Fixed: Mode 7 intermittent lines

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Fixed: Mode 7 intermittent lines

Post by anightin » Sat Aug 17, 2019 8:23 pm

Dear All,

I am fortunate to have 2 BBC Model B's, however one is currently suffering a screen noise problem. It manifests as very short bursts of random lines every 60 seconds or so before settling again to an otherwise noise free solid display.

IMG_4641.jpeg
IMG_4640.jpeg
(I captured the above using video then extracted the corrupted frames for illustration)


I must admit I'm claiming this as a MODE 7 issue but can't be sure I've not seen it in other modes -- I don't think I have.

Apart for this, the machine seems to function fine.

The motherboard is an Issue 4 with an ATPL ROM board:
IMG_4643.jpeg

I've done some obvious checks like swapping RGB cables, supply voltages, re-socketed ICs and have run the machine for an hour or so -- the problem doesn't seem to get worse, and stays consistent.

Switching over to my other Beeb with the same video cables and display hardware etc and the display is rock solid.



I'd be keen to know if anyone has seen this behaviour before, or can recommend a diagnostic plan of where to start.
Last edited by anightin on Sat Aug 24, 2019 10:17 pm, edited 1 time in total.

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Sat Aug 17, 2019 9:31 pm

anightin wrote:
Sat Aug 17, 2019 8:23 pm
I must admit I'm claiming this as a MODE 7 issue but can't be sure I've not seen it in other modes -- I don't think I have.
Just to report, I have been running now in MODE 2 and have also seen the issue there now too.

User avatar
marcusjambler
Posts: 711
Joined: Mon May 22, 2017 11:20 am
Location: Bradford
Contact:

Re: Mode 7 intermittent lines

Post by marcusjambler » Sat Aug 17, 2019 10:31 pm

Hi Anightin

Have you tried it with S25 south?

Marcus

cmorley
Posts: 1121
Joined: Sat Jul 30, 2016 7:11 pm
Location: Oxford
Contact:

Re: Mode 7 intermittent lines

Post by cmorley » Sun Aug 18, 2019 6:20 am

IC14 can snow of random characters in mode 7 but not sure about your symptoms. If IC14 is socketed in both machines you can swap it.

More likely teletext chip I'd have thought.

User avatar
1024MAK
Posts: 9516
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Mode 7 intermittent lines

Post by 1024MAK » Sun Aug 18, 2019 6:25 am

Chris, he says he has also noticed it in MODE 2 as well...

Andy, I’m thinking it’s a malfunction of the cursor. Is the line always between the lines of text in the same place as the cursor would be positioned?

The 6845 generates the cursor and signals this on pin 19 (IC2), it then goes to the VideoProc / Video ULA, IC6 pin 25. So if both are socketed, you can try swapping with another Beeb to see if the fault moves with the chip, or not.

Mark
Last edited by 1024MAK on Sun Aug 18, 2019 6:31 am, edited 1 time in total.

cmorley
Posts: 1121
Joined: Sat Jul 30, 2016 7:11 pm
Location: Oxford
Contact:

Re: Mode 7 intermittent lines

Post by cmorley » Sun Aug 18, 2019 6:30 am

1024MAK wrote:
Sun Aug 18, 2019 6:25 am
Chris, he says he has also noticed it in MODE 2 as well...
I'll move my finger from the teletext chip towards the ULA then in that case! :)

User avatar
tricky
Posts: 3866
Joined: Tue Jun 21, 2011 8:25 am
Contact:

Re: Mode 7 intermittent lines

Post by tricky » Sun Aug 18, 2019 7:59 am

From you video, is the first picture just as the text was being drawn, that is, on the previous frame, is there no text, or only the BBC Computer part?
This is to check whether two lines could have had the cursor on the same frame.

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Sun Aug 18, 2019 10:32 am

Dear All,

Thanks for all the suggestions so far.

