Topics

454 screen brightness issues?


Jack Ohme
 

I received a verified-working Tek 454 scope from eBay, and it seems to now,
after shipping, have an issue where the CRT is incredibly bright, like
we're talking burn-a-hole-in-your-vision bright. Naturally I immediately
powered it off, fearing damage to the CRT, and upon turning it back on to
test it, dialing the trace way off the screen, there is still a sizeable,
though not dangerous, glow from electron splash. The grid appears to be at
-150 to -190 volts depending on the brightness setting, and the grid bias
trimmer does essentially nothing, moving the grid bias by around 8 volts
only (not sure if this is a fault or just the design). I checked for a G-K
short, but its about 25 megaohm between (in-circuit), so looks good. I'm
wondering if maybe the grid assembly was knocked loose during shipping
maybe? I'm not intimately familiar with the CRT construction, so maybe
someone more well-acquainted with Tek's CRTs could give me some idea of
what may be going on...


John
 

I assume your -1960V is checked ok. Ditto all three LV supplies. Have a look at the chain of 10M resistors R1442-6 : ohms law suggests you should have a much larger grid swing from the bias pot?

John


John
 

PS: also the 7M5 in parallel with that pot.


EB4APL
 

My experience with this symptom has been always a DC restorer problem, even I bought a defective R&S scope with this symptom and the problem was the DC restorer diodes as I was figuring out.

Also I had a problem with a non operating focus control and the cause was the focus DC restorer diodes also. I think that these diodes are stressed in some way and with time they develop leaks that make the circuit to fail. I tried to measure these leaks, but it happens when they are subjected to HV and I don't have the right equipment to test them, so I replaced all the diodes in the failing DC restorer (3 or 4, depending on the circuit) and the problem solved. Since most of the work is to reach this part of the scope, changing all of them is a good advice.

The problem could be elsewhere, of course, but if you have right voltages in the CRT the DC restorer is a good candidate.

Regards,

Ignacio

El 06/01/2020 a las 0:29, Jack Ohme escribió:
I received a verified-working Tek 454 scope from eBay, and it seems to now,
after shipping, have an issue where the CRT is incredibly bright, like
we're talking burn-a-hole-in-your-vision bright. Naturally I immediately
powered it off, fearing damage to the CRT, and upon turning it back on to
test it, dialing the trace way off the screen, there is still a sizeable,
though not dangerous, glow from electron splash. The grid appears to be at
-150 to -190 volts depending on the brightness setting, and the grid bias
trimmer does essentially nothing, moving the grid bias by around 8 volts
only (not sure if this is a fault or just the design). I checked for a G-K
short, but its about 25 megaohm between (in-circuit), so looks good. I'm
wondering if maybe the grid assembly was knocked loose during shipping
maybe? I'm not intimately familiar with the CRT construction, so maybe
someone more well-acquainted with Tek's CRTs could give me some idea of
what may be going on...


Mlynch001
 

I'm
wondering if maybe the grid assembly was knocked loose during shipping
maybe? I'm not intimately familiar with the CRT construction, so maybe
someone more well-acquainted with Tek's CRTs could give me some idea of
what may be going on...
While fixing my newest 465 I recall seeing a post where a 465 had a similar problem. The OP found an internally disconnected Grid pin. Here is the post.

https://groups.io/g/TekScopes/topic/tek_465_intensity_problem/7656664

FWIW, I received a 465 in near mint condition as part of the purchase of a 485. I took a 465 and a 2215 as a group to get the 485. The 465 had a similar problem. I usually clean and DeOxit all the switches on any old scope that I get before anything else is attempted. Although the switch felt good, the beam finder worked properly when fully depressed and appeared to be in the correct position when released, in reality the Beam Finder on this scope was evidently "sticking sightly on" and creating some weird intermediate connection in the circuit and this cause a similar issue to what you are seeing on your 454. After cleaning and DeOxit the scope came back to normal, so a very simple fix in this case..

Best of luck
--
Michael Lynch
Dardanelle, AR


Chuck Harris
 

I saw that once in a 2456B's CRT. An internal weld between
a strap and an element broke, leaving an open circuit on
the CRT's intensity (control) grid .

The symptom was the intensity started out low, readout seemed
fine, and the beam could be moved with the intensity control,
but the beam kept getting brighter and brighter.

-Chuck Harris

Mlynch001 wrote:

I'm
wondering if maybe the grid assembly was knocked loose during shipping
maybe? I'm not intimately familiar with the CRT construction, so maybe
someone more well-acquainted with Tek's CRTs could give me some idea of
what may be going on...
While fixing my newest 465 I recall seeing a post where a 465 had a similar problem. The OP found an internally disconnected Grid pin. Here is the post.

https://groups.io/g/TekScopes/topic/tek_465_intensity_problem/7656664


Jack Ohme
 

Update:
Upon resistance testing the cathode and grid, it seems to want to conduct
one way (in negative resistance, according to my meter), and not conduct at
all the other...
So perhaps the grid is not broken after all? Although this was with the
whole socket connected, so for all I know I could just be reading a diode
in the circuit, although the -10Mohm resistance would be odd for that.

