TV for oscilloscope

[Michael A. Terrell]

This TV is recent(maybe 99). It wasn't being used so I tore it apart.

What do you mean struggle? What would be the potential problems in wiring
the coils myself(as they seem to be the issue)?

Do you happen to know about how many turns they use on the coils?

Thanks,
Jon

There is no set number. It varies, to match the inductance required
by the flyback transformer and vertical output stage.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Abstract Dissonance]

Mostly, getting something designed that will drive the yoke windings,
which are quite inductive, to the frequencies you want to play with. You
can't use the existing deflection amps, because they're not designed to do
an arbitrary pattern or Lissajous or anything - they're designed to make a
raster.

At this point in time I'm not interested so much in HF stuff(except looking
at noise, say)... I mainly just want something to view the output of simple
circuits so I can have a visual idea if they are working or not.... such as
from a simple regulated power supply I need to make(instead of guessing that
its working when I measure the output voltage and it happens to be what I
want).

Well, look at the existing yoke. Visualize what its magnetic field must
look like while it's whipping the electron beam around.

Wind some coils, get a current-limited power supply, and poke around
a bit!

yeah, I guess thats what I will do...

Hundreds, maybe thousands - except in the vector displays I've seen,
maybe dozens to a few hundred.

Good Luck!
Rich

Thanks,
Jon

 

[Abstract Dissonance]

There is no set number. It varies, to match the inductance required
by the flyback transformer and vertical output stage.

What does that matter? do you mean that since they drive the coils with
there transformer they have to worry about the inductance of the coil?

I don't plan on using the TV's power supply(atleast at this point) to drive
the coils so I don't see how I have to worry about that...

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Jon

 

[Abstract Dissonance]

Video in == baseband. The blanking goes in there. With TV video, it's in
the signal itself - it just goes black for the retrace.

I don't know if these are DC coupled.

It'd be interesting to see what happens when you turn your channel
selector to "AUX" or whatever - that's what my set calls the yellow
video in, but when I turn mine there, if there isn't a video signal, it
blanks the whole screen. So you'd need a sync generator of some kind,
but ou _should_ still be able to use it for intensity modulation whenever
you get your deflection drivers going.


I don't know what this means. Can you rephrase it?

It means that I just want to visualize whats going on instead of guessing.
I don't care if I'm off a 1V out of 10 or if the plot is distorted due to
the nonlinear deflection of the tube or whatever... Just seeing a simple
plot is good enough for me.. for now atleast. I'll probably try to get me a
real oscilloscope soon but until then I'd like to play around with the TV
one for fun and to see what I can learn from it.

Thanks!

??

Rich

Thanks
Jon

 

[Abstract Dissonance]

I've not done it, but have a few things to add.

PC scopes are free now and do 20Hz-20kHz, and some sound cards will go
above 20kHz. So theres no point building a scope for that frequency
range.

Sure there is... its a good learning experience.

Even the most basic CRT scope can do a few MHz if you use the beam
modulation to indicate amplitude instead of the scan coils. Let the
tube scan as normal, couple your signal to the cathode or grid.
Coupling some signal to the scan oscs might be able to sync them,
havent tried that.

An as-is TV gives you 2 timebases to play with: 50Hz and 16kHz. By
putting signal into the other unused scan coil, you can thus observe
frequencies of upto around 10x those, ie 500Hz on one axis, 160kHz on
the other. Thats if you can drive the frame scan coils at 160kHz, which
I'm not so convinced about.

You've got inductive current driven scanning, so feeding V into the
scan coils will give plenty of distortion. As someone mentioned,
current drive them.

A computer monitor with faster scan frequencies would give you higher
frequency limits than a tv plus original scan yoke. A colour tube also
gives you 3 colour channels to play with. You might want to plug the
monitor into a computer to get it to play ball without a bundle of
mods.

I had a computer monitor but now its gone ;/ really sucks... but the TV will
do for now until I can find a real oscilloscope.

In principle I suppose one could run 2 of the colour channels as a pc
monitor, and use the 3rd to display waveform by brightness modulation.

To take frequency limit even higher, use envelope detection. This is
just a diode, cap, resistor on the input. Thus the tube displays the
envelope of the waveform, not the wave itself. This can be used to
extend the working range of a scope greatly, though of course with less
information displayed.
While this sort of kit is pretty much obsolete here, it is still of
value in countries where buying a nice fast scope is out of the
question.