Had a quick play this morning and:
  • S25 South to enable only 16K has no effect on the symptoms
  • Swapping the Video ULA with the one in my issue 7 Beeb also had no effect on the symptoms

I guess this should rule out ULA and lower 16K memory issues at least

  • Tricky, I need to do more experimenting with the video but the lines seem to be independent of the text or cursor.
What I will add is that every time the interference occurs, the video converter has to re-sync itself as if the video stream is interrupted/corrupted briefly.
D8334BAB-C8CC-43B1-B10D-A68A2EA6842E.jpeg

User avatar
tricky
Posts: 3866
Joined: Tue Jun 21, 2011 8:25 am
Contact:

Re: Mode 7 intermittent lines

Post by tricky » Sun Aug 18, 2019 11:19 am

FYI, the cursor is generated by the 6845 and so are the sync signals that could cause the TV to loose sync.
You could try swapping them if they are not soldered.

roganjosh
Posts: 119
Joined: Sat Dec 10, 2016 6:51 pm
Location: W.Yorks
Contact:

Re: Mode 7 intermittent lines

Post by roganjosh » Sun Aug 18, 2019 11:33 am

anightin wrote:
Sun Aug 18, 2019 10:32 am

What I will add is that every time the interference occurs, the video converter has to re-sync itself as if the video stream is interrupted/corrupted briefly.
I had exactly the same issue on several Model Bs (horizontal lines & sync loss) as I've reported elsewhere. In all cases replacement of the 16MHz crystal (X1) solved the problem.

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Sun Aug 18, 2019 11:50 am

Many thanks, will get a couple in and report back in a few days :)

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Wed Aug 21, 2019 10:38 pm

Dear All,

Got a new crystal in today and tried the machine -- short answer is no change of symptoms :(

I was really hoping that would fix it :?
IMG_4667.jpeg
IMG_4668.jpeg
IMG_4669.jpeg
The red heat-shrink on the new crystal is there as I noticed the crystal touching the underside of the ROM/SWRAM board when dismantling the machine.


All started off fine as I saw no interference/loss of sync but once the machine had warmed up for 5-10 mins it was back :(

I guess the next thing to try is to look at some of the video timing associated ICs .


At least I know it has a new 16MHz crystal now.


Best wishes

Andy

roganjosh
Posts: 119
Joined: Sat Dec 10, 2016 6:51 pm
Location: W.Yorks
Contact:

Re: Mode 7 intermittent lines

Post by roganjosh » Thu Aug 22, 2019 6:32 am

anightin wrote:
Wed Aug 21, 2019 10:38 pm

All started off fine as I saw no interference/loss of sync but once the machine had warmed up for 5-10 mins it was back :(

I guess the next thing to try is to look at some of the video timing associated ICs .


At least I know it has a new 16MHz crystal now.

I suspect that the lines are artefacts coming from the HDMI converter as sync is lost rather than from cursor issues. However, before going further do you have an alternative video setup (e.g. RGB->SCART->TV) you could try? I've been assuming that you're currently using a SCART to HDMI converter. TV RGB circuitry is generally more forgiving. That could help to either discount or lay blame at the cursor side of things. It's worthwhile measuring the oscillator frequency (IC43 pin 6) but in my experience this reports OK. It would be well to follow the sync pulses at the 6845 and the derived csync from there through ICs 41 & 48 to see if anything looks wildly off. Spotting any stability issue is likely to be difficult though. I've never been able to detect differences in scope signals before/after crystal replacement even though the fault had been fixed.

I did have one instance of this problem where changing the crystal initially made the sync problem worse but then, after a minute, the problem cleared. Changing the oscillator IC (74S04 IC43) as well as the crystal cured that. IIRC someone else reported a similar problem with IC43 back in the mists of time. The oscillator resistors and capacitors are unlikely to be at fault. It would be as well to see whether anyone else chips in before doing major IC43 surgery.

As you say, it is as well that the crystal has been replaced. I now treat this as a stock fault and keep a bag of spare crystals. Crystal technology has improved a lot over 30 years in terms of accuracy but, more importantly, stability. To paraphrase something Mark (Retroclinic) said, you'd be lucky to get an old crystal working in a modern USB circuit.