On Mon, Jan 6, 2020 at 8:10 AM Chuck Harris <cfharris@...> wrote:

I saw that once in a 2456B's CRT. An internal weld between
a strap and an element broke, leaving an open circuit on
the CRT's intensity (control) grid .

The symptom was the intensity started out low, readout seemed
fine, and the beam could be moved with the intensity control,
but the beam kept getting brighter and brighter.

-Chuck Harris

Mlynch001 wrote:
I'm
wondering if maybe the grid assembly was knocked loose during shipping
maybe? I'm not intimately familiar with the CRT construction, so maybe
someone more well-acquainted with Tek's CRTs could give me some idea of
what may be going on...
While fixing my newest 465 I recall seeing a post where a 465 had a
similar problem. The OP found an internally disconnected Grid pin. Here
is the post.

https://groups.io/g/TekScopes/topic/tek_465_intensity_problem/7656664



Jack Ohme
 

Where should I be looking for those DC restorers, in the -1920v?

On Mon, Jan 6, 2020 at 11:52 AM Jack Ohme via Groups.Io <machinamancerjack=
gmail.com@groups.io> wrote:

Update:
Upon resistance testing the cathode and grid, it seems to want to conduct
one way (in negative resistance, according to my meter), and not conduct at
all the other...
So perhaps the grid is not broken after all? Although this was with the
whole socket connected, so for all I know I could just be reading a diode
in the circuit, although the -10Mohm resistance would be odd for that.

On Mon, Jan 6, 2020 at 8:10 AM Chuck Harris <cfharris@...> wrote:

I saw that once in a 2456B's CRT. An internal weld between
a strap and an element broke, leaving an open circuit on
the CRT's intensity (control) grid .

The symptom was the intensity started out low, readout seemed
fine, and the beam could be moved with the intensity control,
but the beam kept getting brighter and brighter.

-Chuck Harris

Mlynch001 wrote:
I'm
wondering if maybe the grid assembly was knocked loose during shipping
maybe? I'm not intimately familiar with the CRT construction, so maybe
someone more well-acquainted with Tek's CRTs could give me some idea
of
what may be going on...
While fixing my newest 465 I recall seeing a post where a 465 had a
similar problem. The OP found an internally disconnected Grid pin. Here
is the post.

https://groups.io/g/TekScopes/topic/tek_465_intensity_problem/7656664





Jack Ohme
 

Oh dear. The Z axis blanking doesnt seem to work either. Maybe this is a
grid issue after all

On Mon, Jan 6, 2020 at 11:55 AM Jack Ohme via Groups.Io <machinamancerjack=
gmail.com@groups.io> wrote:

Where should I be looking for those DC restorers, in the -1920v?

On Mon, Jan 6, 2020 at 11:52 AM Jack Ohme via Groups.Io <machinamancerjack=
gmail.com@groups.io> wrote:

Update:
Upon resistance testing the cathode and grid, it seems to want to conduct
one way (in negative resistance, according to my meter), and not conduct
at
all the other...
So perhaps the grid is not broken after all? Although this was with the
whole socket connected, so for all I know I could just be reading a diode
in the circuit, although the -10Mohm resistance would be odd for that.

On Mon, Jan 6, 2020 at 8:10 AM Chuck Harris <cfharris@...> wrote:

I saw that once in a 2456B's CRT. An internal weld between
a strap and an element broke, leaving an open circuit on
the CRT's intensity (control) grid .

The symptom was the intensity started out low, readout seemed
fine, and the beam could be moved with the intensity control,
but the beam kept getting brighter and brighter.

-Chuck Harris

Mlynch001 wrote:
I'm
wondering if maybe the grid assembly was knocked loose during
shipping
maybe? I'm not intimately familiar with the CRT construction, so
maybe
someone more well-acquainted with Tek's CRTs could give me some idea
of
what may be going on...
While fixing my newest 465 I recall seeing a post where a 465 had a
similar problem. The OP found an internally disconnected Grid pin.
Here
is the post.

https://groups.io/g/TekScopes/topic/tek_465_intensity_problem/7656664







 

Jack:

Also, do you have a copy of a 454 manual?  I have a copy (070-0617-00; I
believe this is available "on the web" as a .pdf <possibly at
w140.com>); have you tried doing an initial setup of controls as
described on page 6-8 and 6-9  ("Preliminary Procedure"), followed by a
CRT Grid Bias adjustment (page 6-11) of grid bias potentiometer R1447
(shown on page 6-12), to get the voltage at TP1349 (shown on page 6-12)
to be at +12 volts with INTENSITY at mid-scale?  If the voltage at
TP1349 is too high (too much positive) it will result in very high
intensity which cannot be reduced to a workable value.  And if the
TP1349 voltage cannot be brought into range, this will (I think) mean
that your problem is likely in the Z-axis board area, so you won't have
to mess around with any of the HV resistors.

And if you find this to be the issue, keep us informed as to your
progress in repair.  I am interested because I have a 453A (sn b103169)
which has a similar problem (intensity is too high, and the 453A's
TP1047, which corresponds to the 454's TP1349, can only be adjusted to
values between about +35 to +75 volts) and I am starting to think it's a
Z-axis board issue.  The Z-axis circuitry in the 453A and 454 are very
similar.

Mike D. N4MWP

On 1/6/20 3:36 PM, Jack Ohme wrote:
Oh dear. The Z axis blanking doesnt seem to work either. Maybe this is a
grid issue after all

On Mon, Jan 6, 2020 at 11:55 AM Jack Ohme via Groups.Io <machinamancerjack=
gmail.com@groups.io> wrote:

Where should I be looking for those DC restorers, in the -1920v?

On Mon, Jan 6, 2020 at 11:52 AM Jack Ohme via Groups.Io <machinamancerjack=
gmail.com@groups.io> wrote:

Update:
Upon resistance testing the cathode and grid, it seems to want to conduct
one way (in negative resistance, according to my meter), and not conduct
at
all the other...
So perhaps the grid is not broken after all? Although this was with the
whole socket connected, so for all I know I could just be reading a diode
in the circuit, although the -10Mohm resistance would be odd for that.

On Mon, Jan 6, 2020 at 8:10 AM Chuck Harris <cfharris@...> wrote:

I saw that once in a 2456B's CRT. An internal weld between
a strap and an element broke, leaving an open circuit on
the CRT's intensity (control) grid .

The symptom was the intensity started out low, readout seemed
fine, and the beam could be moved with the intensity control,
but the beam kept getting brighter and brighter.

-Chuck Harris

Mlynch001 wrote:
I'm
wondering if maybe the grid assembly was knocked loose during
shipping
maybe? I'm not intimately familiar with the CRT construction, so
maybe
someone more well-acquainted with Tek's CRTs could give me some idea
of
what may be going on...
While fixing my newest 465 I recall seeing a post where a 465 had a
similar problem. The OP found an internally disconnected Grid pin.
Here
is the post.
https://groups.io/g/TekScopes/topic/tek_465_intensity_problem/7656664





Albert Otten
 

Hi Jack,

Where should I be looking for those DC restorers, in the -1920v?
The 454 has no DC restorers. The HV transformer has separate windings for the grid and the cathode circuit. The rectified voltage for the grid can be reduced somewhat by the grid bias pot R1447 (in the divider chain with R1442-R1446) and is stacked on the voltage supplied by the Z-axis amplifier.
When you follow Mike's suggestion and use a pdf of the manual, you will note that the waveform at TP1349 is almost invisible. The blue picture in the paper manual is also very faint. The voltage switches between the 2 written values 17.7 V and 6.75 V.

Albert


Jack Ohme
 

Mike,

I have a copy of the manual in PDF and on paper. The intensity dial
correctly changes the voltage (although the screen brightness shows no
change), but the grid biasing potentiometer does not change the voltage at
TP1349, which stays at around 6.8v. I'm not sure whats happening here, if
this is the grid biasing pot that is broken or something nearby on the Z
axis board, but I will inspect the schematics. Let me know if you think of
anything to look for.

-Jack

On Mon, Jan 6, 2020 at 2:15 PM Albert Otten <aodiversen@...> wrote:

Hi Jack,

Where should I be looking for those DC restorers, in the -1920v?
The 454 has no DC restorers. The HV transformer has separate windings for
the grid and the cathode circuit. The rectified voltage for the grid can be
reduced somewhat by the grid bias pot R1447 (in the divider chain with
R1442-R1446) and is stacked on the voltage supplied by the Z-axis
amplifier.
When you follow Mike's suggestion and use a pdf of the manual, you will
note that the waveform at TP1349 is almost invisible. The blue picture in
the paper manual is also very faint. The voltage switches between the 2
written values 17.7 V and 6.75 V.

Albert




 

Jack:

In my previous email, I unsuccessfully tried to transcribe the procedure
for CRT grid bias adjustment described on page 6-11 of the manual.  Grid
bias adjustment control R1447 does not directly affect the voltage at
TP1349. Refer to the manual (page 6-11) for the exact process; my
apologies for my error.  I suggest the following:

1.  Setup the scope's controls as described on page 6-8 and 6-9
"Preliminary Procedure".
2.  Try to adjust the INTENSITY control  to get +12V at TP1349, as per
page 6-11 "Adjust CRT Grid Bias."  If this cannot be done, there might
be a Z-axis problem. If you can get +12 volts, then go to step 3 below:
3.  Adjust CRT Grid Bias potentiometer R1447 per paragraph 6.d on page 6-11.
4.  Let us know what the results are. Although at this point I might be
at a loss as to further ideas.

It's also possible that there might be an issue with the grid bias
resistors/potentiometer (R1442 thru R1449) as others have suggested.  If
you have access to a ohmmeter (such as a VTVM) that can detect
variations of about 1.8 megohms in a total resistance of about 52
megohms, then you can check this by measuring the resistance (with power
down) from the CRT pin 3 to TP1349 while varying grid bias potentiometer
R1447 from fully CCW to fully CW.  You might get one measured value with
the ohm meter's leads in one position, and another with the meter leads
"reversed"; if so then go with the higher set of measurements, as the
lower set of measurements would be due to forward bias of D1440.

Mike D N4MWP

On 1/6/20 7:37 PM, Jack Ohme wrote:
Mike,

I have a copy of the manual in PDF and on paper. The intensity dial
correctly changes the voltage (although the screen brightness shows no
change), but the grid biasing potentiometer does not change the voltage at
TP1349, which stays at around 6.8v. I'm not sure whats happening here, if
this is the grid biasing pot that is broken or something nearby on the Z
axis board, but I will inspect the schematics. Let me know if you think of
anything to look for.

-Jack

On Mon, Jan 6, 2020 at 2:15 PM Albert Otten <aodiversen@...> wrote:

Hi Jack,
Where should I be looking for those DC restorers, in the -1920v?
The 454 has no DC restorers. The HV transformer has separate windings for
the grid and the cathode circuit. The rectified voltage for the grid can be
reduced somewhat by the grid bias pot R1447 (in the divider chain with
R1442-R1446) and is stacked on the voltage supplied by the Z-axis
amplifier.
When you follow Mike's suggestion and use a pdf of the manual, you will
note that the waveform at TP1349 is almost invisible. The blue picture in
the paper manual is also very faint. The voltage switches between the 2
written values 17.7 V and 6.75 V.

Albert




Albert Otten
 

Hi Jack,

The grid appears to be at
-150 to -190 volts depending on the brightness setting, and the grid bias
trimmer does essentially nothing, moving the grid bias by around 8 volts
only (not sure if this is a fault or just the design).
So far this finding has has been overlooked I think. I suppose for this you attached a DMM between grid and cathode leads. Even measured with a 10M meter there will be some reduction in voltage, so in reality the grid voltage would be still more negative. The tube T4540 is missing from tek_crt_data.pdf but the cut-off voltage of similar tubes is above -100 V. For instance -80 V for the 453 tube T4530. Your trace should be complete dark, unless there is a broken connection to the grid somewhere. (Are you sure about the sign of the voltage difference?)

Update:
Upon resistance testing the cathode and grid, it seems to want to conduct
one way (in negative resistance, according to my meter), and not conduct at
all the other...
So perhaps the grid is not broken after all? Although this was with the
whole socket connected, so for all I know I could just be reading a diode
in the circuit, although the -10Mohm resistance would be odd for that.
I'm not sure what you mean with negative resistance. Are there still some capacitor voltages present that disturb your resistance measurement? Or does you meter pick up rectified "hum" or so? You can't check for a broken connection to the grid this way; you might discover a short between grid and cathode.

Albert


EB4APL
 

Hi,

You are right, the 454 has not DC restores. Sorry for the misleading indications, I don't have a 454 and I had not checked the schematics.

El 06/01/2020 a las 23:15, Albert Otten escribió:
Hi Jack,
Where should I be looking for those DC restorers, in the -1920v?
The 454 has no DC restorers. The HV transformer has separate windings for the grid and the cathode circuit. The rectified voltage for the grid can be reduced somewhat by the grid bias pot R1447 (in the divider chain with R1442-R1446) and is stacked on the voltage supplied by the Z-axis amplifier.
When you follow Mike's suggestion and use a pdf of the manual, you will note that the waveform at TP1349 is almost invisible. The blue picture in the paper manual is also very faint. The voltage switches between the 2 written values 17.7 V and 6.75 V.

Albert


Jack Ohme
 

Mike-

My meter measures an open circuit from pin 3 to point 1349, I think the
resistance is more than it can measure. I was able to get 12v at the point,
but its VERY touchy, about half of the pot does next to nothing, then the
middle changes so fast you have to brush the dial with your finger to dial
in past 2 or 3 volts difference, then it becomes very slow again. So about
20 volts of the 6.8v -> 32v transit occur on about 8-10% of the pot's wiper
surface. Seems unusual to me, not the source of the problem at least, but
perhaps a clue. It just seems very odd to me that the CRT grid measures
within spec for the datasheet... actually about 25 volts more negative than
its listed typical operating values at intensity pot centre. To the best of
my understanding, a higher negative bias makes the screen darker, so this
is quite confusing indeed, and what led me to believe there may be a busted
grid.