NT

Thanks,
Jon

 

[Abstract Dissonance]

Its a total waste of time unless you can design and build make own
deflection yoke. The existing yoke is the limiting factor, and has been
since the first modification plans for old tube type B&W TVs were
published 45 years ago. Sure, you can rotate the yoke 90 degrees to
improve the vertical response and remove the resonant capacitors to gain
a slight bit more, but you'll need a second original yoke to keep the
flames inside the flyback transformer. Bottom line, you need
electrostatic deflection, not electromagnetic.

In the electrostatic deflection do they just use plates as most examples on
oscilloscopes shows? Could I replace the coils with plates or would they be
to far apart?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Thanks,
Jon

 

[Michael A. Terrell]

What does that matter? do you mean that since they drive the coils with
there transformer they have to worry about the inductance of the coil?

I don't plan on using the TV's power supply(atleast at this point) to drive
the coils so I don't see how I have to worry about that...


Jon

You don't see any of the problems, do you. I've worked on TVs and
video monitors for 40 years.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

In the electrostatic deflection do they just use plates as most examples on
oscilloscopes shows? Could I replace the coils with plates or would they be
to far apart?


Thanks,
Jon

How are you going to open the CRT's glass to add the plates, then
pump it back down, first with a vacuum pump, followed by a diffusion
pump. Don't forget to install new getter material to flash while you're
inside the CRT.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Winfield Hill]

Abstract Dissonance wrote...

Michael A. Terrell wrote ...

And hence the "tuned" nature of the deflection.

Exactly. As spelled out in my freshman electronics book in 1961.

In the electrostatic deflection do they just use plates as most examples
on oscilloscopes shows? Could I replace the coils with plates or would
they be to far apart?

Yes, they'd not only be too far apart (i.e. high drive voltages required)
but they'd have too much capacitance (high capacitive currents required).
You really need those electrodes inside the vacuum display tube.

 

[[email protected]]

How are you going to open the CRT's glass to add the plates, then
pump it back down, first with a vacuum pump, followed by a diffusion
pump. Don't forget to install new getter material to flash while you're
inside the CRT.

Do they have to go inside?

Obviously the further apart, the higher the voltage required, and the
dielectric constant of glass relative to air will change the field a
little bit, so it may not be practical to put them outside. Also there
might be a conductive coating on the inside or the outside?

Are there by any chance any moderately low voltage plates already in
there for geometric correction or anything?

Of course while messing around inside the TV is fun, it's also
hazardous. Here's an idea for something to do outside the box.
Imagine a stable waveform being displayed on a triggered scope, with
the horizontal sweep happening at about the same frequency as the
horizontal scan rate of a TV. Now imagine the rastering TV beam
sampling that frequency.

Seems that for this one horizontal frequency, you could build a box
that had a ramp generator slaved to the vertical deflection, and a fast
window comparator. When the window comparator says the ramp voltage
corresponding to the current scan line equals the signal voltage at the
currant scan X, you turn on the beam. In actuality you will be
displaying a picture made up of one slice each of (#scanlines) periods
of the waveform, but if both the waveform and your trigger circuit are
stable, that's not a probem.

Obviously this is limited in the frequencies it can cover, but at least
the vertical bandwidth is no longer limited by the coil inductance -
instead it's limited only by what you can see on a screen with a
comparatively low, fixed horizontal scan rate. Guess a multisync
monitor would offer more options than a TV.

 

[Michael A. Terrell]

Do they have to go inside?

Obviously the further apart, the higher the voltage required, and the
dielectric constant of glass relative to air will change the field a
little bit, so it may not be practical to put them outside. Also there
might be a conductive coating on the inside or the outside?

Typical deflection voltage is 300 VDC INSIDE the CRT.

Are there by any chance any moderately low voltage plates already in
there for geometric correction or anything?

Why would there be?

Of course while messing around inside the TV is fun, it's also
hazardous. Here's an idea for something to do outside the box.
Imagine a stable waveform being displayed on a triggered scope, with
the horizontal sweep happening at about the same frequency as the
horizontal scan rate of a TV. Now imagine the rastering TV beam
sampling that frequency.

Seems that for this one horizontal frequency, you could build a box
that had a ramp generator slaved to the vertical deflection, and a fast
window comparator. When the window comparator says the ramp voltage
corresponding to the current scan line equals the signal voltage at the
currant scan X, you turn on the beam. In actuality you will be
displaying a picture made up of one slice each of (#scanlines) periods
of the waveform, but if both the waveform and your trigger circuit are
stable, that's not a probem.

