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How to eliminate horizonal overscan when no service adjustment?

P

Peabody

Jan 1, 1970
0
It's a JVC AV-27920 television, circa 1995 or so.

I understand that manufacturers deliberately build in overscan so
that any shrinkage resulting from CRT aging won't produce visible
black borders. But I would like see all the picture.

The service menu has adjustments for vertical size and position, so
I assume I can use those to get rid of all, or at least most, of the
vertical overscan.

But there is no width or horizontal size adjustment, only horizontal
position.

So I would like to think about making changes to the circuit that
would give me this adjustment.

In theory, I guess I don't want to change the horizontal frequency,
just the relationship between the yoke and the guns on each line.
So maybe instead of an actual width adjustment, it would work well
enough to change where the scan begins, or ends, and then use the
position adjustment to center it again.

And I wonder if I could do this by adding additional inductance in
series with the horizontal deflection yoke, with the idea that more
inductance would slow down the rate of change in the yoke, so that
the beams don't progress horizontally as fast as before, and
therefore finish the line before reaching the edge of the screen.

Is this all crazy? It doesn't seem like it should be. I mean,
width/size adjustments are not unknown in the modern world, so it
should be possible to insert one. I just don't know enough about
televisions to know how to do it.

I would appreciate any suggestions anyone might have.

Oh, and I do have the schematic. Sams 4080.
 
J

James Sweet

Jan 1, 1970
0
And I wonder if I could do this by adding additional inductance in
series with the horizontal deflection yoke, with the idea that more
inductance would slow down the rate of change in the yoke, so that
the beams don't progress horizontally as fast as before, and
therefore finish the line before reaching the edge of the screen.



Usually that's precisely how it's done. Look for an inductor in the
horizontal section with an adjustable slug, it may have enough range to do
what you want without even modifying anything.
 
P

Peabody

Jan 1, 1970
0
Usually that's precisely how it's done. Look for an
inductor in the horizontal section with an adjustable
slug, it may have enough range to do what you want
without even modifying anything.

I don't see anything adjustable. There's a big IC that
produces an "HD" output, which goes to the base of the
Horizontal Drive transistor (NPN), the collector of which
connects to the Horizontal Output transistor by way of a
transformer. But there's nothing on the schematic
suggesting anything is adjustable.

Any idea what kind of coil I would need to add? How many
turns? What gauge wire?
 
J

James Sweet

Jan 1, 1970
0
Peabody said:
I don't see anything adjustable. There's a big IC that
produces an "HD" output, which goes to the base of the
Horizontal Drive transistor (NPN), the collector of which
connects to the Horizontal Output transistor by way of a
transformer. But there's nothing on the schematic
suggesting anything is adjustable.

Any idea what kind of coil I would need to add? How many
turns? What gauge wire?

It's usually a coil about 1/2" diameter and an inch tall wound with what
looks to be around #14 Litz wire in series with the yoke. If you can find an
older junk TV you could probably salvage something.
 
I don't see anything adjustable. There's a big IC that
produces an "HD" output, which goes to the base of the
Horizontal Drive transistor (NPN), the collector of which
connects to the Horizontal Output transistor by way of a
transformer. But there's nothing on the schematic
suggesting anything is adjustable.

Any idea what kind of coil I would need to add? How many
turns? What gauge wire?

I cant imagine any mfr producing everything in the chain to tight
enough tolerance to not need a width control.

The IC that produces the horiz scan waveform, are you really sure
theres nothing to control it?


NT

fwiw I cant think of any way crt ageing woud affect pic width.
 
A

Arfa Daily

Jan 1, 1970
0
James Sweet said:
It's usually a coil about 1/2" diameter and an inch tall wound with what
looks to be around #14 Litz wire in series with the yoke. If you can find
an older junk TV you could probably salvage something.

