Original 520ST Video instability

[coonsgm]

Looking for some help here.

I picked up an original 520ST (no internal PSU / floppy drive). Other than some yellowing, it is in fantastic condition and based on copper tape on the RF shielding with no fingerprints on the shielding it looks like it's never been opened up before.

The problem is a new one to me. Within about 5-10 seconds from switching on either with the diagnostic cart, loading a program from disk, or booting to the desktop, the screen begins to dance/shake vertically.

This is on an Atari SC1224 monitor. I checked with a couple other ST's to make sure it wasn't the monitor hadn't developed the disease.

Power Supply is perfect...works just fine with the other 3 520ST's (yes I have a problem).

The one interesting point is in the diagnostic cart menu, when I switch to 50hz video mode, it's rock solid. This is 100% an NTSC machine.

Any thoughts?

[coonsgm]

I've also used contact cleaner on all socketed IC's and the video port out.

Swapped in 2 different shifters (same issue)

[PhilC]

@coonsgm id probably work your way through the video circuit from the shifter. I've had a few diodes, caps and transistors fail in the arse round the shifter before on a couple of old STMs.

[stephen_usher]

Remember that the v-sync signal is generated by the MMU and the h-sync by the GLUE. Neither are generated by the shifter, and this sounds like a sync issue of some kind.

As it's a 520ST and not 520STM there won't be any composite sync, only h-sync and v-sync, and there's nothing between the chips and the video socket.

[coonsgm]

Turns out it was the GLUE. First partly faulty GLUE I've run into.