Black and white CCTV monitor loss of V sync

[aberco]

Hello all,
I have a pair of video monitors, no name brand, which I have purchased to use on my Apple 1 build. They are both PAL, since I'm in Europe, and they work with the Apple1 NTSC output.
Monitors both showed issues that I thought were coming from my Apple 1 build, as I had horizontal (pixel clock) jitter on the video signal. This jitter is dependent over what the computer is doing, it's not visible when the CPU does nothing (held in reset) but it is quite visible when running a program. Then I tried 3 other monitors from friends, CCTV monitors and vintage TVs, and none of them are showing the jitter, which had made me think that the issue was coming from my monitors instead.

I can live with the jitter issue, but I tried to replace the electrolytic capacitors in one of them to potentially solve it and it now does not lock on V sync. The V sync adjustment pot makes the image scroll up or down and is very sensitive, I can get it sometime to stop scrolling but the start position can be anywhere on the display, not necessarily in the upper left where the image should start.

The inside shows what is probably a very typical 1980/1990 CRT circuit using a TDA1170 for vertical and a TBA920 for horizontal drivers. I do not have the schematics, but it seems that they have followed the reference design for the TDA1170 and TBA920. I have attached a photo of the inside of the monitor.

I'd appreciate any lead that would allow me to recover the lock on the V sync. Thanks in advance for the time!

[Psi]

They are both PAL, since I'm in Europe, and they work with the Apple1 NTSC output

This seems somewhat contradictive.

[BrokenYugo]

As I recall, monochrome NTSC/PAL50 are close enough they're sorta interchangeable IF you can get the vertical to lock on the display in question.

[aberco]

Most PAL monitors will indeed work with NTSC signals, the color information is not needed. Then it's a 60Hz scan instead of 50Hz, but as said above it is fine if the monitor can lock on that V sync frequency. Many old monitors do.

Maybe changing the capacitors did improve the tolerance in a way that prevent the monitor to lock onto the signal? as of now the V sync oscillator is free running. The TDA1170 receive the sync signal on pin 8, I checked with an oscilloscope and it is there and looking fine.