On Mon, Jan 6, 2020 at 8:10 PM Mike Dinolfo <@mdinolfo> wrote:

Jack:

In my previous email, I unsuccessfully tried to transcribe the procedure
for CRT grid bias adjustment described on page 6-11 of the manual. Grid
bias adjustment control R1447 does not directly affect the voltage at
TP1349. Refer to the manual (page 6-11) for the exact process; my
apologies for my error. I suggest the following:

1. Setup the scope's controls as described on page 6-8 and 6-9
"Preliminary Procedure".
2. Try to adjust the INTENSITY control to get +12V at TP1349, as per
page 6-11 "Adjust CRT Grid Bias." If this cannot be done, there might
be a Z-axis problem. If you can get +12 volts, then go to step 3 below:
3. Adjust CRT Grid Bias potentiometer R1447 per paragraph 6.d on page
6-11.
4. Let us know what the results are. Although at this point I might be
at a loss as to further ideas.

It's also possible that there might be an issue with the grid bias
resistors/potentiometer (R1442 thru R1449) as others have suggested. If
you have access to a ohmmeter (such as a VTVM) that can detect
variations of about 1.8 megohms in a total resistance of about 52
megohms, then you can check this by measuring the resistance (with power
down) from the CRT pin 3 to TP1349 while varying grid bias potentiometer
R1447 from fully CCW to fully CW. You might get one measured value with
the ohm meter's leads in one position, and another with the meter leads
"reversed"; if so then go with the higher set of measurements, as the
lower set of measurements would be due to forward bias of D1440.

Mike D N4MWP

On 1/6/20 7:37 PM, Jack Ohme wrote:
Mike,

I have a copy of the manual in PDF and on paper. The intensity dial
correctly changes the voltage (although the screen brightness shows no
change), but the grid biasing potentiometer does not change the voltage
at
TP1349, which stays at around 6.8v. I'm not sure whats happening here, if
this is the grid biasing pot that is broken or something nearby on the Z
axis board, but I will inspect the schematics. Let me know if you think
of
anything to look for.

-Jack

On Mon, Jan 6, 2020 at 2:15 PM Albert Otten <aodiversen@...>
wrote:

Hi Jack,
Where should I be looking for those DC restorers, in the -1920v?
The 454 has no DC restorers. The HV transformer has separate windings
for
the grid and the cathode circuit. The rectified voltage for the grid
can be
reduced somewhat by the grid bias pot R1447 (in the divider chain with
R1442-R1446) and is stacked on the voltage supplied by the Z-axis
amplifier.
When you follow Mike's suggestion and use a pdf of the manual, you will
note that the waveform at TP1349 is almost invisible. The blue picture
in
the paper manual is also very faint. The voltage switches between the 2
written values 17.7 V and 6.75 V.

Albert







Jack Ohme
 

Update, everyone

I am a complete idiot. I had my leads wrong, the grid is actually around
150 volts ABOVE the cathode, which explains everything. Thank God it isn't
a broken grid, but... the Z axis board is certainly messed up if something
like this is going on. I will further consult schematics, if anyone knows
what might be wrong, drop me a line

On Tue, Jan 7, 2020 at 10:33 AM Jack Ohme via Groups.Io <machinamancerjack=
gmail.com@groups.io> wrote:

Mike-

My meter measures an open circuit from pin 3 to point 1349, I think the
resistance is more than it can measure. I was able to get 12v at the point,
but its VERY touchy, about half of the pot does next to nothing, then the
middle changes so fast you have to brush the dial with your finger to dial
in past 2 or 3 volts difference, then it becomes very slow again. So about
20 volts of the 6.8v -> 32v transit occur on about 8-10% of the pot's wiper
surface. Seems unusual to me, not the source of the problem at least, but
perhaps a clue. It just seems very odd to me that the CRT grid measures
within spec for the datasheet... actually about 25 volts more negative than
its listed typical operating values at intensity pot centre. To the best of
my understanding, a higher negative bias makes the screen darker, so this
is quite confusing indeed, and what led me to believe there may be a busted
grid.

On Mon, Jan 6, 2020 at 8:10 PM Mike Dinolfo <@mdinolfo> wrote:

Jack:

In my previous email, I unsuccessfully tried to transcribe the procedure
for CRT grid bias adjustment described on page 6-11 of the manual. Grid
bias adjustment control R1447 does not directly affect the voltage at
TP1349. Refer to the manual (page 6-11) for the exact process; my
apologies for my error. I suggest the following:

1. Setup the scope's controls as described on page 6-8 and 6-9
"Preliminary Procedure".
2. Try to adjust the INTENSITY control to get +12V at TP1349, as per
page 6-11 "Adjust CRT Grid Bias." If this cannot be done, there might
be a Z-axis problem. If you can get +12 volts, then go to step 3 below:
3. Adjust CRT Grid Bias potentiometer R1447 per paragraph 6.d on page
6-11.
4. Let us know what the results are. Although at this point I might be
at a loss as to further ideas.

It's also possible that there might be an issue with the grid bias
resistors/potentiometer (R1442 thru R1449) as others have suggested. If
you have access to a ohmmeter (such as a VTVM) that can detect
variations of about 1.8 megohms in a total resistance of about 52
megohms, then you can check this by measuring the resistance (with power
down) from the CRT pin 3 to TP1349 while varying grid bias potentiometer
R1447 from fully CCW to fully CW. You might get one measured value with
the ohm meter's leads in one position, and another with the meter leads
"reversed"; if so then go with the higher set of measurements, as the
lower set of measurements would be due to forward bias of D1440.

