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Hi guys,
First post, please be gentle.
Just ran into this problem with my 2467B, kinda strange. Check the video link with my scope exhibiting the issue.
Ideas?
Thanks
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That's weird, I've never seen anything quite like it. The readout is fine so you know the deflection circuitry for the CRT is working properly.
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Indeed, makes me doubt that the IC U800 may be faulty. I installed a heatsink, no results so I took it off. The IC is Maxim type, I heard theese may not be too reliable. Anyway, thank you for your reply.
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Looks like the horizontal/timebase deflection ramp isn't a ramp anymore at certain positions and runs into a rail or something that distorts it.
If you have a second scope, you can start from the deflection plates (use 10x probe, usually somewhat high voltage) and work the signal backwards towards the fault. I wouldn't fiddle too much with U800 until you're certain that the fault is in there for sure.
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Although I'm not sure what is causing that, but it "looks like" the horizontal is doing something weird.
Check this thread, especially the video to confirm whether your U800 is sick -> https://www.eevblog.com/forum/chat/tektronix-2400-series-scope-hoz-output-ic-failure-howto-confirm/ -
Time to get out the Cold Spray and start cooling components one by one....
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Is voltage at Pin 24 of U800 stable during X10 mode?
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Thanks for the answers guys, I appreciate your time.
I think I found the bad part, it is the U800.
Check the video
Later on I sprayed a bit more the IC and I even magnified by 10 with perfect results.
I already took the scope apart and extracted the IC in perfect conditions. To tell you the truth I was a bit nervous about taking out the main board but it turned out to be reasonably easy. I mean it is not child play but it is not rocket science either.
I'll let you know what will happen further.
Cheers -
I think I found the bad part, it is the U800.
Thanks for the video
, now we have two videos validating on how to detect sick U800.
As you mentioned in the video that you had ordered the U800 replacement chip, if you feeling adventurous, check this -> Alternative U800 replacement
Looking forward to see your result on your scope fixing at U800. -
I'll keep you informed.
By the way, how do you guys feel about mounting the U800 in a socket instead of soldering it directly on the main board?
I don't want to replace the IC only to discover that it is faulty and then to have to desolder it again. Don't know what to say about these second hand IC's available on ebay.
Cheers -
I'll keep you informed.
By the way, how do you guys feel about mounting the U800 in a socket instead of soldering it directly on the main board?
I don't want to replace the IC only to discover that it is faulty and then to have to desolder it again. Don't know what to say about these second hand IC's available on ebay.
That is what I would do. If you solder individual collet pins into the circuit board instead of using a socket, then the additional parasitic elements will be very small.
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@David Hess
Thanks for the message. Please read on.
A quick update on what I did already.
I decided to mount the new chip on strips. I thought that mounting the chip on a conventional socket would somewhat restrict the air flow under the chip. Remember, the chip does not touch the mainboard even factory mounted, there is a space between the chip and mainboard just enough to fit that washer under the cooling sink.
Since the old Maxim chip is still usable until certain speeds, I decided to mount it in the strips and put the scope back together to see if it works. At least until the new chip arrives. I was eager to see if I messed it up during the replacement procedure.
Scope assembly worked ok with no problems. Powered it up and yes, still works.
Set the speed to 10ns on a 50Mhz wave, positioned completely to the left and as expected the wave started to behave exactly like in the videos I already attached.
HOWEVER...
I left it running and went to the kitchen to get a cup of coffee. When I returned in the room I checked again the behaviour by moving the position knob all the way to the left. Surprise surprise, no more drifting.
At first I thought I left it running at a slower speed but when I checked the readout it was at 10ns. Set it to 5ns, no drifting. However, magnification does the same mess to the wave.
This may or may not be good news since I don't have an explanation of the phenomenon.
As I mentioned before, after I put it back together the scope behaved as expected. I mean showing the expected fault. Now only magnification seems to do something bad there. If I magnify a wave displayed at 100ns, the magnification gets it to 10ns with the messed up wave. But without the magnification the scope behaves correctly at all speeds.
Thoughts?
Added three pics showing the chip before desoldering, the strips in place and the chip in place in the strips.
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I would guess there is an intermittent connection within the IC and as various parts heat up the connection occasionally closes and it works. The problem is likely to gradually become worse over time.
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Could be.
I rechecked with the freezing spray. But this time I had to be on magnification to observe the effect.
However, as soon as I sprayed the chip the problem was corrected. Yes, I need to change the IC but I just needed an explanation I guess. It is strange that a connection inside the chip became unstable. Could be that the stress on the chip legs was modified as I mounted on strips? I only installed the retaining nuts using my finger force, very little... -
I have always thought that the problem with the large tab style DIPs in these oscilloscopes is how Tektronix mounted them causing mechanical strain. It is like the physical design was done before checking on the requirements for DIP packages. For instance the widening of the DIP leads prevents the tab from sitting flush with the board.
