Tek 2465B failed with strange display

[rodd]

Dear all,
I was using my scope and, suddenly the display flickered and started to display a bright spot and four horizontal dotted lines with several bright spots.
The display is not affected by the intensity control, but the readout intensity has some effect on its brightness.(see 1st photo)
I ran the self tests and they all seemed to have passed, but the scope is “frozen” with several leds on and another bizarre display.
I have no idea of how to start the troubleshooting of this problem.
Any ideas?
Thanks

[Uunoctium]

my first guess is: faulty Z-Axis Hybrid

[Testtech]

As always, check the power supply first. By frozen, do you mean that it is unresponsive to buttons and knobs? again, check the PSU. This is an earlier s/n, so it won't have the leaking SMD caps on the control board.
The Z-Axis hybrid won't cause it to be locked up.

[rodd]

Thank you for the replies.
I am on vacation until August 5th.
I will post my observations here as soon aa I start the repair.

[Uunoctium]

tl;dr

Here my brief report of repair. My scope (DVS version) failed with excact the same symptoms. Distorted image, near all LED's lit, no reaction on any key or knob.
I've done the mistake in following the TEK fault-tree. At measuring port (empty IC socket) half of the Voltages were missing or too low (esp the negative ones).
The fault-tree always guided me to failures in PSU. So i ripped it out and recapped it. After an isolated burn-in on workbench it runs as expected. But putting it back nothing of instrument behavior changed.
Due to lack of any brilliant idea finding this fault, i decided to focus on abnormal power dissipation of components. In short, which of the hybrids are running hot?
Closest was the vertical output hybrid. After pulling it, most functions were back. So i replaced it with an used from bay.
Very disappointing! Same fault.
And my second mistake. Not consequently following the logic.
After that i pulled Z-axis hybrid. This gives one the chace to observe the function of Input-amps, Attenuators, horizontal and vertical stages (of course with an distorted image, Vertcal Lines, no blanking...). If i had swapped these steps, i had saved 50$.
So my recommendation is clear -pull the Z-axis hybrid and observe scopes behavior.

p.S. at moment the 3rd Z-Axis hybrid working in that scope
p.p.S. if you decide, repairing the PSU, be very careful at removing the fan wheel! It's a very delicate, fagile unit with no replacement. Here in eevblog exists an excellent howto thread.

[rodd]

Hi folks,
I took out the cover and measured the PS volatges at the empty IC socket and they were all within tolerances.
However, while I was making these measurements the display blinked and went blank.
So I restarded the scope and, to my surprise, the problem vanished!
I have been busy with other matters and had no time to work on the project that needs the scope, but today as I read the answer from Uunoctium, I turned the scope on just to check.
It worked as expected however I noticed that the display sometimes flickers and the trace intensity keeps changing a little.
One time the "dots" appeared just for one second but soon desappeared and did not came back during the 5 min the scope was on.
I will come back here if the problem reappears.
Thank you for all the replies.
Roger

[rodd]

Hi folks,
After returning from my vacations and using the scope a few time the problem reappeared.
It does not seem to be related to calibration constants loss since there are values shown for the voltage and  time/div.
Also, the scope seems to be working (see a photo of the display showing the calibration signal)
However, the dots are there!!!
The dots are not affected by intensity, but their "brightness" can be varied by the REDOUT/INTENSITY knob.
Sometimes a very bright vertical lines appears at the beginnig of the sweep (see photo 2)
Its position can be changed turning the horizontal position knob but the its intensity does not respond to either INTENSITY or READOUT/INTENSY knobs
Can these symptoms be related to a bad Z-Hybrid as suggested above?
Regards,
Roger 

[Ground_Loop]

Although it shouldn't show up during normal ops and through a specific key sequence, that looks a lot like one of the calibration displays. You don't have the calibration jumper pulled do you.

[SoundTech-LG]

I have a 2467B that exhibits this same behavior if left on for hours. It is in a rackmount, so may get warmer
than the non rackmount units. If I turn the power off, turn back on, the problem disappears, only to return later.
I also think it is Z Axis IC overheating.

[FrancisM]

Hi Rodd,

Have a look at this thread as it seems to be the same issue, grid bias slightly off :

https://www.eevblog.com/forum/testgear/tektronix-2465b-mystery-dots/msg3910736/#msg3910736

Francis

[rodd]

Thank you very much for your reply, FrancisM.

You were right. The problem was there!!!!!!!!

When trying to perform CAL08, I discovered that I could NOT adjust the grid bias.
The spot was always MUCH BRIGHTER than the reference dots.

Checking the voltages around the HV PCB, I noticed that the voltage at pin 1 of U1956, was -0.87V instead of the +1.4V as shown on the schematics.

Since I have a spare HV PCB for emergencies, I swaped the boards and the dots disappeared!!!!!!!!

This "unusual high" grid bias was causing all sorts of problems on the scope due to improper behavior of teh A5 board.
I spent hours following signals in the A5 board without success.
I would never guess it.
Thank A LOT

[SoundTech-LG]

Usually the A5 board electrolytics are leaking all over the resistors and traces. Check very carefully.

[rodd]

Thank you for your reply.
But that  is not the case.
As I have mentioned before, my A5 board does not have smd components at all.

[james_s]

Nice job, that's a bizarre fault, I don't think I ever would have guessed the cause.

[rodd]

Maybe some  veteran could step in to bring some light and increase our knowledge.

[FrancisM]

Rodd

Nice catch.
As a guess, I would first check the PSU filter cap C1972 for capacitance and ESR.

Francis