Re: 454 screen brightness issues?


Jack Ohme
 

and sorry, but i dont have access to a VTVM :(

surely if my meter cant measure across it, then there at least isnt a
catastrophic short?

On Tue, Jan 7, 2020 at 2:49 PM Jack Ohme via Groups.Io <machinamancerjack=
gmail.com@groups.io> wrote:

also anyone know where B1473-75 are? looks like tek added some neon bulbs
there to make sure the grid didnt go above K, but i dont know what theyre
doing now...

On Tue, Jan 7, 2020 at 2:40 PM Jack Ohme via Groups.Io <machinamancerjack=
gmail.com@groups.io> wrote:

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












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