Obviously this is limited in the frequencies it can cover, but at least
the vertical bandwidth is no longer limited by the coil inductance -
instead it's limited only by what you can see on a screen with a
comparatively low, fixed horizontal scan rate. Guess a multisync
monitor would offer more options than a TV.

Obviously its a total waste of time. Find a surplus or used O-scope
CRT to play with, instead. I've bought old tube type 100 KHz scopes for
$2 to $30 at thrift stores and hamfests. Those worked. I've hauled home
lots of dead scopes since the late '60s and fixed most of them with a
handful of new resistors and caps, and sometimes a couple new tubes.
You could fix most of them with a good VTVM and a schematic.

http://bama.edebris.com/manuals/heath/ has a number of old heathkit
scope schematics, and the site has some other brands as well. The IO
series was scopes, and there may be a couple other series that were
scopes.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[[email protected]]

A horizontal tv timebase is already running at well above audio.
Although the fundamental is af, the harmonics needed to produce the
waveform are multiples of this, so much higher frequency viewing should
be quite workable.

A monitor that does high resolution and high frame rate will give all
the more bandwidth, and colour.


NT

 

[[email protected]]

Blanking? Only jacks the TV has is a coax and the video in/Audio L and R
in(whatever its called... the RCA Red, yellow and white type of connects).

Blanking means switching the beam off to black while the scan coils
whip it back across the screen, which takes longer than zero time. Dont
worry about it, scopes work without blanking.

Do you happen to know the specifics of driving the coils properly? like
about how much voltage and current are needed in general(amps, mA's, etc..).

read your post again!

Could I just make a voltage controlled variable current source and use my
signals to drive it?

Thanks,
Jon

Simplest way to get it going is to take your stereo amp outputs,
connect one channel to one scan coil, other to other. Voila.

When doing this, ensure that either:

1. your tv set is an isolated from the mains type. Many arent, and real
bad things would happen if you connected a non-isolated one to the
stereo. Eg your stereo system and you becoming live / electrocuted.

2. if the tv isnt fully isolated, that there is no connection to scan
coils, or correction coils, other than the one to the stereo. If you
make a mistake on that, big problem, so dont do it until you can be
sure you know exactly whats what in there.

I assume you know that a tv provides more death opportunities than any
other household equipment, and that youre aware of them. If not, ask
first, cos it can be a bit hard to ask after.


NT

 

[[email protected]]

This brought to mind the time that I looked at ordinary NTSC color video
on an actual scope. It was almost spooky. I could turn the sweep way up,
and see the color burst itself, and then crank the sweep down and see
the scan lines - averaged over 1/60 second, of course, and since this
was off-air (I was scoping my TV), the image was continually changing.
The pattern on the scope screen looked almost like a "top view" of the
picture, like a graph of light/dark, averaged. Then I cranked the sweep
down even more, sunc on the vertical, and got a "side view".

It was way cool! %-}

Cheers!
Rich

Sounds like the elephant phenomenon, where you can look at the same
thing from different perspectives and see something quite different.


NT

 

[Spehro Pefhany]

Sounds like the elephant phenomenon, where you can look at the same
thing from different perspectives and see something quite different.


NT

"An elephant is warm and squishy".


Best regards,
Spehro Pefhany

 

[[email protected]]

In the electrostatic deflection do they just use plates as most examples on
oscilloscopes shows? Could I replace the coils with plates or would they be
to far apart?

Metal plates inside the tube, and huge deflection voltages. No, much
too far away outside the tube, plus youd need excessive deflection
voltages. Get it working now, look at weaks later.


But first, a few popular ways to die

- non-isolated supplies usually mean everything in the tv is live,
including chassis, all electronics, speaker, etc.

- supply reservoir capacitor may store 160/330v for a long time after
switch off.

- tube acts as capacitor to the 19-24 kV EHT. The thick red wire with
black rubber cap is connected to this. Try not to get too close. Use
care when shorting it out, and realise it will recharge itself after
shorting, due to weird capacitance effcets, so it needs shorting 3
times.

- focus voltage may on some sets be a few thousand volts.


If the tv has as assortment of external connectors, it will be isolated
from the mains. If it doesnt have these, its more likely to be
non-isolated.

The big supply reservoir cap can be shorted out. There will often be
more than one. If you want to keep the set working, short it using a
low resistance rather than plain metal, to avoid damaging the
capacitor.

