Vertical hold not working using Happ Vision Pro MTG-1901

[MasterControl]

Hi. I have an Atari 1985 Gauntlet-II Arcade. It has a vertical hold issue. I bought it, knowing this issue, thru an auction out of state a few months ago. The CRT and CRT board had been replaced before I bought it. My guess is the upgrade occurred 15-20 years ago or used that technology from then around 2003.

Altho all adjustments work, I cannot get the Vertical hold to stop. The best that I can adjust it, is to slow it down. Rarely locking in.

What the previous owner replaced:
1). CRT is a 19 inch Samsung type A48KRD82X01. Vision Pro. Happ Controls MTG-1901CN.
2). The board is a Vision Pro. Happ Controls MTG-2901.

The service manual:
https://www.arcade-museum.com/manuals-monitors/Vision_Pro_(MTG-XX01_Series_13in_and_19in_CGA).pdf

I did the following in various stages, WITHOUT ANYTHING IMPROVING.
 1). Recap the entire board.
 2). Replaced chip i301 (LA7851) because the VH was affected by cold spray.
 3). Various Mylar caps around that area because the VH was affected by cold spray.
 4). Replaced the 200k VH pot VR303.
Again, there was NO CHANGE.

So I bought a new "working" VisionPro Happ Controls CRT board that was identical to the board I had.
And guess what, NO CHANGE.

My only thought at this time is the VH problem is originating from the main game board.

On the upper left side of the Happ VisionPro CRT board are the inputs from the main game board. R,G,B, gnd, V/S, H/S.
Does anyone know the video pinout for the Atari Gauntlet II game board?

Both the V/S and H/S are connected to pins on this board. If I remove either of them, the picture gets scrambled. If you refer to the link at the top, it was suggested that the V/S (yellow wire) should be disconnected which scrambled the picture.

or Is it possible that the V/S is not working properly? I have a fairly new LED Oscilloscope.  But my expertise ends here.

Thank You. Pete.

[Zebidee]

Some thoughts:

1) use that oscilloscope on the V sync, see if the pulses are there. They should be fairly obvious if present. Horizontal pulses are short (around ~4.7us) and you'll see them easily at 20-100us. H sync pulse every ~64us. Vertical pulses are a lot longer but less frequent (~60/second). You'll need to set the time to around 1ms or more to see them easily. Stabilise the view by locking the trigger onto the sync signal. Then you can reduce the time period to "zoom in" on the signal, as desired. Not sure what your scope can do.

2) try another PCB to see if it is a board problem or monitor problem.

3) try bridging the V sync over to the H sync, with a wire. Easy way, try U shape, cut up a paperclip or something, insert into back of connector so it touches the metal on each pin. Easy to install and easy to remove. Nothing will blow up. This may give you a crude composite sync ("smushy" sync), which hopefully the monitor can lock onto.

I'm also guessing that the monitor is maybe OK. However, you could...

4) check all connections, solders, looks for cold or loose joins. Lightly tap/poke board with something non-conductive (like a wooden spoon or chopstick) around the sync and vertical sections especially.

5) get out multimeter into diode mode and check those two zener diodes (D311,D312) on the sync inputs. They would be for input protection against over-voltage. While you are there and in diode mode already, check those sync transistors (Q304, Q305).

6) check resistance to ground from the Vsync input. There should be some resistance (like that 10K resistor) but a dead short means a problem somewhere.

Disclaimer: I've never worked on that monitor, just what I can see from the schematic.

But yeah, scope out that sync.

[MasterControl]

 Zebidee, thanks for the very helpful information!  I found the Gauntlet schematic and got the video pinouts from the game board. IT'S FIXED!!! HOORAY!

The yellow V/S wire from the VisionPro board was connected to composite out. I moved it to V/S and it fixed the issue!

I will definitely do a few of your suggestions just to gain more knowledge. And I do appreciate your quick response.

I have a Discs of Tron environmental arcade that definitely needs recapping due to an A/C hum in the audio. I am also looking for artwork to make it look great again. I think this site is a great site for those resources.       

Take care. Pete.