Tektronix 2221A Screen/OSD issue?

[Fractal]

Hello!

Following my old original thread from last year where I replaced the blown-up paper capacitor, the main screen issue remains unsolved and I'd like to see if anyone could help me out with this one specific problem.

Here is the failure mode of the scope:

• On cold start, it boots up and functions perfectly well; all buttons, all knobs, store as well as non-store, everything works like a dream
• After about 5mins, the screen dies. store/non-store makes no difference, and adjusting brightness makes no difference. Dead screen. BUT, pressing beam find shows a bright spot. Weird.
• Upon turning it off and on again, the screen comes back, seemingly in non-store only (analog) BUT there is no OSD of any kind. Pressing store/nonstore does nothing, it seems to stay in a fully functional analog mode except without OSD. Horizontal and vertical scales can be changed without any issue. Turning the intensity knobs, only the non-store changes the signal strength.
• After turning off giving it some time to cool down, it will work perfectly fine again, getting back to the 1st point above.

I would guess towards a thermal issue? But it would need to be something specifically related to the digital part of the circuit?


Any pointers would be greatly appreciated; I love using this scope but having only a few minutes to get a measurement before it goes dark makes it quite unusable unfortunately!


Thanks!


I've linked below a gallery of all the work performed so far, what the internals look like as well as some videos of the failure mode.

https://photos.app.goo.gl/5R1W59mDrAtyf3R36


Cheers!

__Yves

[Fractal]

Bump!

No proposal?

Could the caps from 1984 cause anything like that? Thinking of starting with a full recap but the issue doesn't seem consistent with what bad caps would produce?

Any help much appreciated!

[bdunham7]

I have one of these that I repaired from poor condition and I still use it on occasion.  It's been a few years, but it can be quite challenging to get into the guts and really poke around.  Another challenge for me was that there wasn't a specific 2221A service manual available, I had to work from the 2221, 2230 and 2232 manuals.  I see that a good scan of the 2221A manual is now available on Tekwiki. 

I'd suggest first verifying all of the power supply voltages when the failure occurs.  After that I'd have to think a bit.  I'm busy right now but if you don't get anywhere I'll take a look in the next few weeks.  There's a way to bypass the digital board entirely IIRC, but I don't remember the specifics.  Here's the manual if you don't already have it:

https://w140.com/tekwiki/images/e/e5/070-8157-00.pdf

[Fractal]

Woah my first attempt was based on what I could figure out from the only other 22 series scope service manuals available back then

Finally having a matching one is a tremendous find; I see now that the top board appears to be dedicated to store more; which makes me think I should definitely start by monitoring power to it specifically as the issue appears

I will try that and report back; thank you

[Fractal]

Ok!

So following the manual's PSU limits I went on to measure the reference outputs and here is what I've got:



Test Pt. /    Ref    / Measured
W961   /  -8.6V   /  -8.599 OK
W9020 /  -5.0V   /  5.124 OK?? Why positive?
W9068 /  +5.0V  /  0 - SHORT TO GND
W960   /  +8.6V  / +8.616 OK
W956   /  +30V   / +29.99 OK
W954   /  +102V / +101.8 OK

So I'm seeing 2 issues; the +5v being short and the -5V being inverted

A complete rail failure seems weird as to why would the score operate perfectly fine for a while, but makes sense as to why it would fail following up.

Getting it in failure mode was in fact not easy with the cover off!

I had it run for 2h without the issue appearing

It was only after blocking the fan exhaust that I could reproduce the fault, which led to no change in the PSU rail voltages.


So I guess the next step would be to find that short on the 5V rail; should be somewhere on the PSU area;

Any tips on finding it out? I didn't find any visually obvious sign of faulty component under there the first time

Cheers!

__Yves

[bdunham7]

The first step would be to unload the + and -5V supplies to see if they are faulty or simply being overloaded by downstream circuitry.  There's a section marked 'Power Supply Isolation' in the beginning of the Diagrams section.  I haven't done this myself and don't know the details.  I'm surprised the scope would work properly (cold) with no +5V supply, but perhaps it wasn't completely OK?

[Fractal]

Nice; I could find the PSU Isolation mote but couldn't quite find a suitable break point

On the other hand I measured that C635 cap that seems directly in line with the 5V probe point and it came out at 5K

Could that guy be the culprit?