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Vertical Bars on Display

Posted: 09 May 2017 19:57
by wyerd
The display on my MTX500 has never been that good as there are faint vertical bars on the display. My recently acquired US spec MTX500 doesn't display these bars and is very clean. Any thoughts on what could be causing it?
MTX512
MTX512
39A9088A-0A3C-439C-A80C-3C01DA0EB8AC.JPG (40.51 KiB) Viewed 2604 times
MTX500
MTX500
55923EA3-C70C-4F5F-8D27-937034273F7F.JPG (73.49 KiB) Viewed 2604 times

Re: Vertical Bars on Display

Posted: 09 May 2017 20:50
by Martin A
Both my MTX's have the bars on the display, so does the MTXplus although that uses a different VDP/Encoder combination.

I hadn't realised there were any MTX's out there that didn't have them !

Re: Vertical Bars on Display

Posted: 09 May 2017 22:01
by Dave
Of course, the US model uses a TMS9918, as opposed to the UK models TMS9929, that could be the difference.

As Martin say's, all of the UK models seem to have the bars

Regards
Dave

Re: Vertical Bars on Display

Posted: 10 May 2017 03:52
by wyerd
Oh, okay. Good to know.

As the TMS9918 is running at 60Hz, I could replace it with an F18A. Decisions, decisions.......

Re: Vertical Bars on Display

Posted: 11 May 2017 11:48
by 1024MAK
I suspect this problem may be a limitation of the circuitry. But it may also be due to limitations of the supply rail decoupling. As others have said, it's very common.

On other systems (e.g. ZX Spectrum 48k) vertical bars can sometimes be due to inadequate decoupling of the +5V and +12V supply rails in the area of the video generator chip and it's (video) RAM.

However, I have not investigated on any of my MTX machines. If you want to try renewing the ceramic capacitors in a MTX, it's the ceramic capacitors near the VDP and the 4116 DRAM chips. Use 1uF multilayer ceramic types.

Mark

Re: Vertical Bars on Display

Posted: 11 May 2017 15:07
by wyerd
1024MAK wrote:If you want to try renewing the ceramic capacitors in a MTX, it's the ceramic capacitors near the VDP and the 4116 DRAM chips. Use 1uF multilayer ceramic types.Mark
Are they C22 and the one in between LNK10 & LNK11?

Re: Vertical Bars on Display

Posted: 11 May 2017 15:41
by 1024MAK
wyerd wrote:
1024MAK wrote:If you want to try renewing the ceramic capacitors in a MTX, it's the ceramic capacitors near the VDP and the 4116 DRAM chips. Use 1uF multilayer ceramic types.Mark
Are they C22 and the one in between LNK10 & LNK11?
I'll have to look at a MTX board...

Mark

Re: Vertical Bars on Display

Posted: 11 May 2017 19:08
by wyerd
They're in this shot
FullSizeRender.jpg
FullSizeRender.jpg (169.61 KiB) Viewed 2571 times

Re: Vertical Bars on Display

Posted: 11 May 2017 22:01
by 1024MAK
The ones circled in red ;)
I may have missed some under the video PCB.
MTX video decoupling caps
MTX video decoupling caps
MTX video decoupling caps.png (609.55 KiB) Viewed 2569 times
Basically, if it is near the VDP or the 4116 DRAM chips and it connects to either the +5V PCB tracks or the +12V PCB tracks, renewing with a 1uF cap may help. With the MTX, keep in mind that this is just theory at the moment.

Mark

Re: Vertical Bars on Display

Posted: 12 May 2017 03:54
by wyerd
Ah! Just a few then :shock: :shock: :shock: