How To Measure CRT Voltages? DC or AC?

R

Rock Braud

Guest
I am chasing down a problem with my Emerson 27" model MS2700RDA.

The picture has about 20 "retrace" lines across it, the image is
barely visible, and red dominates the color.

I have done some research and am trying to determine if there is a CRT
short and if so where it is.

When measuring voltages on the CRT pins should I have my DMM set on DC
or AC.
Also, I am placing the negative lead on the CRT Header Element ground
pin; is this correct?????

When I measure the cathodes, Red, Green, and Blue, wrt heater filament
ground, I get the proper voltages; 155-165 VDCon all three and they
are usually within 1 volt of each other.

However, when I measure VDC "across" the heater filament I get 0 VDC.
But, when I measure it for AC voltage, I get around 2 or 3 VAC. What
is the correct way to measure heater filament voltage???

Finally, when I measure Pin 5, a.k.a. screen grid, I get 0 volts, both
AC and DC settings. Shouldn't I be getting 21-26 Volts? How should I
measure screen grid voltage?

I have measured resistence across all combinations of pins. All
combinations show infinite resistence EXCEPT across the header
filament AND Pin 4 ("blanK") and the filament which is 150 ohms if I
recall correctly. Or is it 150 kohms? Anyway what resistence should I
get across the heater filament and what is Pin 4???


Thanks.
 
"Rock Braud" <rnbraud@yahoo.com> wrote in message
news:a46be137.0404021221.2e630cd2@posting.google.com...
I am chasing down a problem with my Emerson 27" model MS2700RDA.

The picture has about 20 "retrace" lines across it, the image is
barely visible, and red dominates the color.

I have done some research and am trying to determine if there is a CRT
short and if so where it is.

When measuring voltages on the CRT pins should I have my DMM set on DC
or AC.
Also, I am placing the negative lead on the CRT Header Element ground
pin; is this correct?????

When I measure the cathodes, Red, Green, and Blue, wrt heater filament
ground, I get the proper voltages; 155-165 VDCon all three and they
are usually within 1 volt of each other.

However, when I measure VDC "across" the heater filament I get 0 VDC.
But, when I measure it for AC voltage, I get around 2 or 3 VAC. What
is the correct way to measure heater filament voltage???

Finally, when I measure Pin 5, a.k.a. screen grid, I get 0 volts, both
AC and DC settings. Shouldn't I be getting 21-26 Volts? How should I
measure screen grid voltage?

I have measured resistence across all combinations of pins. All
combinations show infinite resistence EXCEPT across the header
filament AND Pin 4 ("blanK") and the filament which is 150 ohms if I
recall correctly. Or is it 150 kohms? Anyway what resistence should I
get across the heater filament and what is Pin 4???


Thanks.
Click to expand...
All voltages except the filaments should be DC, they will be AC.
 
All voltages except the filaments should be DC, they will be AC
Click to expand...
Correct, you should have 6vac across the filiments, but you need a decent meter
to measure it accurately because its at 15.7khz. A cheap meter isn't capable of
accurately measuring AC at higher frequencys.
 
On 02 Apr 2004 23:05:17 GMT, ronkz650@aol.comnospam (RonKZ650) wrote:

All voltages except the filaments should be DC, they will be AC

Correct, you should have 6vac across the filiments, but you need a decent meter
to measure it accurately because its at 15.7khz. A cheap meter isn't capable of
accurately measuring AC at higher frequencys.
Click to expand...
You need true RMS capability also.

Tom
 
On 2 Apr 2004 12:21:29 -0800, rnbraud@yahoo.com (Rock Braud) wrote:

I am chasing down a problem with my Emerson 27" model MS2700RDA.

The picture has about 20 "retrace" lines across it, the image is
barely visible, and red dominates the color.
Click to expand...
Is the FBT screen too high? Does it need a colour setup as well?

I have done some research and am trying to determine if there is a CRT
short and if so where it is.
Click to expand...
I think you can verify a CRT roughly by something like this (I read
this here years ago, and I don't know how well I remember it, but)...

After removing the CRT board, apply 6.3 volts to the heater pins on
the CRT. Measure the voltage with respect to ground on the 3 cathodes.
They should be roughly the same, and about 1.5 volts (I think) for a
healthy CRT.

When measuring voltages on the CRT pins should I have my DMM set on DC
or AC.
Also, I am placing the negative lead on the CRT Header Element ground
pin; is this correct?????

When I measure the cathodes, Red, Green, and Blue, wrt heater filament
ground, I get the proper voltages; 155-165 VDCon all three and they
are usually within 1 volt of each other.

However, when I measure VDC "across" the heater filament I get 0 VDC.
But, when I measure it for AC voltage, I get around 2 or 3 VAC. What
is the correct way to measure heater filament voltage???
Click to expand...
That's what I'd expect to see with a garden variety (not true RMS)
meter. The best way is with a scope and/or true RMS meter. If I
remember correctly, the scope will yield readings of about 27 V P-P.

Finally, when I measure Pin 5, a.k.a. screen grid, I get 0 volts, both
AC and DC settings. Shouldn't I be getting 21-26 Volts? How should I
measure screen grid voltage?
Click to expand...
If my memory can be trusted at all, 0 is correct.

Tom

I have measured resistence across all combinations of pins. All
combinations show infinite resistence EXCEPT across the header
filament AND Pin 4 ("blanK") and the filament which is 150 ohms if I
recall correctly. Or is it 150 kohms? Anyway what resistence should I
get across the heater filament and what is Pin 4???


Thanks.
Click to expand...
 
ronkz650@aol.comnospam (RonKZ650) writes:

All voltages except the filaments should be DC, they will be AC

Correct, you should have 6vac across the filiments, but you need a decent meter
to measure it accurately because its at 15.7khz. A cheap meter isn't capable of
accurately measuring AC at higher frequencys.
Click to expand...
Also note that the screen/G2 voltage is from a very high
impedance source (several hundred Megohms). So, a normal DMM will
load it excessively. You need a high impedance meter or HV probe.
The focus is also from a high impedance source and is several kV
so you need a HV probe. If the focus looks OK, no need to measure it.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.
 
Thanks, that is exactly what I was looking for.

Later.

On Fri, 2 Apr 2004 15:26:43 -0600, "Patch" <bob_acord@hotmail.com>
wrote:

"Rock Braud" <rnbraud@yahoo.com> wrote in message
news:a46be137.0404021221.2e630cd2@posting.google.com...
I am chasing down a problem with my Emerson 27" model MS2700RDA.

The picture has about 20 "retrace" lines across it, the image is
barely visible, and red dominates the color.

I have done some research and am trying to determine if there is a CRT
short and if so where it is.

When measuring voltages on the CRT pins should I have my DMM set on DC
or AC.
...


Thanks.

All voltages except the filaments should be DC, they will be AC.
Click to expand...
 
On Fri, 02 Apr 2004 23:28:49 GMT, Tom MacIntyre
<tom__macintyre@hotmail.com> wrote:

The picture has about 20 "retrace" lines across it, the image is
barely visible, and red dominates the color.

Is the FBT screen too high? Does it need a colour setup as well?
Not sure I follow. What should I measure for the FBT and what are the
Click to expand...
expected values.

What is "colour setup??"

I think you can verify a CRT roughly by something like this (I read
this here years ago, and I don't know how well I remember it, but)...

After removing the CRT board, apply 6.3 volts to the heater pins on
the CRT. Measure the voltage with respect to ground on the 3 cathodes.
They should be roughly the same, and about 1.5 volts (I think) for a
healthy CRT.
I have measured the cathode voltages and ther were all within 1 VDC of
Click to expand...
each other. However, when I measured again last night the red was 20+
VDC lower. I plan to re-measure tonight both with the PCB connected
and not.

I am not sure what you mean by apply 6.3 volts to the heater pins.
Isn't this 6.3 VAC or 6.3 VRMS? Did you mean to jumper the PCB heater
connections to the CRT Pins??

However, when I measure VDC "across" the heater filament I get 0 VDC.
But, when I measure it for AC voltage, I get around 2 or 3 VAC. What
is the correct way to measure heater filament voltage???

That's what I'd expect to see with a garden variety (not true RMS)
meter. The best way is with a scope and/or true RMS meter. If I
remember correctly, the scope will yield readings of about 27 V P-P.

This is actually very good info for me. I hadn't realized the heater
Click to expand...
filament was being fed by a varying source. I just received my
Photofact schematics, and sure enought the show a waveform across the
heater filament which shows 23 volts p-to-p!

I plan to obtain an RMS meter this weekend to better measure these
values.

I also now have the correct pinout on the CRT pins. I plan to
re-measure tonight to see if things are correct.
 
The best way to measure CRT voltages is: very carefully!!

You are dealing with DC potentials of 15 to 20KV/

"RonKZ650" <ronkz650@aol.comnospam> wrote in message
news:20040402180517.22051.00000503@mb-m22.aol.com...
All voltages except the filaments should be DC, they will be AC

Correct, you should have 6vac across the filiments, but you need a decent
meter
to measure it accurately because its at 15.7khz. A cheap meter isn't
capable of
accurately measuring AC at higher frequencys.
Click to expand...
 
Well, I went to Fry's and couldn't find a cheapo Vrms meter. I was
looking for one of the "old" meter types. I know, I know.

Anway, I really enjoy building my own tools and was wondering if
anyone knew of a good schematic for a Vrms meter. I have seen various
schematics for in-circuit Cap testors and so fourth.

Anyone can point me in the right direction.

Thanks.
On 2 Apr 2004 12:21:29 -0800, rnbraud@yahoo.com (Rock Braud) wrote:

I am chasing down a problem with my Emerson 27" model MS2700RDA.

The picture has about 20 "retrace" lines across it, the image is
barely visible, and red dominates the color.

I have done some research and am trying to determine if there is a CRT
short and if so where it is.

When measuring voltages on the CRT pins should I have my DMM set on DC
or AC.
Also, I am placing the negative lead on the CRT Header Element ground
pin; is this correct?????

When I measure the cathodes, Red, Green, and Blue, wrt heater filament
ground, I get the proper voltages; 155-165 VDCon all three and they
are usually within 1 volt of each other.

However, when I measure VDC "across" the heater filament I get 0 VDC.
But, when I measure it for AC voltage, I get around 2 or 3 VAC. What
is the correct way to measure heater filament voltage???

Finally, when I measure Pin 5, a.k.a. screen grid, I get 0 volts, both
AC and DC settings. Shouldn't I be getting 21-26 Volts? How should I
measure screen grid voltage?

I have measured resistence across all combinations of pins. All
combinations show infinite resistence EXCEPT across the header
filament AND Pin 4 ("blanK") and the filament which is 150 ohms if I
recall correctly. Or is it 150 kohms? Anyway what resistence should I
get across the heater filament and what is Pin 4???


Thanks.
Click to expand...
 
"Arthur W." <abw0628@optonline.net> writes:

The best way to measure CRT voltages is: very carefully!!

You are dealing with DC potentials of 15 to 20KV/
Click to expand...
Well, only one of them should be that high - or higher. The ones on the
neck PCB are 200 V or so for the cathodes, a few hundred for the screen/G2
and a few kV for the focus. But the last two are extremely low current.

Still, I agree, very carefully is most definitely important!

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.
 
rnbraud@yahoo.com writes:

Well, I went to Fry's and couldn't find a cheapo Vrms meter. I was
looking for one of the "old" meter types. I know, I know.

Anway, I really enjoy building my own tools and was wondering if
anyone knew of a good schematic for a Vrms meter. I have seen various
schematics for in-circuit Cap testors and so fourth.

Anyone can point me in the right direction.
Click to expand...
If it's to measure the filament voltage, really not necessary. What color
is the glow?

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.
 
Yes, it is to measure the filament voltage. I get around 2-3 Vdc when
measured. I am pretty confident it is correct but just wanted to be
completely sure.

And, yes there are three "glowing" sets of tiny wires in the neck of
the CRT.

Thanks for your help.

On 04 Apr 2004 19:13:52 -0400, Sam Goldwasser <sam@saul.cis.upenn.edu>
wrote:

rnbraud@yahoo.com writes:

Well, I went to Fry's and couldn't find a cheapo Vrms meter. I was
looking for one of the "old" meter types. I know, I know.

Anway, I really enjoy building my own tools and was wondering if
anyone knew of a good schematic for a Vrms meter. I have seen various
schematics for in-circuit Cap testors and so fourth.

Anyone can point me in the right direction.

If it's to measure the filament voltage, really not necessary. What color
is the glow?

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.
Click to expand...
 
rnbraud@yahoo.com writes:

Yes, it is to measure the filament voltage. I get around 2-3 Vdc when
measured. I am pretty confident it is correct but just wanted to be
completely sure.

And, yes there are three "glowing" sets of tiny wires in the neck of
the CRT.

Thanks for your help.
Click to expand...
If they are visibly glowing, they are working well enough to get a picture.

I don't recall what your original problem was but if it's no picture or
a color missing, the filaments aren't the cause.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.
 
Hello Sam,

I agree with you. I am now planning to troubleshoot a possible Screen
Grid (G1) to Red Cathode short.

Thanks for your input.

Sam Goldwasser <sam@saul.cis.upenn.edu> wrote in message news:<6wzn9rfc6w.fsf@saul.cis.upenn.edu>...
rnbraud@yahoo.com writes:

Yes, it is to measure the filament voltage. I get around 2-3 Vdc when
measured. I am pretty confident it is correct but just wanted to be
completely sure.

And, yes there are three "glowing" sets of tiny wires in the neck of
the CRT.

Thanks for your help.

If they are visibly glowing, they are working well enough to get a picture.

I don't recall what your original problem was but if it's no picture or
a color missing, the filaments aren't the cause.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.
Click to expand...
 
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