Topics

Unable to display cursors and diagnostic messages after LPVS "recapping"


Rogerio O
 

Dear all,
I am working on this 2465BCT for a while.
First I had to recap and fix the A5 board, which I managed to succeed thanks to the help of this forum.
I managed to adjust DAC and run CAL08 to try to have a better display, which was a little jittery.
Since I could not reduce the jitter, I thought it would be a good idea to replace all the electrolytic capacitor in the LPVS since they all have 1991 date codes.
I replaced one by one at a time to avoid errors (C1115 / C1132 for example).
All new caps were bought from Mouser.

when I powered up the scope I could see the error message PUSH A/B TRIGGER once.
After pushing this button the message display vanished, and it is impossible to display any message no matter what is the position of the READOT INTENSITY control.

I noticed also that it is not possible to display the cursors.

To this moment I have checked the following:
1 - A5 +1.36V and -1.25V ==> OK
2 - J119 voltages ===> all on spec. (specially +10.00V)

I think the problem is that the calibration constants are not valid anymore since the power supply voltages have changed.
However it is NOT POSSIBLE to start any calibration routine since I CAN'T READ THE MESSAGE DISPLAY.

How to I proceed?
Thank you in advance,
Roger


satbeginner
 

Hi,

First, I would increase grid bias a bit to see if there is any beam.

After that, I would check all the "combs" that connect the two LVPS boards together.
I've seen the sometimes one pin was misaligned, or a comb not properly inserted.

Good luck,

Leo


Craig Cramb
 

I doubt that it is a PS issue as unless you turned the 10v pot. Go back thru all of your PS to main board connections and carefully review every thing that was possibly disturbed during removal and reinstalling the PS.

Craig


Rogerio O
 

Thank you for the replies.

It is possible to display all four traces, and the vertical and horizontal position knobs work as expected.
SEC/DIV and horizontal position work as well.

If I put the scope manually in test mode and keep fumbling with the READOUT INTENSITY is possible to "flash" de DIAGNOSTIC message for one second or so.

The problem seems to affect the readout display and cursors only.

I tried to start the scope with all the NVRAM images I had tested before (they all caused the error message TEST4 FAIL10 to be displayed) but none of them produces a visible anymore.

Is there a circuit the is responsible displaying both the messages AND the cursors?

Thanks


Rogerio O
 

After some more testing I found that there is a bright spot near the center of the screen at almost every position of the READOUT INTENSITY control knob.
If I move it slowly, a figure like a rectangle appears when the knob is about the center position.
I have uploaded a photo (https://groups.io/g/TekScopes/album?id=258165)


Craig Cramb
 

I would suggest that you disassemble it again remove the option rack and boot the scope without the option rack assembly. Yes you will see option errors but see what you get then is a good starting point. The options rack assembly alignment and getting everything plugged back in correctly can be problem. Leave cover off when you do this test but run a fan across the main board hybrids to prevent overheating.

Craig


Siggi
 

On Wed, Dec 16, 2020 at 8:09 AM Rogerio O <rodd414@gmail.com> wrote:

when I powered up the scope I could see the error message PUSH A/B
TRIGGER once.
And this is a reasonable intensity?


After pushing this button the message display vanished, and it is
impossible to display any message no matter what is the position of the
READOT INTENSITY control.
I would suspect the intensity pot, or the MUX in the potentiometer
readback. The pot itself is easy enough to test, just find the pot signal
ahead of the MUX on the A5 board and see whether it tracks with the pot as
you move it.

I noticed also that it is not possible to display the cursors.
The cursors and the OSD are generated from the same machinery, and they
share the same intensity setting.


Chuck Harris <cfharris@...>
 

Since the failure happened after he recapped the power supply,
I think the problem is almost 100% certainly a result of that
job.

Most likely, it is a cabling issue, as it is easy to forget
one, or to put one in the wrong place... usually with dire
results.

-Chuck Harris

Siggi wrote:

On Wed, Dec 16, 2020 at 8:09 AM Rogerio O <rodd414@gmail.com> wrote:

when I powered up the scope I could see the error message PUSH A/B
TRIGGER once.
And this is a reasonable intensity?


