Lowering B+ voltage to reduce horizontal over scan

[Shun]

I am messing around with a 14 inch portable tv and I have a pac-man board hooked up to it via scart.
I have the normal over scan issues with it, but the TV has Vertical width and height adjustment pots which allow me to position the screen correctly. However there are no controls for the horizontal width so the top and bottom of the screen are missing (credits and high score in this case).

I have poked around in the back of the TV and there is a B+ adjustment pot. If I increase the resistance on this, the picture shrinks (and I assume the B+ voltage is lower as a result) and I can now see the credits and high score. Unfortunately, when the pot is set at the highest resistance the top and bottom (horizontal width because pac-man is rotated 90 degrees) are still slightly off.

I'm thinking that I could replace the pot with a slightly higher resistance rated potentiometer.

The pot is marked 471 which I think is a 470 ohm pot. It looks like the one in this picture picture: http://ecx.images-amazon.com/images/I/41wfl20Zb2L._SX342_.jpg

I was wondering if I could get away with replacing it with this 1K ohm pot from Maplin:

http://www.maplin.co.uk/p/1k-andohm-cermet-horizontal-preset-potentiometer-wr40t

Would I be right to assume that it is pretty safe to have a lower than normal B+ as apposed to a higher one?

 

[PaulSwan]

The width is often a coil rather than a pot. Dropping B+ affects everything so they'll be a limit that will start to cause other problems such as low HV and poor focus & brightness. Many TV manuals are scanned online now so it might be worth getting a service manual for the TV and seeing what it says about width adjustment options.
Paul.

 

[obcd]

Isn't that horizontal width coil in serie with the horizontal deflection coil? It's a rather big one with a ferrit core you can screw in or out the coil center. (Just be carefull as ferrit breaks easily.)

Turning the core more in increases the inductance of the coil so that deflection coil get's less power, so that should decrease horizontal width. (Or should I say vertical width in your case)

 

[Shun]

Thanks Paul, Obcd,

I actually have two televisions that I am trying to get working with the Midway pac-man board

The one that playing with at the moment is a pacific pvtv361 14 inch crt tv
I can't find a service manual or schematic anywhere for this. The chassis is a vmb290a.
I can get into to the service mode, however there are no options for the geometry.
Also I can't see any width coils like you would have on a standard arcade monitor.
There are coils on the power board, however there don't seem to be any holes for adjustment.

The other tv I do have a service manual for however there are no controls for vertical height.
The only controls are for vertical width and horizontal position. I'll dig the model number and info on that one when I get home.

 

[Shun]

Ok, I replaced the pot for one with higher resistance, however the horizontal width wouldn't get any smaller at a certain point.

I've heard that you can change the capicitors in the width circuit to reduce the size:

http://arcadecontrols.com/BBBB/width.html

However without a schematic its not easy to work out what is currently installed . There only seems to be one capacitor that looks like one of the ones in the link above however it doesn't seem to be marked as a normal capacitor (see below). Does anyone know the rating for this capacitor?

 

[gunblade]

the orange one is 8.6nf 1250v

 

[joe34]

The other tv I do have a service manual for however there are no controls for vertical height.
The only controls are for vertical width and horizontal position.

just double checking, there is a vertical 'width' option?

 

[joe34]

The one that playing with at the moment is a pacific pvtv361 14 inch crt tv

I know the model number is not exact but have a look at the link below, might get lucky and it's the same tv (or failing that, the same service adjustments procedures)
http://www.mediafire.com/download/c5qajz8anze6yb5/Pacific+PTV3606.pdf

joe342015-05-28 23:36:16

 

[Shun]

Thanks for the responses guys.

the orange one is 8.6nf 1250v

Ah, that appears to be too big then, I think it should be less .56uf while that one is .86uf i think.