Mike D N4MWP

On 1/6/20 7:37 PM, Jack Ohme wrote:
Mike,

I have a copy of the manual in PDF and on paper. The intensity dial
correctly changes the voltage (although the screen brightness shows no
change), but the grid biasing potentiometer does not change the voltage
at
TP1349, which stays at around 6.8v. I'm not sure whats happening here,
if
this is the grid biasing pot that is broken or something nearby on the
Z
axis board, but I will inspect the schematics. Let me know if you think
of
anything to look for.

-Jack

On Mon, Jan 6, 2020 at 2:15 PM Albert Otten <aodiversen@...>
wrote:

Hi Jack,
Where should I be looking for those DC restorers, in the -1920v?
The 454 has no DC restorers. The HV transformer has separate windings
for
the grid and the cathode circuit. The rectified voltage for the grid
can be
reduced somewhat by the grid bias pot R1447 (in the divider chain with
R1442-R1446) and is stacked on the voltage supplied by the Z-axis
amplifier.
When you follow Mike's suggestion and use a pdf of the manual, you
will
note that the waveform at TP1349 is almost invisible. The blue picture
in
the paper manual is also very faint. The voltage switches between the
2
written values 17.7 V and 6.75 V.

Albert









Albert Otten
 

Hi Jack,

I had my leads wrong, the grid is actually around
150 volts ABOVE the cathode, which explains everything. Thank God it isn't
a broken grid, but... the Z axis board is certainly messed up if something
like this is going on.
Now that you confirm that you made a sign mistake (question in my previous response) the fault is certainly not in the Z-axis board, diagram <16>, or in the CRT itself. In y opinion it must be a fault in the CRT supply circuitry diagram <17>. Probably in the grid supply. Probably an open "bottom" end of the grid bias pot R1447. Did you measure the resistance between wiper and joint of R1447/D1440? Does it go to zero in one detend of the wiper?

Albert


 

Jack, in response to your email of 1:33 pm:

I think it's possible that your "excessive trace brightness" problem
could be an issue with HV circuit component failure (as others have
suggested), and/or Z-axis circuitry or adjustment.  I should also add
"and/or something else that I don't know".  It's good that you've
managed to get +12V at TP1349, because I think this pretty much
indicates that the Z-axis circuitry is OK.  But more on that below...

Regarding your resistance measurement: I believe that probably the
majority of "garden variety" multimeters in use today do not allow for
measurement of resistances in the 50 megohm range (they only indicate
"open circuit" or the equivalent of "no measurement possible"), but I
also believe most of the VTVM's of 50 or 60 years ago would do so. 
Although a VTVM's accuracy and resolution for those measurements is only
fair, it would be sufficient for the measurement that I tried to
describe; that's why I suggested a VTVM.  So your multimeter/ohmmeter
might be indicating "open circuit."  But look at the schematic for the
454 CRT circuit and the Z-axis board and you will see the resistors
(R1442 thru R1449) that I mentioned; measuring from TP1349 (on the
Z-axis board) to CRT pin 3 is effectively a measurement of the
resistance across the terminals of C1449.  Or you could do a "deep dive"
into the HV compartment (removing a few layers of components) and
measure the resistances individually.  Note that I've never done such a
"deep dive"measurement, but I think I've seen emails from other
Tekscopes group members saying that a "power down" measurement of these
resistances might not be valid because the resistances could change
under applied voltages during "power up" conditions.  It's also a lot of
work (my opinion) to do such a "deep dive" measurement, and even more
work to replace these components in a shotgun approach.  So I've been
trying to encourage you to verify the Z-axis circuitry is OK before you
zero in on the HV circuitry as the culprit.

Regarding your "12V at the point" measurement: When doing the "Adjust
CRT Grid Bias" adjustment on page 6-11 of the manual, you have adjusted
INTENSITY to get +12 volts at TP1349, correct?  When you then go to the
next step, to adjust CRT Grid Bias R1447, can you then reduce the
visible dot or trace on the screen to get it "turned down" to a
reasonable level of brightness?  When you talk about "the pot" and "the
dial" and "8-10% of the pot's wiper", I admit that I'm confused as to
whether you are talking about the INTENSITY control (front panel) or
grid bias trimmer pot R1447.

Finally, my earlier emails did not correctly state the process described
on page 6-11 of the manual, and for that I apologize. But I'm kind of at
a dead end as far as any further suggestions.