Is there not a set HT pot in the power supply ? Usually, turning this down
by a few volts will shrink the picture all round. You can then just correct
the vertical via the menu. I would not personally try adding inductance into
the H-OP stage or scan coil path. This is a very stressed and very carefully
designed bit of circuitry, and some extra L that it doesn't want, could lead
to all sorts of unwanted effects such as ringing, poor linearity, or even
overheating and ultimate failure of the H-OP transistor. If you were to
start adding L, then you would probably also have to alter the value of the
S correction cap in series with the scan coils, as this forms a tuned
circuit with the scan coils to deliberately make them ring in order to
achieve the velocity modulation of the beam required to get a linear sweep
on a flat(ish) tube face. I'm actually surprised that you feel that you have
that degree of overscan that it's affecting your viewing. If you really are
missing actual picture, then it must be significantly more than the 'normal'
7 - 10% raster overscan. Remember that the broadcast picture is not as wide
as the scanned raster, and a lot of the relationships that we used to use to
judge picture width, have now gone out of the window, with all the different
'widescreen' formats that they keep broadcasting now, and which many older
TVs just don't look good on. Have you actually looked at the picture on a
proper 4:3 test pattern ? It's the only way really to make a valid judgement
on the picture geometry. You may of course have an actual fault. It's not at
all uncommon for the values of the components in the set HT circuit to
drift, resulting in a slowly 'growing' scan.

Arfa
 
P

Peabody

Jan 1, 1970
0
[email protected] says...
I cant imagine any mfr producing everything in the chain
to tight enough tolerance to not need a width control.
The IC that produces the horiz scan waveform, are you
really sure theres nothing to control it?

I'm not really sure of anything. :)

The IC that provides the signal to the Horizontal Drive
transistor is a TA1242N, 56 pins. It's pin 32 of that chip,
labeled HD. The waveform shown on the schematic shows it
going high (3.5V) for about 40% of the time, then back low
for the remaining 60%. I don't know if that's what you mean
by "horizontal scan waveform." It really looks pretty much
like a square wave. By the time you get to the Horizontal
Output transitor, the waveform looks completely different.

The only datasheet I've found for the TA1242 is in Japanese,
but there is a logical diagram of the chip in the schematic.
There are inputs that it appears my have some effect on that
pin 32 output, including 32fH, HP, XRAY, and a number of
others that aren't labeled.

I just don't have enough information to answer your
question. There doesn't appear to be anything adjustable in
this part of the circuit, although there may well be parts
whose value could be changed to affect the waveform. I just
don't know.
 
A

Arfa Daily

Jan 1, 1970
0
Peabody said:
[email protected] says...



I'm not really sure of anything. :)

The IC that provides the signal to the Horizontal Drive
transistor is a TA1242N, 56 pins. It's pin 32 of that chip,
labeled HD. The waveform shown on the schematic shows it
going high (3.5V) for about 40% of the time, then back low
for the remaining 60%. I don't know if that's what you mean
by "horizontal scan waveform." It really looks pretty much
like a square wave. By the time you get to the Horizontal
Output transitor, the waveform looks completely different.

The only datasheet I've found for the TA1242 is in Japanese,
but there is a logical diagram of the chip in the schematic.
There are inputs that it appears my have some effect on that
pin 32 output, including 32fH, HP, XRAY, and a number of
others that aren't labeled.

I just don't have enough information to answer your
question. There doesn't appear to be anything adjustable in
this part of the circuit, although there may well be parts
whose value could be changed to affect the waveform. I just
don't know.
Square wave drive is correct. It's integrated into a sawtooth scan waveform,
by the inductance of the scan coils.

Arfa
 
P

Peabody

Jan 1, 1970
0
Arfa Daily says...
Is there not a set HT pot in the power supply ? Usually,
turning this down by a few volts will shrink the picture
all round. You can then just correct the vertical via
the menu.

I have a fair amount of experience with digital circuits,
but not analog, and particularly not television. So I know
you'll understand when I ask:

Is HT the flyback transformer? If so, are you talking about
the source that supplies the primary of that transformer as
well as the deflection coil? It's 134V on the schematic.

If so, that is controlled by a STR30134 regulator. There
are no pots there, but there is a pair of fixed resistors
as a divider that appear to control the output voltage, and
I'm sure I could insert an adjustment there if that would do
it.
I'm actually surprised that you feel that you have that
degree of overscan that it's affecting your viewing. If
you really are missing actual picture, then it must be
significantly more than the 'normal' 7 - 10% raster
overscan.

It has been this way since it was new, so it's not an aging
problem. I've always had the sense that I was missing part
of the picture, and when I got a DVD player that also lets
me zoom OUT, the difference became apparent. It may
not be more than 10%, and perhaps others would not object to
it, but I would just like to see what's actually in the
picture, rather than a cropped version.
Remember that the broadcast picture is not as wide as
the scanned raster, and a lot of the relationships that
we used to use to judge picture width, have now gone out
of the window, with all the different 'widescreen'
formats that they keep broadcasting now, and which many
older TVs just don't look good on. Have you actually
looked at the picture on a proper 4:3 test pattern ?
It's the only way really to make a valid judgement on
the picture geometry.