After pushing this button the message display vanished, and it is
impossible to display any message no matter what is the position of the
READOT INTENSITY control.
I would suspect the intensity pot, or the MUX in the potentiometer
readback. The pot itself is easy enough to test, just find the pot signal
ahead of the MUX on the A5 board and see whether it tracks with the pot as
you move it.

I noticed also that it is not possible to display the cursors.
The cursors and the OSD are generated from the same machinery, and they
share the same intensity setting.






Rogerio O
 

Thank you for the replies.

The cables I had to remove were connected to A5 (J251, J421) and to the GPIB and CTT board.
However, I have checked the all the voltages required by A5 board and they were all present and on spot.

AFAIK, a problem in the cables connected to the GPIB/CTT would not cause this problem .

Consulting the chart " Readout Troubleshooting Procedure", is a little tricky since I can't say if the messages TEST 03 FAIL 01 or TEST 03 FAIL02 were displayed (I can see only the traces, but no messages/cursors) , so I moved the case
NO CURSORS PRESENT.
In this case I should check lines RA6, CA6 , DLY REF1 and CURSOR0 which were:
RA6 - Active
CA6 - Stuck @ 0V
CURSOR0 - Stuck @ -1.3V
DLY REF 1 - sTUCK @ -1.6V

The is a bright spot which flickers in the screen (my guess is that it would be the starting point of the cursor/message display).
It moves from time to time.
Also, a message flickers every 3 min (but it is to fast to be read).

I do not understand how it is possible to have activity in pins 3 and 8/9/10 U2521/U2530 and no activity at DLY REF1 and CURSOR0 since I have already replaced U2620 and U2630.
Have they fried again?

Roger


 

Rogerio,

I don't actually know anything about the scope you're working on, but looking at the schematic I can see that you would not only need "activity" on pins 8-10 of U2521, but also activity on pins 2, 4, and 5 in order to have any effect on U2620 (and similar for U2530/U2630).

Have you verified that the signals from the MUX chips to the op amps are active?

Have you looked at the output from U2301 (pins 12, 15, and 16) to see if it's counting all 8 possible values?

Have you checked continuity between pins 12, 15, and 16 on U2301 to pins 9, 10, and 11 on U2521?

-- Jeff Dutky


Rogerio O
 

Thank you for your reply Jeff

I decided to test a extra A5 board that I have keep for emergencies and the scope worked as expected so the problem is really in the A5 board I was using.

Then a installed the "old board" back, but I decided to use the NVRAM that was in the "spare A5" .
To my surprise, the display and cursors were back!!!

It seems that the NVRAM data was corrupted and was causing all the problems.
I don't know how this happened and how corrupted data would cause these problems

I can't discard a cabling problem as pointed by Chuck, although I have reseated all A5 cables after reading his reply.

I will begin DAC calibration again and move forward if possible, since the problem started after I finished this step.


Rogerio O
 

On Sat, Dec 19, 2020 at 04:07 AM, Rogerio O wrote:

Wrong conclusion.

The problem reappeared as soon as the scope warmed up.

Back to square 1.


satbeginner
 

Try warming the Analog multiplexers related to the cursor position on the A5 PCB.

I have seen these go bad.

Also the related opamps and capacitors that form a sample-and-hold circuit are suspects.

The fly-by-wire controls if the 246x use a DA converter to generate a Analog representation of eg. a cursor position, or vertical position, etc.
That Analog value is redirected to the relevant sample-and-hold circuit.
These values are refreshed in a round robin loop.

If the sample capacitor is shorted, or the voltage follower opamp is kaput, that could cause the things you see too.

Just my 2cnts, good luck,

Leo


 

Rogerio,

If the problem is coming and going when the scope is warm or cold, that suggests a broken connection somewhere, maybe a bad solder joint, or (worse) a cracked PCB trace. You can try heating or cooling the board to exercise the bad joint (using a hair dryer, for example, to warm the board. At one time we had canned freon to cool things down, but some the Tek gear specifically warns against using CFCs, and all the freon stuff is no longer available anyhow).

