Btw: cutting a bit of the key membrame solved that. I now have all keys up and running

Be careful with next opening, i reached that time few years ago when i could not cut more , fortunately since then i have new membrane.

@ZoZo: Thank you very much for the explanation. Makes a lot of sense!

I will start by "upgrading" the power supply to a new one (still have not recieved the polarity changer) and keep and eye out for the ePower.

Has any body changed the heat sink on their Nick? Mine was very firmly connected.

Btw: cutting a bit of the key membrame solved that. I now have all keys up and running

 

Great explanation, Zozo.

Sorry for being insistent with the external cause. The Enterprise video output was made analogic and  for CRTs, and so, the images we actually see on our LCD monitors are always conversions. You Zozo, logically have sugested making tests with a CRT TV first, because it would show the real image before being converted.

On the near future it will be harder and harder to connect the Enterprise on the TVs that will come. I think that this same problem or similar will arise more frequently.

And I think that most likely the problem is EP related since it comes after 3-5 min. of use. When I turn the EP on there is almost no display problem at all.

@ZoZo. You mentioned a "small inaccuracy in Nick clock.". Is that something that can be fixed?

I thought about the jittering.
First look the internal 12V circuit.
Then the Nick clock circuit, which is use the previously generated 12V.
This is why I guessing about power parts.
It is a well know problem, when something weak at power side, the two half screen start to make a sine dance on the screen.
With a L2 and C43 can adjusted the Nick clock, if the voltages are ok, then these can be used for stabilize the screen.

When the components warmed up, these two complex circuits are little change their frequencies. Especcialy the 12V circuit components near to the two 7805, which are generate lot of heat. And the 12V used by the Nick clock circuit...
On a some machine see the problem, screen start sine dance after few minutes, then used L2 for a fine adjustment, it is solved the problem.
Sometimes see machine if turned on below 20 celsius, start dancing screnn until few minutes warm up. Also fine adjustment solved it.

Another factor the Nick heat shink, especcialy at the 08-04 type Nick chips, which are more sensitive for a heat.

I thought about something similar happen at your machine, just in a minimal size dance, one pixel, or a less. Possible on a real CRT don't see anything, because it is very minimal. But at digital sampling can the minimal differences magnified. If one pixel started little earlier, then it is moved as one full pixel on the screen.

Nice, you find the right setting on the OSSC!

"which may be useful with sources that use off-spec horizontal rate."
IstvanV wrote about the Nick freq:

For the NICK chip, the clock frequency is derived from the PAL chroma sub-carrier (17734475 / 4 Hz), such that one line is exactly 284 cycles of that, which is slightly different from the PAL standard. Thus, the horizontal refresh rate is 17734475 / 4 / 284 = 15611.3336 Hz in theory, instead of the standard 15625 Hz. Although the circuit that generates the clock can be broken in some of the machines, resulting in an inaccurate frequency and "waving" picture or other artifacts.

I found the solution to the problem.

In the OSSC settings you have to turn this on:

Allow upsample2x
Controls whether 2x samplerate is used instead of pixel repetition in certain modes, e.g. 384pX2, 480pX2, 480iX3, 480iX4, 960iX2.

On: 2x samplerate is used to double output horizontal resolution, which may be useful with sources that use off-spec horizontal rate. Alternatively, the option can be used to generate slightly more analog-esque (i.e. less pixellated) picture.
Off: Pixel repetition is used to double output horizontal resolution, which regenerates source image most faithfully if sampling matches dot clock. [default]
So as gflorez suggested it had something to with the horizontal resolution.

Unfortunately I messed up my keymembrame when I opened the machine to check the cooling on Nick. First some of the keys did not work and after a couple of tries to fix it now A LOT of the keys do not work. I am trying to find a new .... Until then I can not really use the machine