Rolling picture...sometimes

About original Memotech hardware.
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mgarnett
Posts: 2
Joined: 08 Sep 2020 12:20

Rolling picture...sometimes

Post by mgarnett »

Hi Everyone,

I have an MTX 512 (with the extra ram board rather than all memory on the main board), and I am having some minor problems with the video image.

When the computer is first turned on, the image suffers from some intermittent rolling and tearing, but then after four or five minutes the image becomes stable. However, when I run certain games (Worm for example), the image will begin to roll constantly, but when I exit out of the game, the rolling stops and the image returns to normal at the basic prompt. Other games, such as space invaders and many others, work absolutely fine with no tearing or rolling. Only a few games have the problem.

From the behaviour, it looks like the issue gets better as components get warmer (other than the games that cause the image to roll, they roll no matter how much time has passed).

I have replaced every electrolytic capacitor in the system, some were way out spec, both on the video board and the power section. As prevention, I also replaced both regulators and the power transistor. This has had no effect on the video issue. I’ve checked all voltages and they are very good and solid.

The monitor is a JVC PVM CRT which also has a Commodore 128 connected to it, and the picture from that machine is rock solid. However, I am trying to source a different monitor to rule that out of the equation.

I notice that there looks to be some sort of trimmer capacitor on the PAL video board and wondered if that has any effect before I start turning it.

I think that the sync signal comes from the video chip, could this sort of intermittent issue be caused by a faulty VDP chip? I’m going through the machine to give everything socketed a thorough clean as well.

Thanks

Mark
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Dave
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Joined: 11 Aug 2012 18:16
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Re: Rolling picture...sometimes

Post by Dave »

Hi Mark,

the most obvious cause might be a missing ground strap between the back panel and the regulator ground. Have a look at page 48 of the Service Manual available here

http://primrosebank.net/computers/mtx/mtxmanuals.htm

Also note the 47pF capacitor on the back panel

You could rule out most of the components on the video board by pinning it for mono, rather than colour. (More details if required, but the pin/staple is shown on the PAL video board PCB schematic - again, the schematics can be found on the page referenced above.)

(I assume that you are using the Composite video output with your TV/Monitor? The RF/TV output is often poor as the modulator will likely have deteriorated after all these years - they were never that good in the first place! The modulator output does often change as it warms up, the AV output, not so much.)

regards
Dave
mgarnett
Posts: 2
Joined: 08 Sep 2020 12:20

Re: Rolling picture...sometimes

Post by mgarnett »

Hi Dave,

Thanks for the response, the ground strap is indeed missing from the machine.

I'll cobble one up and see if that makes a difference. The capacitor is there and checks out, and yep, I'm running through the composite out rather than the RF.

I'll post some results shortly.

Cheers

Mark
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1024MAK
Posts: 627
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: Rolling picture...sometimes

Post by 1024MAK »

In answer to your other questions, yes the TMS9929A VDP produces all the video signals. The sync. signals are included in the composite monochrome video output (Y) on pin 36.

A rolling picture normally is caused by either insufficient signal level / poor signal to noise ratio, or a problem with the monitor not being able to lock to the sync. signals for some reason.

It is also possible that there is a poor or high resistance contact or pin between the VDP socket and the chip legs.

All video timing for the VDP comes from the 10.7MHz crystal.

The PAL board takes the Y signal, and two colour difference signals, and uses these to encode the colour to produce a PAL colour composite video signal. The variable capacitor (CV1) on the PAL board is for trimming the colour frequency oscillator (4.433619MHz). It’s not normally recommended to fiddle with this unless you have a suitable oscilloscope or frequency counter. And this being off frequency would result in colour problems rather than a rolling picture.

Having said that, as Dave says, all the colour circuitry can be bypassed by cutting the ‘a’ link wire and making the ‘b’ link wire. Then the video output will be composite monochrome.

The PAL video board runs off the +12V supply, which is then decoupled by C63 (shown as C52 on some diagrams). So you do need a good (smooth, ripple free) +12V rail. For some of the circuitry, the +12V is dropped to a +5V supply by R47 (470R), ZD4 (5.1V) and C41 (100uF).

Mark
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