It's still worth tracing the signals between U2301 and U2521, so you know where the bad joint or trace is. Once you know exactly where the fault is you can fix the bad connection, either by reflowing the solder joint, or by bridging the bad trace with a length of wire.

-- Jeff Dutky


Rogerio O
 

Progress!!
I decide to touch all the IC's on A5 board to check for "abnormal heating" and as I touched U2890 the message display appeared.

I repeated this test several times and it always worked, so I decide to replace U2890 as well as R2890 and C2831 as they are close to the IC.

The the message display is "readable" and I can display the cursors.

There are still a lot of dots being displayed on the message area and I don't know is it is a calibration issue or what is described in the troubleshooting chart as "misplaced dots".

I will check U2810 and U2900 as suggested by the chart.


Rogerio O
 

Progress!!
I was able to complete DAC adjustment followed by CAL08 and the dots disappered!!!

I will start the other automated calibration routines.

Thank you all for the help that made it possible for me to get to this point.


Jean-Paul
 

Needs recalibration if dots are displayed. Send a phot.

Run self tests, you will find the notorious fail 05 test 04 loss of CAL constants

Jon


Rogerio O
 

thank you for the reply.
The only error I get is relative to CTT :TEST 87 FAIL 41 (not sure about the fail #).
However, I could not finish CAL01.
I was not able to understand the "highlight" the nth pulse as well as superimpose the DLYED trace,
The display I get is shown in the album Tek 2465BCT CAL01 weird behaviour (https://groups.io/g/TekScopes/photo/257368/15?p=Name,,weird,20,1,0,00.

Can anyone show me pictures of the display that is supposed to appear during the CAL01 routine highlighting the "expected appearance) of the upper and lower traces?
Thank you.


Chuck Harris <cfharris@...>
 

Ok, a couple of things:

The highlighting they talk about is often very smeared... particularly
at the higher speeds. You want the spike from the time-mark generator
to be somewhere in the middle of the highlighted section.

When doing steps 7-16, you want the Delta REF knob to place the highlight
on the 2nd graticule line on the left side of the screen, and the Delta
knob to place the highlight of the specified marker on the 2nd graticule
line from the right side. (it is always the 2nd graticule line from the
edges of the screen )

Note, you are always trying to make the time marks evenly divide and overlay
the lines on the graticule. The 2nd time-mark overlays the 2nd graticule
line (count from left), the 10th time-mark overlays the 2nd graticule line from
the right.

When you do this, there should appear a pair of sped up versions of the
time-mark overlays that move when you turn the Delta REF, and Delta knobs.
You want them to overlay each other as best as you can.

When doing steps 17 - 34, you need a good comparable scope to the 2465B you
are calibrating, to display the signals... This is the "bench" scope.

In this section, they are using your "bench" scope to trigger off of the
"B-GATE" output from your 2465B being calibrated, and both scopes to
display the time-mark generator.

The time-mark generator triggers the 2465B being calibrated's B timebase.
This causes the "B-Gate" to be generated, which triggers the "bench" scope.
This will result in two time marks alternately being displayed on the "bench"
scope. One at the beginning of the gate signal, and one at the end of the
B timebase. Your eye will make you see both on the screen... if you have
everything set up correctly.

The "bench" scope must have a 10x horizontal magnifier.

-Chuck Harris
Rogerio O wrote:

thank you for the reply.
The only error I get is relative to CTT :TEST 87 FAIL 41 (not sure about the fail #).
However, I could not finish CAL01.
I was not able to understand the "highlight" the nth pulse as well as superimpose the DLYED trace,
The display I get is shown in the album Tek 2465BCT CAL01 weird behaviour (https://groups.io/g/TekScopes/photo/257368/15?p=Name,,weird,20,1,0,00.

Can anyone show me pictures of the display that is supposed to appear during the CAL01 routine highlighting the "expected appearance) of the upper and lower traces?
Thank you.