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I got a faulty Tektronix 2213 analog oscilloscope and am hoping to repair it. When it should be showing a straight line, it shows this crazy waveform instead. This results in severe distortion when viewing any signal. distorted trace

That picture should be a straight line, the input coupling is set to GND. Here is a picture of the settings: oscilloscope settings

The trace looks different on different settings, it gets bigger at smaller volts/division and gets wider with smaller seconds/division, exactly how a real signal would respond. It can even trigger off of this "signal". It looks the same on channel 2 as well. It looks like it might even be affecting the Z-axis, notice the varying intensity, although it does not seem to correlate with the amplitude of the signal the way you would expect if it were indeed being modulated by the same signal.

I checked the voltages inside and they are all slightly high (except for the regulated 5V line), out of tolerance according to the service manual. I think this is due to my power line being 120V instead of the 115V the manual specifies for testing. Unfortunately I do not have a variac to test this theory.

Any ideas would be greatly appreciated, I'm about to start just trying each test point from the manual and seeing if the waveforms match, but knowing what area to focus on would be a big help. Here is the service manual: http://w140.com/tektronix_2213_service.pdf

I do have another good oscilloscope to troubleshoot this with. I've hooked up a function generator and am trying to trace the signal to see where the distortion begins but it is very difficult to probe the front end from all the stuff in the way. I would need to remove the attenuator board to reach most of the points and it plugs straight into the front panel board- I don't see a way to probe most of it while it is operating... but I'll keep trying!

Something is definitely also affecting the 1KHz calibration output. The trace will not stay still on the scope I'm using to measure it and it is triggering at around 45KHz and the top and bottom of the square wave look incredibly noisy. I can't tell if it is exactly the same noise modulating it...

I've taken some noise measurements of the power rails- they look reasonable except for the -8.6V line having 700mV of ripple at 50kHz, and the 5V line having 1.2V of ripple at 50kHz!! I think this severe ripple could be the source of the problem and I'm going to investigate the power supply now.

Thanks!

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closed as off-topic by Leon Heller, Bruce Abbott, laptop2d, R Drast, PlasmaHH Nov 13 '17 at 9:05

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the repair of consumer electronics, appliances, or other devices must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being repaired. See also: Is asking on how to fix a faulty circuit on topic?" – Leon Heller, Bruce Abbott, laptop2d, R Drast, PlasmaHH
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ In David Jones voice Beauty! \$\endgroup\$ – winny Nov 12 '17 at 19:30
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    \$\begingroup\$ To my opinion, this isn't the right place to post this question. Please consider eevblog.com instead, there are plenty of threads on scope fixing. Thank you. \$\endgroup\$ – Verbal Kint Nov 12 '17 at 19:33
  • \$\begingroup\$ I'd start tracing signals and seeing if the waveforms match, myself. Do you have another scope you can troubleshoot this one with? \$\endgroup\$ – ThreePhaseEel Nov 12 '17 at 19:37
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    \$\begingroup\$ What I can tell you is that the line voltage isn't the problem. US line voltage is 120V±5%; 115V is within that specification. \$\endgroup\$ – duskwuff Nov 12 '17 at 20:43
  • \$\begingroup\$ Something you could do from outside the enclosure is to use the other scope to see if this high frequency noise is also modulating the 1khz calibration output. \$\endgroup\$ – Chris Stratton Nov 12 '17 at 20:52
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Check if the unwanted waveform is 50/60/100/120Hz.

If this is the case, suspect a huge ripple on the power supply of a specific stage, perhaps due to a dry electrolytic capacitor.

If it is in the 20-50 kHz range, suspect a dry/dead cap at the output of a SMPS.

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    \$\begingroup\$ Nope, look at the settings- that waveform is ~62kHz. :/ I am checking the power rails for noise though now, don't know why I didn't think of this before, thanks for giving me the idea. :) \$\endgroup\$ – HOLOGRAPHICpizza Nov 12 '17 at 23:22
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    \$\begingroup\$ Television set nearby? That's close to 4x the flyback frequency of PAL/NTSC. \$\endgroup\$ – matja Nov 12 '17 at 23:47
  • \$\begingroup\$ @matja Nope, the scope is the only CRT running in the house. That would blow me away if a TV was causing interference this severe, but I wouldn't be too surprised. I worked at an analog TV station during college and spent a week trying to track down some strange interference. It turned out it was the only CRT in the building not genlocked to the rest of them- the one ordinary TV set we had to monitor our broadcast on the cable network! I turned it off and our broadcast signal immediately became crystal clear!! \$\endgroup\$ – HOLOGRAPHICpizza Nov 12 '17 at 23:57
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    \$\begingroup\$ I've just realized the seconds/division fine adjustment pot is messed up and I don't know where the "home" position is so my frequency measurement is meaningless. I found 50kHz ripple on the power rails so now I'm pretty sure the bogus signal is also 50kHz. \$\endgroup\$ – HOLOGRAPHICpizza Nov 13 '17 at 0:10
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    \$\begingroup\$ OK, it has a switching supply - eevblog.com/forum/repair/tektronix-2213-power-supply - so a dry or dead capacitor would explain 50kHz ripple on one of the rails... \$\endgroup\$ – peufeu Nov 13 '17 at 8:14
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You are lucky and unlucky at the same time.You have a service manual. Unfortunately you need another oscilloscope to fix this.

Obviously the sweep and the y deflection work at least somehow, but some bad unwanted reqular signal exists for some reason. Be it a faulty component, broken joint, short circuit or dirt - you must find it.

Fix this yourself or get local help. An analog oscilloscope doesn't get more convenient to use. I used it's brother TEK465 as long as there were proper maintenance available. One day my boss brought it away because the job couldnt stand unreliable instruments and keeping TEK465s in full condition had become too costly.

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