How to narrow picture

[apple2dude]

Hi all, this is my first post here. I've got an Admiral KMC13-100 from 1978 that works like a charm. My primary use for it is with a roku, which I've built into it to make it a sort of vintage style smart tv.

The only problem is that the overscan is great enough that it cuts off maybe 1/8 the picture on each side, so for example I can't see the first column of letters when I go to search something. I've got a service manual, but can't find any reference to a way to adjust the horizontal width. Most of the low voltage h-sync circuitry is housed in a chip that I can't find anywhere online.

I thought to try putting a low value, high wattage resistor in line with the horizontal deflection coil, but haven't really tried yet. Any ideas here?

[zeno]

Most B&W sets have no width adj.
If it has a regulated power supply be sure the output voltage
is OK.
Changing anything in the hoz / HV is a big risk to the hoz output.
It could be a failure though. Also all sets are overscanned, some quite a bit.

73 Zeno
LFOD !

[JohnCT]

Originally Posted by apple2dude

The only problem is that the overscan is great enough that it cuts off maybe 1/8 the picture on each side,

In the old days, TVs were always overscanned, particularly the vacuum tube models as it absorbed some aging of the sweep tubes. There weren't an awful lot of graphics on screen when these TVs were new as they were optical in nature. Today, inserting graphics is child's play, and because there is no scanning at all in flat panel displays, they can shove the graphics right to the very edge of the frame.

But on to your problem. Zeno is right in that playing around with the sweep can cause problems, but there is an easy way that can get your sweep down without causing trouble. The good news is that it will reduce your sweep, the potentially bad news is that the HV goes up at the same time and may stress components, but a *little* increase won't hurt.

Locate the retrace capacitor across the horiz output transistor (goes from collector to ground). Remove it and replace it with a *slightly* lower value making sure to keep the same voltage *and* type of capacitor in your sub. You will have to experiment but a small change of value downward will have a fairly large effect on the width and of course, HV.

If your selected capacitor sub is too low, you can leave it in place and add a smaller value disc capacitor across it to "trim" it.

John

[Electronic M]

Some here have used an Exitron IN1502 and a VGA to composite adapter to rescale the video to compensate for the overscan of the TV.

[old_tv_nut]

Originally Posted by JohnCT

But on to your problem. Zeno is right in that playing around with the sweep can cause problems, but there is an easy way that can get your sweep down without causing trouble. The good news is that it will reduce your sweep, the potentially bad news is that the HV goes up at the same time and may stress components, but a *little* increase won't hurt.

Locate the retrace capacitor across the horiz output transistor (goes from collector to ground). Remove it and replace it with a *slightly* lower value making sure to keep the same voltage *and* type of capacitor in your sub. You will have to experiment but a small change of value downward will have a fairly large effect on the width and of course, HV.

If your selected capacitor sub is too low, you can leave it in place and add a smaller value disc capacitor across it to "trim" it.

John

Solid state sets were sometimes designed with longer retrace time than standard to reduce the pulse height on the output transistor. In other words, your problem may be more than just too wide sweep, but may also be too long a retrace. Using a smaller cap reduces the retrace time, but for this to work it has to also compensate for any remaining overscan, so a double whammy. So, take baby steps so you just barely get enough change.

Broadcasters used to rag on TV makers about how much image was wasted by overscan; camera viewfinders often had a safe title graticule attached, and safe title area specs were published by SMPTE. When TV sets went backwards in tube shape (from rectangular tubes for monochrome to round tubes for color), color studios might even use a "double D" safe area graticule.

https://www.nab.org/xert/scitech/pdfs/tv031510.pdf

[apple2dude]

I get the gist of what you all are saying. I'm not super good with knowing exactly what different things do in a circuit though.
I've attached a screenshot of the bit around the horizontal driver. Could one of you enlighten me as to which cap I should play around with?
Capture.PNG

[apple2dude]

My bad, it looks like that was the horizontal output.
Here is the horizontal driver section. Would I be correct in thinking its c810 or c811?

[old_tv_nut]

Originally Posted by apple2dude

My bad, it looks like that was the horizontal output.
Here is the horizontal driver section. Would I be correct in thinking its c810 or c811?

No, you don't want to change the driver circuit.

Post more of the horizontal output including the flyback and yoke, so the correct cap can be pointed to.

[apple2dude]

Its split across two nonconsecutive pages, but I think this is mostly what you're looking for.

[old_tv_nut]

If C100, 4-lead safety cap, is the main tuning cap, I don't know what you can do, as the value is not specified and there won't be a selection of substitute values available anywhere even if you did take it out and measure it.

I'm stumped. Anybody else got an idea?

[zeno]

Somehow I thought it was a B&W. Anyhows check the power
supply, I bet its running too high.

73 Zeno

[apple2dude]

There are two 2.7 ohm voltage dropping resistors in the power supply. Bypassing one of them increases the overscan, so logically adding another would decrease it.

I don't have any of usable value or wattage at the moment, but in looking for one, is it a huge problem if the line voltage drops below a certain point?

[JohnCT]

Originally Posted by apple2dude

There are two 2.7 ohm voltage dropping resistors in the power supply. Bypassing one of them increases the overscan, so logically adding another would decrease it.

I don't have any of usable value or wattage at the moment, but in looking for one, is it a huge problem if the line voltage drops below a certain point?

You need to verify the B+ is both on target *and* regulating with brightness. I'm assuming your regulator is OK because you never mentioned the overscan changing between light and dark scenes.

Adding or subtracting high wattage resistors is not the way to adjust B+. If you can post the power supply we can tell you how to lower B+ if you want to try it that way. If you fudge the pass resistors, you risk the B+ dropping under high demand scenes and the regulator running hotter.

Some of these use a self contained series/pass regulator IC like an STR30135 (guess). The way to lower this is to replace that regulator with an STR30130 which will lower you B+ from 135 to 130V.

If it uses all discrete components instead of the IC regulator, changing values of resistors and zeners in the regulator drive circuit will lower the B+ and still allow it to regulate tightly.

Regarding the sweep, there is a 330pf cap right on the collector of the horiz out to ground. You can remove that and see if your sweep is where you want it and your HV stays within the design specification.

Lowering B+ will reduce scan and lower HV
Reducing the value of the retrace capacitor will reduce the scan and raise the HV, so there is a choice to make.

John

[zeno]

Not a time to re engineer ! I went to my Sams but to my horror the folder is missing the Admiral ! It only has a Panasonic & two copies of a Sylvania B&W.
So post the power supply part of the schematic. Its either on the bottom left or on a separate page as a rule. Check the main B+ it should be in the range of 120 - 130 V as a rule. It should stay constant running at the 100-140 VAC
input. ( see schematic ).

It would be nice to get this set saved. Its one of the last Admiral engineered
TV sets & very well built. IIRC they folded up in 1979

Originally Posted by apple2dude

There are two 2.7 ohm voltage dropping resistors in the power supply. Bypassing one of them increases the overscan, so logically adding another would decrease it.

I don't have any of usable value or wattage at the moment, but in looking for one, is it a huge problem if the line voltage drops below a certain point?

[apple2dude]

I'm having trouble figuring out where to take readings to figure out what the B+ is at right now. In the meantime, here's the power supply. Which of the output voltages are actually B+?