. Minerva sometimes starts with 64k but very often reports an issue that always corresponds with RAM chip IC8. So now I'll change that while I have the m/b out.


https://QXL.WIN
Collector of QL related computers, accessories and QL games/software.
Ask me about felt pads - I can cut them to size and they have proved excellent for mdv data recovery.
User avatar
Pr0f
QL Wafer Drive
Posts: 1298
Joined: Thu Oct 12, 2017 9:54 am

Re: CRT syncing issue, any ideas?

Post by Pr0f »

You were lucky you didn't cook off all your chips - most of them will tolerate 6V - but no that much more...


Post Reply