Mike D N4MWP

On 1/7/20 1:33 PM, Jack Ohme wrote:
Mike-

My meter measures an open circuit from pin 3 to point 1349, I think the
resistance is more than it can measure. I was able to get 12v at the point,
but its VERY touchy, about half of the pot does next to nothing, then the
middle changes so fast you have to brush the dial with your finger to dial
in past 2 or 3 volts difference, then it becomes very slow again. So about
20 volts of the 6.8v -> 32v transit occur on about 8-10% of the pot's wiper
surface. Seems unusual to me, not the source of the problem at least, but
perhaps a clue. It just seems very odd to me that the CRT grid measures
within spec for the datasheet... actually about 25 volts more negative than
its listed typical operating values at intensity pot centre. To the best of
my understanding, a higher negative bias makes the screen darker, so this
is quite confusing indeed, and what led me to believe there may be a busted
grid.

On Mon, Jan 6, 2020 at 8:10 PM Mike Dinolfo <@mdinolfo> wrote:

Jack:

In my previous email, I unsuccessfully tried to transcribe the procedure
for CRT grid bias adjustment described on page 6-11 of the manual. Grid
bias adjustment control R1447 does not directly affect the voltage at
TP1349. Refer to the manual (page 6-11) for the exact process; my
apologies for my error. I suggest the following:

1. Setup the scope's controls as described on page 6-8 and 6-9
"Preliminary Procedure".
2. Try to adjust the INTENSITY control to get +12V at TP1349, as per
page 6-11 "Adjust CRT Grid Bias." If this cannot be done, there might
be a Z-axis problem. If you can get +12 volts, then go to step 3 below:
3. Adjust CRT Grid Bias potentiometer R1447 per paragraph 6.d on page
6-11.
4. Let us know what the results are. Although at this point I might be
at a loss as to further ideas.

It's also possible that there might be an issue with the grid bias
resistors/potentiometer (R1442 thru R1449) as others have suggested. If
you have access to a ohmmeter (such as a VTVM) that can detect
variations of about 1.8 megohms in a total resistance of about 52
megohms, then you can check this by measuring the resistance (with power
down) from the CRT pin 3 to TP1349 while varying grid bias potentiometer
R1447 from fully CCW to fully CW. You might get one measured value with
the ohm meter's leads in one position, and another with the meter leads
"reversed"; if so then go with the higher set of measurements, as the
lower set of measurements would be due to forward bias of D1440.

Mike D N4MWP

On 1/6/20 7:37 PM, Jack Ohme wrote:
Mike,

I have a copy of the manual in PDF and on paper. The intensity dial
correctly changes the voltage (although the screen brightness shows no
change), but the grid biasing potentiometer does not change the voltage
at
TP1349, which stays at around 6.8v. I'm not sure whats happening here, if
this is the grid biasing pot that is broken or something nearby on the Z
axis board, but I will inspect the schematics. Let me know if you think
of
anything to look for.

-Jack

On Mon, Jan 6, 2020 at 2:15 PM Albert Otten <aodiversen@...>
wrote:
Hi Jack,
Where should I be looking for those DC restorers, in the -1920v?
The 454 has no DC restorers. The HV transformer has separate windings
for
the grid and the cathode circuit. The rectified voltage for the grid
can be
reduced somewhat by the grid bias pot R1447 (in the divider chain with
R1442-R1446) and is stacked on the voltage supplied by the Z-axis
amplifier.
When you follow Mike's suggestion and use a pdf of the manual, you will
note that the waveform at TP1349 is almost invisible. The blue picture
in
the paper manual is also very faint. The voltage switches between the 2
written values 17.7 V and 6.75 V.

Albert






Jack Ohme
 

I am fairly certain its the HV by this point, mike, since my -1960v is only
around 1800v. I meant the front panel intensity control with the touchy
wiper. I'm currently trying to ascertain the best way of getting a look
inside the HV circuitry, the 12KV multiplier for the anode seems to be on
top of the -2kv section for cathode and grid. I'm awful with loose parts so
I'd rather not tear the whole thing down, currently scouring the manual for
test points I could use to point out which diode or cap is bad in the bias
supply.

On Tue, Jan 7, 2020 at 2:36 PM Mike Dinolfo <@mdinolfo> wrote:

Jack, in response to your email of 1:33 pm:

I think it's possible that your "excessive trace brightness" problem
could be an issue with HV circuit component failure (as others have
suggested), and/or Z-axis circuitry or adjustment. I should also add
"and/or something else that I don't know". It's good that you've
managed to get +12V at TP1349, because I think this pretty much
indicates that the Z-axis circuitry is OK. But more on that below...

Regarding your resistance measurement: I believe that probably the
majority of "garden variety" multimeters in use today do not allow for
measurement of resistances in the 50 megohm range (they only indicate
"open circuit" or the equivalent of "no measurement possible"), but I
also believe most of the VTVM's of 50 or 60 years ago would do so.
Although a VTVM's accuracy and resolution for those measurements is only
fair, it would be sufficient for the measurement that I tried to
describe; that's why I suggested a VTVM. So your multimeter/ohmmeter
might be indicating "open circuit." But look at the schematic for the
454 CRT circuit and the Z-axis board and you will see the resistors
(R1442 thru R1449) that I mentioned; measuring from TP1349 (on the
Z-axis board) to CRT pin 3 is effectively a measurement of the
resistance across the terminals of C1449. Or you could do a "deep dive"
into the HV compartment (removing a few layers of components) and
measure the resistances individually. Note that I've never done such a
"deep dive"measurement, but I think I've seen emails from other
Tekscopes group members saying that a "power down" measurement of these
resistances might not be valid because the resistances could change
under applied voltages during "power up" conditions. It's also a lot of
work (my opinion) to do such a "deep dive" measurement, and even more
work to replace these components in a shotgun approach. So I've been
trying to encourage you to verify the Z-axis circuitry is OK before you
zero in on the HV circuitry as the culprit.

