Screen Flicker

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rodders
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Screen Flicker

Post by rodders » Wed Jul 01, 2020 5:47 am

I've recently been resurrecting my old Model B and have been plagued with a problem where the display randomly flickers off and on. This happens in all modes but the computer seems to operate fine. Poking around it seemed to me that I could invoke the flicker by moving the 0v wire connected to the SW corner of the board, and this could even cause a crash. I thought I'd found the problem as the resistance of that wire would go open circuit if you wiggled it and I traced this to a dry joint in the connection to the PSU. However, its started doing it again and the wire continuity is fine now.
I have the Beeb connected to a Celcus LCD tv via an RGB - Scart lead from Retro Computer Shack, the TV is set to game mode.
Any ideas?
BBC Model B 32k issue 3, WE ROM/RAM board, Speech Processor, 6502 Second Processor

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richardtoohey
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Re: Screen Flicker

Post by richardtoohey » Wed Jul 01, 2020 5:51 am

Hardware not my thing, but have you tried another TV/cable to eliminate those?

From what you've said seems unlikely, but just to eliminate them.

rodders
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Re: Screen Flicker

Post by rodders » Wed Jul 01, 2020 7:23 am

I'd have to make one up but worth a try.
I've just checked the PSU output and while the 5v leads are showing 5.1v the -5v one is only showing -4.4. Anyone know whether this is significant and what it would affect?
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tricky
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Re: Screen Flicker

Post by tricky » Wed Jul 01, 2020 7:38 am

-5v is audio amp and serial (I think) only and mine are also not 5v.
Can you try a composite lead or even RF to the same TV?
There are quite a few threads here where LCD TVs have occasional drop outs.
It seems that they are just more sensitive to signals from old HW.

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Re: Screen Flicker

Post by Kazzie » Wed Jul 01, 2020 8:23 am

What issue board do you have? Earlier boards had their 5V power supply cut into three parts (hence three wires) to suit the early linear power supplies, though they were mostly shipped with a switching power supply. These were joined together on later boards, so those won't usually suffer from one loose power connection.
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Re: Screen Flicker

Post by rodders » Wed Jul 01, 2020 8:28 pm

Its an Issue 3 board which has had all the upgrade mods done.

In trying to disconnect the leads from the board to examine the spade posts I managed to snap one off (that's two now, Doh!). The spade was extremely tight, I had to use considerable force with pliers to get the broken bit out. These spade connectors are a pain so I decided to solder a flying lead into the PCB instead and have the connector midway to the PSU. All back together and Voila! seems to be OK until <flash> Nooo. At this point I was running out of ideas so I rebooted the TV and Hey Presto the flickering stopped. I'm pretty sure its nothing to do the the BeeB (except for the dry joint in the PSU - the lead just pulled out of the connector to the PCB), it must be the TV.

Anyway, my long awaited Chinese SCART to HDMI box arrived today so I'm running on an old Benq T905 screen - no problem.

Thanks for your input guys.

One final thing, the display on both the TV and monitor is narrower than the screen, is there any way to adjust this?
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Re: Screen Flicker

Post by Kazzie » Wed Jul 01, 2020 9:52 pm

Glad you've got your power connections sorted. The spade connectors used in the Beeb are still available to buy, if you'd prefer to replace your broken ones. I'm sure someone here could quote the part number for you.

As for the narrow screen, are you using a modern, widescreen display, or is it an old 4:3 ratio?
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
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Re: Screen Flicker

Post by rodders » Thu Jul 02, 2020 8:53 pm

Its a Benq T905 which is 4:3 running at 1280x1024 @ 60Hz
Last edited by rodders on Sun Jul 05, 2020 6:14 am, edited 1 time in total.
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danielj
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Re: Screen Flicker

Post by danielj » Thu Jul 02, 2020 9:41 pm

rodders wrote:
Thu Jul 02, 2020 8:53 pm
Its a Benq T905 which is 4:3 running at 1280x1024 @ 50Hz
A few of the BenQs support 15KHz natively (even though not advertised) - if you can rig up a beeb RGB-VGA cable, with the same resistors in as you'd use in the SCART, you might find you don't need an adaptor at all? (and the beeb's display doesn't extend all the way to the edges, it's sort of squarer in the middle!)

d.

