Putting a Vertical (or horizontal) Size POT on a chassis that never had one...

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Putting a Vertical (or horizontal) Size POT on a chassis that never had one...

I picked up a couple TVs for future tube swap attempts, and there was a 20" in with them. I'd
like to put it next to the arcades with the NES hooked up to it, but it has a crazy amount of
overscan!

The model doesn't have a "service mode" to adjust the screen, so I pulled it apart to adjust
the vert/horiz pots, but it had none.

I've been figuring out the traces (without a schematic available), and it seems like these TVs
were made so that they could use the same board with the different sizes that the model came
in. There's a spot for a 2nd Width Cap, which I've got to the proper size, and there appears to
be a spot for a surface-mount resistor for the Vertical Size (red square in the image).

I hesitated to post this question here, but maybe one of you Monitor Gurus could verify
my findings on this 'monitor'... :) Maybe point me to the proper location, if I'm way off.

Thanks!

EDIT: The model didn't appear to have a service mode. Modifying the remote gave me access to SM.
 

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Since you have the pinout of the vertical frame IC I guess the datasheet doesn't have an application circuit showing height/size adjustment? What's the IC? TDA8170? Try to find the schematics of some other TV chassis that has the same IC and see how they did it there.
 
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I've got a couple scrapped tv boards that I kept for my nephews to learn soldering on, and they use ICs with the same pinout and a very similar circuit (found a data sheet for one of the ICs - A78040A). None of the tvs have had schematics online, though, and they use the same IC in a slightly different manner.

I replaced the 4.7kohm resistor (r582 off of the non inv. input pin) with a 10k variable resistor. Changing the value introduced vertical pinching and stretching (see illustration).
I'm not sure why the extra surface mount pads are there, because it'd parallel the r582 resistor between the Non Inv. Input pin and the capacitor.

Maybe it's a capacitor-to-ground like on the horizontal deflection. C552 on the pump up out circuit is a 0.1uf Film cap.
 

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This chassis appears to be from a time when geometry adjustments were no longer analog but made by a microcontroller through service mode. The LA7804 datasheet confirms this so I wouldn't try to mod the chassis and look for a way to access service mode.
 
Thanks for your help! In looking for the datasheets for the microcontroller, I stumbled upon a service manual for a tv that used a very similar chassis (the microcontroller was listed in the service manual, so it popped up in the search). I was able to modify the remote and adjust the screen! I will still have to populate the extra width capacitor in order to erase that horrible 15% overscan, but the Vert adjusted perfectly via the microcontroller.

EDIT: Still looking for a way to adjust the horizontal size. Changing the cap (C571/C571 - labeled in the original post) did not work.

This chassis appears to be from a time when geometry adjustments were no longer analog but made by a microcontroller through service mode. The LA7804 datasheet confirms this so I wouldn't try to mod the chassis and look for a way to access service mode.
 
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