User avatar
1024MAK
Posts: 9516
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Mode 7 intermittent lines

Post by 1024MAK » Thu Aug 22, 2019 8:44 am

With respect to the 16MHz crystal used in the Beeb, the standard part used in the manufacture of these machines was not a high tolerance part, hence when they were used for provisional video use, often a separate higher quality 16MHz crystal controlled oscillator would be used.

I do agree that if any type of video converter is being used, then you do really need to see the direct RGB or composite video output on a separate display at the same time, so that you can compare the direct video output with the converted video...

With regards to crystal oscillator circuits made with logic circuit chips, these are a ‘black art’ because they are hard to get right. However, once such a circuit starts up, if it manages to run in a stable manner for more than 5 minutes, normally it should be fine until switched off. Of course this assumes it is not subject to thermal heat changes from external heat sources.

Mark

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Thu Aug 22, 2019 9:00 am

Thanks again folks, I will go down the route of:
  • Trying the Beeb on a SCART TV to assess the quality of the direct output vs captured
  • Source some components to swap out one at a time:
    • IC41 SN74ALS02
    • IC43 SN74ALS04
    • IC48 SN74LS86AN
  • Report back

I also tried the OSSC I have that I usually run my Amiga and RiscPC from and it doesn't do a great job of handling Mode 7 on any of my machines compared to the above SCART->HDMI converter -- instead of characters with strong edge definition, they are appear quite ragged in comparison like they are being interlaced. I guess that's another thread once this one has concluded.
Last edited by anightin on Thu Aug 22, 2019 9:14 am, edited 2 times in total.

roganjosh
Posts: 119
Joined: Sat Dec 10, 2016 6:51 pm
Location: W.Yorks
Contact:

Re: Mode 7 intermittent lines

Post by roganjosh » Thu Aug 22, 2019 9:46 am

anightin wrote:
Thu Aug 22, 2019 9:00 am
Thanks again folks, I will go down the route of:
  • Trying the Beeb on a SCART TV to assess the quality of the direct output vs captured
  • Source some components to swap out one at a time:
    • IC43 SN74ALS04
  • Report back

I also tried the OSSC I have that I usually run my Amiga and RiscPC from and it doesn't do a great job of handling Mode 7 on any of my machines compared to the above SCART->HDMI converter -- instead of characters with strong edge definition, they are appear quite ragged in comparison like they are being interlaced. I guess that's another thread once this one has concluded.
As Mark said, oscillators are a black art. I'd therefore substitute like with like i.e. get a 74S04 and not a 74ALS04.

Alan

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Thu Aug 22, 2019 10:36 am

roganjosh wrote:
Thu Aug 22, 2019 9:46 am
As Mark said, oscillators are a black art. I'd therefore substitute like with like i.e. get a 74S04 and not a 74ALS04.
Good hint, I might fit a socket first and experiment with both for a bit of fun.

User avatar
1024MAK
Posts: 9516
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Mode 7 intermittent lines

Post by 1024MAK » Thu Aug 22, 2019 11:00 am

There are normally only two occasions when I don’t recommend sockets, sensitive high frequency circuits is one of them...
[The other being where the chip relies on using the copper of the PCB as a heatsink.]

Mark

User avatar
tricky
Posts: 3866
Joined: Tue Jun 21, 2011 8:25 am
Contact:

Re: Mode 7 intermittent lines

Post by tricky » Thu Aug 22, 2019 11:51 am

Mode 7 (teletext) is the hardest mode for a converter to get right is the "pixels" are not equal width and often have a little noise!

See the rgb to HDMI with a pi for details.

PS if you want to try the timing on the TV most still have a co.posited input even if the scart has gone.
Last edited by tricky on Thu Aug 22, 2019 11:52 am, edited 2 times in total.

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Thu Aug 22, 2019 1:12 pm

1024MAK wrote:
Thu Aug 22, 2019 11:00 am
... high frequency circuits is one of them...

Mark
Fair enough, I guess I'll just have to swap 1 IC at a time in that case starting with no. 43 :)

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Thu Aug 22, 2019 1:13 pm

tricky wrote:
Thu Aug 22, 2019 11:51 am
PS if you want to try the timing on the TV most still have a co.posited input even if the scart has gone.
Yup, I think we have a TV somewhere with a SCART input

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Thu Aug 22, 2019 9:29 pm

So tonight I plugged the RGB output into the SCART input on a telly to see what was actually being presented to the SCART->HDMI box.