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Re: Screen Flicker

Post by Sampull-MC » Thu Jul 02, 2020 10:38 pm

Since I had a similar problem with my Elk, is it a straight on-off flicker? Or does it shudder up/down and disappear?
Last edited by Sampull-MC on Sat Jul 04, 2020 9:11 pm, edited 2 times in total.

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Re: Screen Flicker

Post by rodders » Sat Jul 04, 2020 5:50 am

Its a straight on/off flash, like the video output dropped momentarily
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Re: Screen Flicker

Post by rodders » Sat Jul 04, 2020 7:26 pm

Hmm, I think there is still a problem. With the Beeb connected to the Benq monitor via a SCART-HDMI converter I occasionally get the OSD from the converter box pop up. I suspect this is a glitch in the sync line causing the converter to reinitialise and maybe what was causing the problem with the TV which took longer to recover so the flicker was more noticeable.
Ideas please on what could cause such a glitch and how I could fix it..
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anightin
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Re: Screen Flicker

Post by anightin » Sat Jul 04, 2020 7:32 pm

rodders wrote:
Sat Jul 04, 2020 7:26 pm
Hmm, I think there is still a problem. With the Beeb connected to the Benq monitor via a SCART-HDMI converter I occasionally get the OSD from the converter box pop up. I suspect this is a glitch in the sync line causing the converter to reinitialise and maybe what was causing the problem with the TV which took longer to recover so the flicker was more noticeable.
Ideas please on what could cause such a glitch and how I could fix it..
I had to replace the X1 crystal on my Beeb to stop the screen from flicking off then on every few seconds:

viewtopic.php?f=3&t=17709&hilit=osd#p246099

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Sampull-MC
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Re: Screen Flicker

Post by Sampull-MC » Sat Jul 04, 2020 9:11 pm

rodders wrote:
Sat Jul 04, 2020 7:26 pm
I suspect this is a glitch in the sync line causing the converter to reinitialise and maybe what was causing the problem with the TV which took longer to recover so the flicker was more noticeable.
Ideas please on what could cause such a glitch and how I could fix it..
This is similar to my Electron, which turned out to have a quacked V-sync. This little box sorted it.

https://www.ebay.co.uk/itm/PAL-NTSC-SEC ... SwNbRes7bQ

These work really well.

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Re: Screen Flicker

Post by rodders » Sun Jul 05, 2020 5:57 am

Thanks anightin that sounds exactly the problem I have. 16 MHz crystal ordered and fingers crossed.
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Re: Screen Flicker

Post by rodders » Thu Jul 09, 2020 9:39 am

I fitted a new 16MHz crystal this morning. No more flicker, no more OSD popping up. FIXED!
You guys are brilliant!
Should this be added to the Fault Finding Index?
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Re: Screen Flicker

Post by Kazzie » Thu Jul 09, 2020 9:59 am

rodders wrote:
Thu Jul 09, 2020 9:39 am
I fitted a new 16MHz crystal this morning. No more flicker, no more OSD popping up. FIXED!
You guys are brilliant!
Should this be added to the Fault Finding Index?
Please do!

(And congratulations!)
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

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Re: Screen Flicker

Post by rodders » Thu Jul 09, 2020 10:34 am

anightin has already done it.
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Re: Screen Flicker

Post by anightin » Thu Jul 09, 2020 10:35 am

Kazzie wrote:
Thu Jul 09, 2020 9:59 am
rodders wrote:
Thu Jul 09, 2020 9:39 am
I fitted a new 16MHz crystal this morning. No more flicker, no more OSD popping up. FIXED!
You guys are brilliant!
Should this be added to the Fault Finding Index?
Please do!

(And congratulations!)
Well done on getting it sorted. I may have beaten you to it on the Fault Finding entry but please feel free to update :)

viewtopic.php?f=3&t=2190&p=270865&hilit ... ng#p246304

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