Vertical Bars on Display

[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

39A9088A-0A3C-439C-A80C-3C01DA0EB8AC.JPG (40.51 KiB) Viewed 18182 times

MTX500

55923EA3-C70C-4F5F-8D27-937034273F7F.JPG (73.49 KiB) Viewed 18182 times

[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 !

[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

[wyerd]

Oh, okay. Good to know.

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

[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

[wyerd]

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?

[1024MAK]

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

[wyerd]

They're in this shot

FullSizeRender.jpg (169.61 KiB) Viewed 18149 times

[1024MAK]

The ones circled in red
I may have missed some under the video PCB.

MTX video decoupling caps

MTX video decoupling caps.png (609.55 KiB) Viewed 18147 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

[wyerd]

Ah! Just a few then