Re: 454 screen brightness issues?


 

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





Join TekScopes@groups.io to automatically receive all group messages.