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.