Regarding your "12V at the point" measurement: When doing the "Adjust
CRT Grid Bias" adjustment on page 6-11 of the manual, you have adjusted
INTENSITY to get +12 volts at TP1349, correct? When you then go to the
next step, to adjust CRT Grid Bias R1447, can you then reduce the
visible dot or trace on the screen to get it "turned down" to a
reasonable level of brightness? When you talk about "the pot" and "the
dial" and "8-10% of the pot's wiper", I admit that I'm confused as to
whether you are talking about the INTENSITY control (front panel) or
grid bias trimmer pot R1447.

Finally, my earlier emails did not correctly state the process described
on page 6-11 of the manual, and for that I apologize. But I'm kind of at
a dead end as far as any further suggestions.

Mike D N4MWP

On 1/7/20 1:33 PM, Jack Ohme wrote:
Mike-

My meter measures an open circuit from pin 3 to point 1349, I think the
resistance is more than it can measure. I was able to get 12v at the
point,
but its VERY touchy, about half of the pot does next to nothing, then the
middle changes so fast you have to brush the dial with your finger to
dial
in past 2 or 3 volts difference, then it becomes very slow again. So
about
20 volts of the 6.8v -> 32v transit occur on about 8-10% of the pot's
wiper
surface. Seems unusual to me, not the source of the problem at least, but
perhaps a clue. It just seems very odd to me that the CRT grid measures
within spec for the datasheet... actually about 25 volts more negative
than
its listed typical operating values at intensity pot centre. To the best
of
my understanding, a higher negative bias makes the screen darker, so this
is quite confusing indeed, and what led me to believe there may be a
busted
grid.

On Mon, Jan 6, 2020 at 8:10 PM Mike Dinolfo <@mdinolfo> wrote:

Jack:

In my previous email, I unsuccessfully tried to transcribe the procedure
for CRT grid bias adjustment described on page 6-11 of the manual. Grid
bias adjustment control R1447 does not directly affect the voltage at
TP1349. Refer to the manual (page 6-11) for the exact process; my
apologies for my error. I suggest the following:

1. Setup the scope's controls as described on page 6-8 and 6-9
"Preliminary Procedure".
2. Try to adjust the INTENSITY control to get +12V at TP1349, as per
page 6-11 "Adjust CRT Grid Bias." If this cannot be done, there might
be a Z-axis problem. If you can get +12 volts, then go to step 3 below:
3. Adjust CRT Grid Bias potentiometer R1447 per paragraph 6.d on page
6-11.
4. Let us know what the results are. Although at this point I might be
at a loss as to further ideas.

It's also possible that there might be an issue with the grid bias
resistors/potentiometer (R1442 thru R1449) as others have suggested. If
you have access to a ohmmeter (such as a VTVM) that can detect
variations of about 1.8 megohms in a total resistance of about 52
megohms, then you can check this by measuring the resistance (with power
down) from the CRT pin 3 to TP1349 while varying grid bias potentiometer
R1447 from fully CCW to fully CW. You might get one measured value with
the ohm meter's leads in one position, and another with the meter leads
"reversed"; if so then go with the higher set of measurements, as the
lower set of measurements would be due to forward bias of D1440.

Mike D N4MWP

On 1/6/20 7:37 PM, Jack Ohme wrote:
Mike,

I have a copy of the manual in PDF and on paper. The intensity dial
correctly changes the voltage (although the screen brightness shows no
change), but the grid biasing potentiometer does not change the voltage
at
TP1349, which stays at around 6.8v. I'm not sure whats happening here,
if
this is the grid biasing pot that is broken or something nearby on the
Z
axis board, but I will inspect the schematics. Let me know if you think
of
anything to look for.

-Jack

On Mon, Jan 6, 2020 at 2:15 PM Albert Otten <aodiversen@...>
wrote:
Hi Jack,
Where should I be looking for those DC restorers, in the -1920v?
The 454 has no DC restorers. The HV transformer has separate windings
for
the grid and the cathode circuit. The rectified voltage for the grid
can be
reduced somewhat by the grid bias pot R1447 (in the divider chain with
R1442-R1446) and is stacked on the voltage supplied by the Z-axis
amplifier.
When you follow Mike's suggestion and use a pdf of the manual, you
will
note that the waveform at TP1349 is almost invisible. The blue picture
in
the paper manual is also very faint. The voltage switches between the
2
written values 17.7 V and 6.75 V.

Albert