Tube EHT stored can be shorted, but use a well insulated tool to do it,
and ensure your shorting wire from screwdriver shaft to chassis and
tube outer is securely attached, not just wapped round etc. Reason is
if this comes loose you'll get bit. It is 24,000 volts, but the
capacitance is small. People normally survive a bite from this, but not
always. B&W sets are a lot friendlier in this area.

Focus can be shorted similarly. Focus connects to the tube base, its
the well insulated one.


One last question for someone: how will the line output stage behave if
its load is removed? You need that going to produce the EHT.


NT

 

[[email protected]]

Typical deflection voltage is 300 VDC INSIDE the CRT.



Why would there be?



Obviously its a total waste of time. Find a surplus or used O-scope
CRT to play with, instead. I've bought old tube type 100 KHz scopes for
$2 to $30 at thrift stores and hamfests. Those worked. I've hauled home
lots of dead scopes since the late '60s and fixed most of them with a
handful of new resistors and caps, and sometimes a couple new tubes.
You could fix most of them with a good VTVM and a schematic.

I dont think the OP could. This is a fine opportunity to make something
fun and learn lots. And end up with a basic scope.


NT

 

[Don Lancaster]

Anyone have any experience converting a TV into an oscilloscope?

I have found the deflection coils and have tested them but I'm not really
sure where to go from there. I know that for the horizontal I need a
trigger(a sawtooth) but I'm not sure at what currents and voltages I
need(with my variac hooked directly to it about 3 volts gives almost full
deflection(which I measured about 0.05A).. but I'm not sure if thats right.)


Is it simply a matter of putting an variable gain opamp on the vertical and
a variable sawtooth on the horizontal? Seems like it would be kinda hard to
measure frequency that way since I'd have to convert the resistance of the
sawtooth into frequency.

Any ideas?

Thanks,
AD

The concept of tv as oscilloscope is ludicrously absurd.

It was a standard classified ad scam of the 1950's.

TV sets use a very special and highly efficient fixed frequency
recurrent phase locked horizontal scan. They cannot deviate far from
15,735 Hertz. Nor can they be randomly triggered.

Use an A/D ahead of a personal computer instead.
Or buy any of hundreds of stock pc oscilloscopeadd-ons.

Or pick up scopes surplus.
A Tek 7904 should cost you $12 tops.
A Tek 5440, they should pay YOU $12 to haul it away.

More details at http://www.tinaja.com/auct01.asp


--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Michael A. Terrell]

I dont think the OP could. This is a fine opportunity to make something
fun and learn lots. And end up with a basic scope.

NT

You could spend the time working with a real scope tube and end up
with two useful things. A low end scope and your life. TV CRTs without
the proper drive to the deflection coils have been known to develop a
hotspot on the neck of the CRT and implode.

Zenith had to recall a whole year's worth of their early solid state
TVS because of this. If the loading on the horizontal output stage
changed the HV would rise and cause the tube to implode. I lost count of
how many TV sets i personally modified with the new parts and applied
the federally mandated safety sticker to the back of the cabinets.

Also, the glass at the neck of a TV or monitor CRT is thin and easily
damaged. Have you ever been around a CRT that imploded? They have
killed a number of people who happened to be in front of the tube.

It is dangerous to play with old CRTs when you don't know what you
are doing.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Spehro Pefhany]

Anyone have any experience converting a TV into an oscilloscope?

I have found the deflection coils and have tested them but I'm not really
sure where to go from there. I know that for the horizontal I need a
trigger(a sawtooth) but I'm not sure at what currents and voltages I
need(with my variac hooked directly to it about 3 volts gives almost full
deflection(which I measured about 0.05A).. but I'm not sure if thats right.)


Is it simply a matter of putting an variable gain opamp on the vertical and
a variable sawtooth on the horizontal? Seems like it would be kinda hard to
measure frequency that way since I'd have to convert the resistance of the
sawtooth into frequency.

Any ideas?

Thanks,
AD

1 Old Commodore 64 or Atari 800
1 FPGA (lightly salted with VHDL or Verilog)
1 programmable clock generator with crystal
1 or 2 fast SRAM (optional, depending on FPGA)
1 fast ADC

Mix together on PCB or well-constructed breadboard, fold in the
bitstream to the FPGA through a JTAG port, let stand while you prepare
suitable firmware for the Commodore 64 or Atari. Burn firmware (or at
least sear well on both sides) into game EPROM, insert RF modulator
output into TV input.

Enjoy.



Best regards,
Spehro Pefhany