As commented above, the TV did seem to be more forgiving in that there was no visible loss of sync or OSD being displayed.

However what I saw was a band of interference that would roll up the frame and then roll back from the bottom again over a period of about 30 seconds or so.

I noticed this changed direction from a top to bottom roll when I put different text of the screen.
IMG_4687.jpeg
IMG_4689.jpeg
Every so often there would be a wider band of interference for a fraction of a second which I am guessing would correspond with the OSD coming up on the SCART->HDMI as it re-established sync.

This seems to be independent of the cursor, or the screen mode, I would assume noise getting into in the video circuit which would also explain why the machine itself seems to run fine otherwise.

I am waiting on a 74S04N to replace IC43 as a next step.
Last edited by anightin on Thu Aug 22, 2019 9:41 pm, edited 2 times in total.

User avatar
1024MAK
Posts: 9516
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Mode 7 intermittent lines

Post by 1024MAK » Thu Aug 22, 2019 9:50 pm

Is this machine using an original PSU? If yes, which type? By this I mean, an ASTEC SMPSU in the gold coloured case, or a conventional type in a black metal case.

What else is connected to it? Are any expansions (internal or external) or peripherals connected? Do any of these have their own PSU?

A band of interference that slowly moves up or down (or both) on screen means that it is occurring at around 50Hz (or a multiple of this frequency).

If it was a problem with the master 16MHz oscillator or any of the frequencies derived from it, I would expect any problems to be static in relation to the video picture, not slowly moving... As any signal derived from it will be locked to the same frequency, or a division of it.

Mark

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Thu Aug 22, 2019 10:39 pm

1024MAK wrote:
Thu Aug 22, 2019 9:50 pm
Is this machine using an original PSU? If yes, which type? By this I mean, an ASTEC SMPSU in the gold coloured case, or a conventional type in a black metal case.
Gold PSU
What else is connected to it? Are any expansions (internal or external) or peripherals connected? Do any of these have their own PSU?
Internally there is the ATPL Sidewise ROM/RAM board with 16K of RAM and Acorn 1770 DFS 2.25.

Looks like the ROM/RAM board is taking its 5v power from the motherboard, and hooks into the OS ROM socket, IC73 and S21.

There are no external expansions or peripherals connected.

IMG_4667.jpeg
Last edited by anightin on Thu Aug 22, 2019 10:44 pm, edited 1 time in total.

User avatar
BigEd
Posts: 2749
Joined: Sun Jan 24, 2010 10:24 am
Location: West
Contact:

Re: Mode 7 intermittent lines

Post by BigEd » Fri Aug 23, 2019 6:42 am

I've a feeling one of my machines had a band of noise, and it turned out there was a dry joint inside the power supply. One of the big capacitors sent the message that it still had lots of charge when Dave (hoglet) tried to resolder it.

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Fri Aug 23, 2019 6:52 am

I guess that would be an easy one for me to try, having two Beebs, thanks for the suggestion @BigEd :D

User avatar
1024MAK
Posts: 9516
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Mode 7 intermittent lines

Post by 1024MAK » Fri Aug 23, 2019 7:59 am

Yes, the SMPSU is definitely a suspect.

Basically, what we are looking for, is any circuitry that operates from an independent oscillator. The SMPSU is the next (and most likely) suspect.

As well as possible dry joints, you should also be looking for broken or cracked PCB tracks and any capacitor that has suffered any distress or distortion. Look carefully at the top of each electrolytic capacitor, it should be completely flat and not domed. The bottom should also be flat to the board and not looking like the bottom plastic cap is emerging from the outer can.