No, I don't have any service equipment beyond a multimeter.
The problem shows up more with the DVD player than on
broadcast, particularly when playing things like mpeg1
videos that I've downloaded. Probably because in that
situation, the full raster is used.
You may of course have an actual fault. It's not at all
uncommon for the values of the components in the set HT
circuit to drift, resulting in a slowly 'growing' scan.

I don't think so, and I don't have the sense that it's
getting worse. I think it's just the design of this set.
But if it's possible to fix it, I would like to.

Thanks very much for your help.
 
A

Arfa Daily

Jan 1, 1970
0
Peabody said:
Arfa Daily says...


I have a fair amount of experience with digital circuits,
but not analog, and particularly not television. So I know
you'll understand when I ask:

Is HT the flyback transformer? If so, are you talking about
the source that supplies the primary of that transformer as
well as the deflection coil? It's 134V on the schematic.

If so, that is controlled by a STR30134 regulator. There
are no pots there, but there is a pair of fixed resistors
as a divider that appear to control the output voltage, and
I'm sure I could insert an adjustment there if that would do
it.


It has been this way since it was new, so it's not an aging
problem. I've always had the sense that I was missing part
of the picture, and when I got a DVD player that also lets
me zoom OUT, the difference became apparent. It may
not be more than 10%, and perhaps others would not object to
it, but I would just like to see what's actually in the
picture, rather than a cropped version.


No, I don't have any service equipment beyond a multimeter.
The problem shows up more with the DVD player than on
broadcast, particularly when playing things like mpeg1
videos that I've downloaded. Probably because in that
situation, the full raster is used.


I don't think so, and I don't have the sense that it's
getting worse. I think it's just the design of this set.
But if it's possible to fix it, I would like to.

Thanks very much for your help.

"Set HT" refers to the 134v. Some manufacturers call this the 'HT' and the
output of the flyback transformer 'EHT' or 'HV'. Others call the output of
the flyback simply 'HT' so there's no 'definitive' answer to that question,
but it all boils down to the same thing in the end, and this variation in
terms is just accepted. Altering the supply voltage to the H-OP stage - ie
making the 134v something different - will affect the overall raster size,
as well as affecting the high voltage output from the flyback Tx. Tv sets
normally have an overvolts protection circuit, to stop the flyback voltage
becoming excessive, as this could result in the CRT emitting x-rays beyond
the maximum allowed limit. I think that before getting complicated with
adding L and so on in the deflection current path, I would first try making
the 134v slightly adjustable, and just seeing whether that does it for you.

Arfa
 
P

Peabody

Jan 1, 1970
0
Arfa Daily says...
"Set HT" refers to the 134v. Some manufacturers call
this the 'HT' and the output of the flyback transformer
'EHT' or 'HV'. Others call the output of the flyback
simply 'HT' so there's no 'definitive' answer to that
question, but it all boils down to the same thing in the
end, and this variation in terms is just accepted.
Altering the supply voltage to the H-OP stage - ie
making the 134v something different - will affect the
overall raster size, as well as affecting the high
voltage output from the flyback Tx.

And there are other supply voltages provided by the
secondary of the flyback. In my case a 197V and a 25V, plus
some more that are regulated. So I assume I'm not going to
be able to change it much before things really start to go
wacky. But then, just a little may be enough.

Well, I can certainly make the 134V adjustable. I'll do
that and see what happens.

Thanks again for your help.
 
P

Peabody

Jan 1, 1970
0
Peabody says...
Well, I can certainly make the 134V adjustable. I'll do
that and see what happens.

Best laid plans.....

I found the data sheet for the regulator, and it turns out
it isn't adjustable. Hence the name STR30134 for 134V. The
voltage divider I found that feeds pin 2 just provides a
source to drive the darlington base, but it is further
clipped internally. I would have to reduce the voltage
there below the clipped value to affect the output voltage,
but then I would probably lose all regulation.

There is already a fixed 1500 ohm, 3W, resistor between the
134V supply and the deflection coil. What about making that
a higher value?

Or, inserting one or more forward bias diodes in series at
that point to drop the voltage about .6V each. Maybe even a
1N400x would work ok at this frequency.
 
J

James Sweet

Jan 1, 1970
0
Peabody said:
Peabody says...


Best laid plans.....