The one that playing with at the moment is a pacific pvtv361 14 inch crt tv

I know the model number is not exact but have a look at the link below, might get lucky and it's the same tv (or failing that, the same service adjustments procedures)
http://www.mediafire.com/download/c5qajz8anze6yb5/Pacific+PTV3606.pdf

I had found this one before, but dismissed it due to it being the wrong model number.

Thank you so much for bringing it to my attention again as I managed to use this to display the service mode!

With this I was able to move the picture horizontally (vertically in my case with it being pacman) so that the high score could now be seen. The credits just fit in at the bottom as well.

just double checking, there is a vertical 'width' option?

Regarding the other TV, it is a Philips 14pt1342/25 with a l6.1 chassis. This only has pots for vertical size and horizontal movement when for this TV I needed horizontal size and vertical movement pots to centre the image. There is a service mode for this TV however it only seems to display error codes.

There is only one slight niggle now that I have the Pacific TV setup is that when the TV turns on, it is standby and I have to press the power button to turn it on. This is a problem because I am using the TV and the pacman board to build a Pac-Man cabaret replica from scratch (videos of my progress are here if anyone is interested: http://www.youtube.com/playlist?list=PLodL6wnVHl6cv5nV2t8qxG9ZxmoRpgbsw).

So I may have to knock up some kind of delayed relay to bridge the switch on the standby button. Annoyingly I have the Philips TV already fitted in place as that displays the AV channel as soon as it is switched on. Grrrr!

 

[joe34]

I feel your pain m8 I've been through loads of TV's in the past looking for good ones suitable for arcade/mame use. TV's not allowing for horizontal size changes and powering on in standby we're quite common unfortunately. regarding you said you have a 'power' button you have to press after you turn on the tv to get it out of standby. what happens if the tv is off, you hold down the 'power' button, and you turn on the tv. (perhaps you've tried that already, but if not, give it a go)joe342015-05-29 00:49:09

 

[Shun]

I feel your pain m8 I've been through loads of TV's in the past looking for good ones suitable for arcade/mame use. TV's not allowing for horizontal size changes and coming on in standby we're quite common unfortunately. regarding you said you have a 'power' button you have to press after you turn on the tv to get it out of standby. what happens if the tv is off, you hold down the 'power' button, and you turn on the tv again (perhaps you've tried that already, but if not, give it a go)

Yep tried that, but it just sits there taunting me with its red standby light

.

I'm sure there should be someway of creating a small circuit that has a delay on power up and then clicks a relay on and then off. I have had a quick look on the t'internet and i seems that it might possible.

 

[joe34]

have you looked into sending some volts into pin 8 of your scart connector. that might stop your tv being in standby mode (long shot perhaps but maybe worth a look)
eg. send 10v-12v to scart pin 8, might get away with 5v (although it might put your tv in widescreen mode if it's only 5v-8v

 

[joe34]

have a look at page 22 of that service manual I linked, there's a large IC chip thing, I notice pin 10 there is labelled 'power' (and next to it is the 'standby led' pin (pin 9) .. maybe someone more techie than me might be able to suggest something u can do to those pins to bypass standby mode altogether?

 

[Shun]

have you looked into sending some volts into pin 8 of your scart connector. that might stop your tv being in standby mode (long shot perhaps but maybe worth a look)
eg. send 10v-12v to scart pin 8, might get away with 5v (although it might put your tv in widescreen mode if it's only 5v-8v

Yeah I have 12v going to pin 8 and 5v with a resistor going to pin 16 but it still gets stuck in standby.

have a look at page 22 of that service manual I linked, there's a large IC chip thing, I notice pin 10 there is labelled 'power' (and next to it is the 'standby led' pin (pin 9) .. maybe someone more techie than me might be able to suggest something u can do to those pins to bypass standby mode altogether?

Sounds interesting. May have to have a look into this over the weekend. Thanks!

 

[joe34]

cool, good luck with that and if you do come up with a solution please post it here as this dreaded 'tv starts in standby' issue has plagued many of us for ages