The only other independent oscillators that I can think of are:
  • Crystal X2 (17.73MHz), Q10, IC50, IC46 and associated circuitry.
  • The Modulator
It’s fairly easy to disable the modulator, just desolder the +5V supply wire. But it is also the least likely cause.
The X2 crystal and it’s circuitry is also unlikely to be the cause, as I can’t see how it can get anywhere near to being 50Hz. But it is still worthwhile connecting this Beeb to a TV via UHF to see if it does produce a colour picture on a TV. If it does produce a colour TV picture, then the frequency of X2 is okay, and the oscillator in the modulator is also okay. If you are only going to be using the RGB output, the X2 oscillator section can be disabled by lifting one end of resistor R145 (1kΩ).

Mark

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Fri Aug 23, 2019 5:16 pm

1024MAK wrote:
Fri Aug 23, 2019 7:59 am
Basically, what we are looking for, is any circuitry that operates from an independent oscillator. The SMPSU is the next (and most likely) suspect.
Mark
Today I pulled the PSU from my other Beeb and installed it into the patient.

After 5 mins no OSD so I thought this was great. :)


However after about 5-10 mins I still get the occasional glitch and the OSD comes up -- but not as frequently as before.

So changing the PSU definitely improved the issue if not fixing it. :?

Hooking up to the telly showed that the noise generated from the other PSU was gone:
IMG_4700.jpeg
However there was some faint noise still present on the top few lines of the display -- I suspect this was what was still causing the occasional glitch on. the SCART->HDMI. The telly didn't seem to be bothered too much as with the old PSU tests yesterday.

Next course of action now I think is to fully refurb both PSUs to get them as noise free as possible including re-flowing each solder joint. Looking at the PSUs, visual inspection didn't reveal anything untoward at this stage -- no cracked paper caps or bulging electrolytics.

I may still also replace IC43 after re-testing the PSU.


The quest continues :D
Last edited by anightin on Fri Aug 23, 2019 5:18 pm, edited 1 time in total.

Kazzie
Posts: 1025
Joined: Sun Oct 15, 2017 7:10 pm
Location: North Wales
Contact:

Re: Mode 7 intermittent lines

Post by Kazzie » Fri Aug 23, 2019 7:09 pm

At least you're making some (positive) progress. Keep going!
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

User avatar
anightin
Posts: 368
Joined: Thu Aug 23, 2018 1:03 pm
Location: Cambridge UK
Contact:

Re: Mode 7 intermittent lines

Post by anightin » Sat Aug 24, 2019 10:43 pm

Kazzie wrote:
Fri Aug 23, 2019 7:09 pm
At least you're making some (positive) progress. Keep going!
Thanks Kazzie, your motivation was enough to help me drive this to a conlcusion -- The Beeb is now good as new :D :D :D

It turned out to be the 16MHz crystal after all. I re-fitted a 'new' old stock component to replace the original and that turned out to exhibit exactly the same symptoms as the old one. I fitted a new SEI 16MHz one (below) and the symptoms dissapeared.
roganjosh wrote:
Sun Aug 18, 2019 11:33 am
I had exactly the same issue on several Model Bs (horizontal lines & sync loss) as I've reported elsewhere. In all cases replacement of the 16MHz crystal (X1) solved the problem.
@roganjosh, I'm pleased to say your assertion held true for me too -- eventually! :lol:

I guess on the bright side, I also identified a noisy PSU that I didn't know I had :)
IMG_4723.jpeg
I think I've spotted the culprit! :? (Apologies for focus quality -- too hasty in my snapping)


Sequence of events leading to solution:
  • Replaced 16MHz crystal based on collective input from forum members -- no change observed
  • Run RGB signals direct into SCART TV -- based on input from forum
  • Observed noisy picture -- 50Hz based noise, which every now and again seemed to get worse for a fraction of a second which coincided with the issue
  • Swapped out PSU removing most if not all noise from above picture and improved problem -- but still an intermittent OSD being displayed on the SCART->HDMI
  • Replaced IC43 -- no change
  • Replaced IC41 -- no change
  • Replaced IC48 -- no change
  • Fitted new crystal and no loss of sync or OSD after 15 mins of use :D


I'd just like to thank everyone who has contributed to a spot-on diagnosis -- despite being thrown off the scent at the start #-o
Last edited by anightin on Wed Aug 28, 2019 6:17 pm, edited 5 times in total.

Post Reply