I found the data sheet for the regulator, and it turns out
it isn't adjustable. Hence the name STR30134 for 134V. The
voltage divider I found that feeds pin 2 just provides a
source to drive the darlington base, but it is further
clipped internally. I would have to reduce the voltage
there below the clipped value to affect the output voltage,
but then I would probably lose all regulation.

There is already a fixed 1500 ohm, 3W, resistor between the
134V supply and the deflection coil. What about making that
a higher value?

Or, inserting one or more forward bias diodes in series at
that point to drop the voltage about .6V each. Maybe even a
1N400x would work ok at this frequency.


I still think adding inductance to the horizontal yoke circuit is the
easiest way to go. I first saw this method described for converting a B&W TV
to a computer monitor in a book from the late 1970s, monitors designed for
use in arcade games use the same method. If you go changing the voltage
around it will affect other things.
 
F

Franc Zabkar

Jan 1, 1970
0
It's a JVC AV-27920 television, circa 1995 or so.

I understand that manufacturers deliberately build in overscan so
that any shrinkage resulting from CRT aging won't produce visible
black borders. But I would like see all the picture.

The service menu has adjustments for vertical size and position, so
I assume I can use those to get rid of all, or at least most, of the
vertical overscan.

But there is no width or horizontal size adjustment, only horizontal
position.

So I would like to think about making changes to the circuit that
would give me this adjustment.

In theory, I guess I don't want to change the horizontal frequency,
just the relationship between the yoke and the guns on each line.
So maybe instead of an actual width adjustment, it would work well
enough to change where the scan begins, or ends, and then use the
position adjustment to center it again.

And I wonder if I could do this by adding additional inductance in
series with the horizontal deflection yoke, with the idea that more
inductance would slow down the rate of change in the yoke, so that
the beams don't progress horizontally as fast as before, and
therefore finish the line before reaching the edge of the screen.

Is this all crazy? It doesn't seem like it should be. I mean,
width/size adjustments are not unknown in the modern world, so it
should be possible to insert one. I just don't know enough about
televisions to know how to do it.

I would appreciate any suggestions anyone might have.

Oh, and I do have the schematic. Sams 4080.

Assuming your TV is faulty, I would check the tuning capacitor
attached to the collector of the horizontal output transistor.

- Franc Zabkar
 
P

Peabody

Jan 1, 1970
0
James Sweet says...
I still think adding inductance to the horizontal yoke
circuit is the easiest way to go. I first saw this
method described for converting a B&W TV to a computer
monitor in a book from the late 1970s, monitors designed
for use in arcade games use the same method. If you go
changing the voltage around it will affect other things.

Below is the circuit for the horizontal deflection. (You
would have to view it using a non-proportional font.)

When Q522 turns on, it grounds everything - the yoke circuit
and the flyback primary in parallel, both fed by the 134V
supply.

Does it matter where I insert the coil? I had thought maybe
immediately before or after R526, but I'm not sure if it
matters which one.

I assume the value of L521 should not be messed with.

Placing it immediately before or after the yoke would
require cutting one of the wires going to the yoke, which I
would rather not do.
 
Arfa Daily says...

And there are other supply voltages provided by the
secondary of the flyback. In my case a 197V and a 25V, plus
some more that are regulated. So I assume I'm not going to
be able to change it much before things really start to go
wacky. But then, just a little may be enough.

Well, I can certainly make the 134V adjustable. I'll do
that and see what happens.

Thanks again for your help.

Thats not going to work. HT voltage affects both scan voltage/current
and EHT voltage in equal measure, so it has no effect on picture size.
This is why old TVs with no HT regulation stayed at more or less the
same picture size as the voltage wandered all over the shop. The only
thing that altered pic size on those was loading of the EHT, which
depended on total picture brightness.

If you've played with tv psus you'll be aware of how much one can 'get
away with' when it comes to HT lines.

Boosting HT was formerly used as a last ditch method to get more
emission from dying tubes. It works, but is not recommendable.

If you want to adjust HT V to tweak scan, you'd need to just adjust
the voltage to the horiz scan output stage. That could be done with a
string of diodes, each of which will drop around a volt IRL. Due to
lack of cct details, wise to add an ultrafast diode pointing the other
way across your diode string, and maybe an RC too.


NT
 
A

Arfa Daily

Jan 1, 1970
0
Thats not going to work. HT voltage affects both scan voltage/current
and EHT voltage in equal measure, so it has no effect on picture size.
This is why old TVs with no HT regulation stayed at more or less the
same picture size as the voltage wandered all over the shop. The only
thing that altered pic size on those was loading of the EHT, which
depended on total picture brightness.