Adding low profile collet sockets and then a thermally conductive spacer would allow flexing of the leads and provide better thermal performance from the exposed pad on the bottom of the package. On the other hand, Tektronix could have used a spacer and then solder the part to the board but they did not do that either.
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It would be interesting to know more about the failure analysis of integrated circuits, it's not something I have a lot of knowledge about. Given they are physical devices that expand and contract with thermal cycles though it would not surprise me if conductive layers and junctions can separate over time. An IC die has many layers of different materials, it's not just a monolithic chunk of silicon. There are also the bonding wires and leads, it's probably possible to have bonding failures.
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Well, you're not gonna believe it, it works perfectly at all frequencies, speeds and magnification.
Wild guess: could be that the chip being in a relaxed position from the mechanical stress point of view actually found it's best position? I mean, I don't wanna talk cartoons here but it's strange at least.
I will keep this thread alive and let you know the future developments.
Cheers -
It would be interesting to know more about the failure analysis of integrated circuits, it's not something I have a lot of knowledge about. Given they are physical devices that expand and contract with thermal cycles though it would not surprise me if conductive layers and junctions can separate over time. An IC die has many layers of different materials, it's not just a monolithic chunk of silicon. There are also the bonding wires and leads, it's probably possible to have bonding failures.
These are actually encapsulated hybrids and not integrated circuits. External signs of failure include cracks where the encapsulation meets the pins and tab. Internally I think the problem was separation of the die attachments which makes sense if the tab was being bent.
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Is voltage at Pin 24 of U800 stable during X10 mode?
Drifts can be from elsewhere, the spray may have cleansed it wiping out the trails. -
Don't know about you but I feel like I discovered warp engine.
I imagine many scopes ultimately failed because of this notorious U800 and in fact remounting the chip in a correct manner may be a solution. I cycled the power a few times to observe the behaviour at different temperatures but the scope operates perfectly... -
One day later.
Powered on the scope, checked all frequencies and magnification, flawless.
I'm working on my Deluxe Memory Man delay pedal with the old bucket brigade chips to perform the trims alignment.
Sadly, I only need a 250Hz sine wave...
What a beautiful day!
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Last video of the 2467B U800 failure saga.
Hope this will be useful to other users too.
Cheers,
Adrian
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Thanks for the update video.
While watching your video, I was a bit worry on heating up those hybrids as you don't have a big fan blowing the A1 board while without the case, even though you mentioned it at the video.
Its just its way too long for my personal taste
, as usually it was less than 3 minutes without the fan for my 2465B when turned on naked like that, guess I worry too much. 
Honestly, this thing was keeping pop out in my mind when watching that video ..
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This type of defect happens especially to U800 manufactured by Maxim.
In desperate cases, if you can't find the U800 (Tek p.n. 155-0241-02), you can resort to this possibility also:
http://www.sphere.bc.ca/test/tek-parts/tek-info.html
(look in the middle of the page about for 'The Tek 155-0241-02 Re-Bake Program') -
Thanks Adrianni for the videos and the information. I also get the same problems from my Tektronix 2465b which I bought a year ago and I barely used it; and although its number of hours on duty reported is reasonable, I forgot it.
As soon as I got it from ebay, I clad it with a heat sink and replaced a lot of caps.
Now I also get among trig'd flashing, error 04, Fail12??? A lot of
? Down the readout, and no sooner I turn it on, than the trace disapears.
I wonder what sense it makes placing a precious and vital Chip that glows hot during operation in such a poor cooling environment! What sense does it make fastening it with bolts and screws when it is soldered and while the glowing bottom is seperated from the cooling surface! It is instead connected with the PCB copper area through two star washers!??? Hanging in the hot stream of air!
Last nigt I made an adventure and dismantled it to get the A1 board, not so easy especially managing to loosen the jammed screws in the front and keep the Delay Line laid on the pins of the Chip out of the way! I managed to desolder the Chip. I intend to place it on a socket and to thermally couple it to the PCB underneath it by a copper plate bound to an Aluminum heat sink on top.
If I still get problems I will think of designing a horizontal amplifier no matter the digital sacrifice, and modifications, I have no other choice get the best of its analog merits!
Last night I said to myself I could be happier with a "plug and go" and reliable purely analog Tek485 or a Kukisui COS6100. I miss the Philips PM3295 which had a minor repairable power failure, I sold it to get this Tektronix!