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I'm using an old analog oscilloscope (Metrix ITT instruments OX 725).
(I don't have a printed manual for it, but you can find scans of a French user's manual and service manual (with circuit diagrams) online.)

At some point, it has developed a fault, where the visible image seems to wobble and flicker vertically. Sometimes, it's a smooth movement up and down, sometimes it's quick flickering, sometimes both: (animated gif, may take a moment to load)

Here, both inputs are set to GND (instead of DC or AC coupling), and it affects both of them almost the same way, so I suspect the problem is somewhere with the vertical deflection of the CRT and not the signal input.

The strength of this effect varies while the oscilloscope is on (sometimes, it's not visible at all for a minute, then it only slightly wobbles, then it flickers strongly again).

Does anyone with more experience have an idea what could be wrong or how to troubleshoot this? (If it's just a loose connector or faulty capacitor I would trust myself to be able to fix it, but anything else is probably out of my league.)


Edit: If it's helpful, here are some photos of the inside of the 'scope. I can't spot any obviously faulty components, and I haven't found the courage to poke around in there.

Overview:

Mainboard & power supply:

CRT board:

Front panel boards:

Underside:

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    \$\begingroup\$ Your suspicion of an old capacitor should be investigated. Power supplies in these old analog 'scopes are complex, containing many electrolytic filter capacitors. A shotgun approach to replacing them would be onerous. \$\endgroup\$
    – glen_geek
    Oct 26, 2023 at 15:01
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    \$\begingroup\$ Old 'scopes also suffer from noisy front-panel variable resistors. If you tap, or rotate the vertical position control, is display affected? Volts/Div switch is also suspect for noisy contacts. Sprays are available - they might help. \$\endgroup\$
    – glen_geek
    Oct 26, 2023 at 15:16
  • \$\begingroup\$ How is synchronized the scope? Don't see the right of the scope ... \$\endgroup\$
    – Antonio51
    Oct 26, 2023 at 17:50
  • \$\begingroup\$ Thanks for your suggestions. @glen_geek: The front panel knobs seem to have no effect on the noise (beside the act of switching the volt/div producing a short burst of noise when it clicks into place, which I think is normal). Have not dared to open it yet 😅 \$\endgroup\$
    – Fii
    Oct 26, 2023 at 20:47
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    \$\begingroup\$ Do you see any electrolytic capacitors that appear to be bulging? Or any film capacitors with yellowing plastic branded RIFA? (Rifa capacitors are notoriously poor quality and should always be replaced as a matter of course--they tend to crack and even sometimes explode after a few decades, even when their ratings aren't exceeded.) \$\endgroup\$
    – Hearth
    Oct 27, 2023 at 15:19

1 Answer 1

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Those two vertically-mounted PCB contain front-end circuits for:

  • Channel #1 vertical amplifier
  • Channel #2 vertical amplifier
  • Timebase for sweeping
  • Triggering?

Those two vertically-mounted PCB are likely powered by low-voltage DC - fairly safe to probe.

CRT traces are jumping vertically in intermittent-fashion. However, note that they jump in tandem, so whatever is making them jump is common to Channel 1 and to Channel 2. It is not apparent if horizontal sweep has been set to alternate or to chop. OP's camera video may also affect this.

Tandem-jumping may mean that this intermittent noise is introduced later in the Y-channel signal chain (after alternate/chop multiplexing)...right up to the connection to CRT's Y-deflection plates.
Tandem-jumping might also involve a DC supply to CRT at its back end (a dangerous spot to probe). It might also involve low-voltage DC supply to those two vertical boards.
annotated OP image Much dust has collected - ingress into switches and into those small black trimpots might cause intermittents.

Any wiping contacts might have oxidized - simply exercising these (breaking through the oxide) might be a fix. It is possible that vertical CRT plates are connected directly through the CRT's glass envelope (under the metal shield) rather than through its back-end. Hard to see from those excellent photos.
You might risk re-seating C.R.T. connections (V-plates) and back-end CRT socket. Wait a long time after the power plug has been removed before attempting this.

Are those two vertically-mounted PCB plugged into the horizontal PCB (rather than soldered)? If so, re-seat these. If soldered, carefully check that flexing hasn't damaged these joints.

Satisfy yourself where 2kV voltage exists. Seems that most danger is confined to the 'scopes back-panel area - you might shield this region with some non-conductive cover to keep hands, wayward tools from contact while you troubleshoot.

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  • \$\begingroup\$ Thank you for this comprehensive reply and the advice. After carefully wiggling components in the running oscilloscope with non-conducting pliers and turning some of the relevant trimmers back and fourth, I can't identify any component that has influence on the noise (although it's hard to tell, since the noise pattern changes so erratically). Unfortunately, the vertical PCBs behind the front panel are soldered on the main board (I've also added a picture from the bottom now), connected with a bunch of shielding and seem very hard to dis- and reassemble without damaging something. \$\endgroup\$
    – Fii
    Nov 3, 2023 at 21:17
  • \$\begingroup\$ I noticed one more clue though: activating the beamfinder (which reduces the image to a flat, central line) produces a very stable image. This will probably help to narrow the problematic area down, but I don't understand some of the symbols used in the circuit diagrams and have to post another question about them. I fear though that the problem ends up being in these front PCBs. I think I could reach a few more parts by removing the front cover, but that could also be important shielding? I may also try to contact the company who owns Metrix now, maybe they still have old service docs. \$\endgroup\$
    – Fii
    Nov 3, 2023 at 21:27
  • \$\begingroup\$ Two more points regarding your answer: - The front PCBs barely flex even by hand, since they are fixed to the chassis with a metal rod going through the center of their top sides. The solder joints look fine to me. - In your annotations of the image, you said to clean the rotary switches. How would you do this, besides exercising the switches? Compressed air? Maybe try to get some isopropanol droplets in there and then rotate them again? \$\endgroup\$
    – Fii
    Nov 3, 2023 at 21:31
  • \$\begingroup\$ Fii, Rotary V/cm switches get a lot of exercise - sometimes get noisy. Disassembly would be a royal pain. Simultaneous noise on both channels suggests a noisy switch or trimpot or contact elsewhere. You might work backwards from vertical deflection plates (stage by stage). That noise is jumping slowly enough to show up on a multimeter, making its needle twitch. Do be careful around vertical deflection plates - they are at a somewhat high voltage (but not kilovolts). I'd be reluctant to tackle this without a schematic. So close to useable - worthwhile poking around. \$\endgroup\$
    – glen_geek
    Nov 4, 2023 at 1:58

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