If you've played with tv psus you'll be aware of how much one can 'get
away with' when it comes to HT lines.

Boosting HT was formerly used as a last ditch method to get more
emission from dying tubes. It works, but is not recommendable.

If you want to adjust HT V to tweak scan, you'd need to just adjust
the voltage to the horiz scan output stage. That could be done with a
string of diodes, each of which will drop around a volt IRL. Due to
lack of cct details, wise to add an ultrafast diode pointing the other
way across your diode string, and maybe an RC too.


NT

What you say, is of course, fundamentally true, but I think that the key
here is your "more or less" statement. I've never known the relationship
between scan size and HT setting to be so tight as to maintain the picture
size constant. With most sets that have adjustable HT, altering the setting
will have some effect on the picture size, and it was just something that
the OP could have tried as perhaps a 'simple' fix for what appears to be a
fairly minor amount of overscan that he's trying to correct. As it happens,
the HT is not adjustable on his set, so it's a moot point anyway.

I think that a couple of diodes in the HT feed to the OP stage, as you
suggest, might be a simple way of giving it a try.

Arfa
 
F

Franc Zabkar

Jan 1, 1970
0
James Sweet says...


Below is the circuit for the horizontal deflection. (You
would have to view it using a non-proportional font.)

When Q522 turns on, it grounds everything - the yoke circuit
and the flyback primary in parallel, both fed by the 134V
supply.

Does it matter where I insert the coil? I had thought maybe
immediately before or after R526, but I'm not sure if it
matters which one.

I assume the value of L521 should not be messed with.

Placing it immediately before or after the yoke would
require cutting one of the wires going to the yoke, which I
would rather not do.

That doesn't look right. The LH end of the yoke normally goes to
ground via another capacitor (not the tuning cap, C524).

- Franc Zabkar
 
P

Peabody

Jan 1, 1970
0
Franc Zabkar says...
That doesn't look right. The LH end of the yoke normally
goes to ground via another capacitor (not the tuning
cap, C524).

I left out one cap (C526) back near the 134V source. So it
should look like this:

|---------------FLYBACK---------|
| |
| |
| |
134V--|--R526-|----L521------|--YOKE--|---Q522---|--GND
1.5K | | | H.OUT |
| | | |
| | | |
|--R530--C527--| |---C524---|
|
|
C526
|
|
GND


But somehow I don't think that's what you had in mind. The
"bottoms" of both the flyback primary and the horizontal
deflection coil are tied together as shown. They go to
ground through the low-impedance path to ground when Q522
turns on, and through C524. at other times.
 
P

Peabody

Jan 1, 1970
0
Arfa Daily says...
What you say, is of course, fundamentally true, but I
think that the key here is your "more or less"
statement. I've never known the relationship between
scan size and HT setting to be so tight as to maintain
the picture size constant. With most sets that have
adjustable HT, altering the setting will have some
effect on the picture size, and it was just something
that the OP could have tried as perhaps a 'simple' fix
for what appears to be a fairly minor amount of overscan
that he's trying to correct. As it happens, the HT is
not adjustable on his set, so it's a moot point anyway.
I think that a couple of diodes in the HT feed to the OP
stage, as you suggest, might be a simple way of giving
it a try.

And where would you put these diodes so that nothing "tuned"
would be screwed up? With reference to the diagram:



|----------------FLYBACK---------|
| |
| |
| |
134V--|---R526-|----L521------|--YOKE--|---Q522---|--GND
1.5K | | | H.OUT |
| | | |
| | | |
|--R530--C527--| |---C524---|
|
|
C526
|
|
GND

It would seem that putting one or more diodes immediately to
the left, as shown, of R526 would drop the voltage only
through the deflection coil circuit, not the flyback.

Then the question would be what kind of diodes to use for
the voltage drop, and what to use as a reverse protection
diode, if any. Where I live now, I'm pretty much limited to
Radio Shack, my junque box, or mail order. RS has signal
diodes like the 1N914, Zeners, rectifier diodes like the
1N400x (1 amp) and 1N540x (3 amp),and a 1KV, 2.5 amp PTC205,
whatever that is. I assume only the signal or zener diodes
would be at all fast.

It's not clear to me that there will be any spiking at that
point that needs to be protected against. By the way, R526
is a 3W resistor. I don't know enough about TV's to guess
how much current goes through the horizontal deflection
coil, but all the parts in this part of the circuit have
pretty hefty ratings.
 
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