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I recently acquired a C1-5 soviet-made oscilloscope in unknown condition that has been sitting in a workshop for 18 years. I was not able to find a manual.

When turning it on (y-input and x-input disconnected) a single dot appears on the screen. Changing the time-base setting does not make the dot sweep horizontally. However the y position of the dot responds as usual to frequency inputs. The only way of changing the x position currently is using the x shift knob.

I tried to diagnose the problem by connecting a 100Hz square signal. This input results in two dots that are vertically apart in one single x position.

My current assumption is that some of the hardware responsible for the x-deflection is broken. This would explain why the shown signal is in a reasonable y-position but in a semi-constant x-position.

Any help is greatly appreciated since I am not very familiar with the analog kind of oscilloscopes. Let me know if I should provide pictures of the output on some settings that could help anyone diagnose the problem.

(A picture of the front control panel) picture of control panel

(A picture of the back control panel) picture of back control panel

Johann Höpfner
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    Is it stuck in xy mode? – Bryan Aug 20 '23 at 15:21
  • “X-input disconnected?” Put a signal on the x-input. See what happens. – RussellH Aug 20 '23 at 15:50
  • [It looks like this is the manual.](https://elektrotanya.com/russian_c1-5_0,3vrms-25mmp-p,10mhz_universal_oscilloscope_sm.pdf/download.html) There's also a service manual. – JRE Aug 20 '23 at 16:14
  • Oops. That only the service manual. – JRE Aug 20 '23 at 17:04
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    @Bryan I edited the question to include an image of my front panel. I do not see a x-y mode select. I might be missing something... – Johann Höpfner Aug 20 '23 at 17:29
  • @RussellH I will try for diagnosing purposes as soon as i get my hands on a waveform generator or anything to drive the x input. However I would assume that given the fact that the scope has a trigger select it should be able to provide its own sweep signal. – Johann Höpfner Aug 20 '23 at 17:30
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    True, But this test (in x-y mode) bypasses all the horizontal time base circuitry and tests the horizontal amplifier and drive circuitry separately. If it works, the problem is with the time base. If not then the problem is in the horizontal amplifier/drive circuitry. – RussellH Aug 20 '23 at 17:37
  • @RussellH is your suggested test as simple as connecting a signal to the x input (picture above) or can you spot any mode select? – Johann Höpfner Aug 20 '23 at 17:40
  • From the front panel picture, what I call x-y mode is probably the AMPL (amplifier) setting of the triggering mode. – RussellH Aug 20 '23 at 17:41
  • @RussellH I wrongly assumed it meant triggering at a specific amplitude. So the steps are simply: 1. select Ampl. on triggering mode 2. connect signal to x – Johann Höpfner Aug 20 '23 at 17:44
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    You can check the active elements to see if the their built-in indicator light has gone out (aka vacuum tube filaments). Be careful, there's HV (>1kV) inside at substantial available current. Interesting service manual translation- there's even winding instructions for the power transformer. – Spehro Pefhany Aug 20 '23 at 17:49
  • @RussellH I set the Trigger to amp and connected a 6V DC source (I currently do not own any waveform generator) to the x probes. The dot does not move. – Johann Höpfner Aug 20 '23 at 17:52
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    Ok. From the manual it appears to be an external syncronizing/triggering input. The "X" labeling is interesting. That's all I have. I would start by replacing the tubes in the timebase path. – RussellH Aug 20 '23 at 18:01
  • Is it correct that with "auto" Trigger mode and a low frequency selected in "time base" i shoud not see a single point but instead a horizontal line across the screen? Or is there any way i could have messed up the settings to cause this behaviour? – Johann Höpfner Aug 20 '23 at 18:08

1 Answers1

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If you have another oscilloscope… anyway.

Usually the things that go bad on old gear are switches and potentiometers. Thus, check the h-mode mode select switch and clean its contacts.

The next thing to look at is the h-oscillator. This makes a sawtooth waveform to generate the sweep (it also should have a blanking signal to black out the retrace.) It could have a failed component, like a capacitor or such.

The x-deflect amplifier may also have failed. Put the scope in XY mode and drive H with your square wave. You should see two horizontal dots instead of two vertical ones like when you drove V. If not, the amp is bad.

hacktastical
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  • I added a photo of my control panel to the question. I am so far not seing any "h-mode select" but might be missing something. Thank you for your suggestions so far. I am going to try them when i find a way of changing modes. – Johann Höpfner Aug 20 '23 at 17:33
  • I tested driving the X-deflection. It has no effect. Are there any settings for the X-Amp gain that might be wrong? I will try testing the contacts of the switches next and then move on to the tubes as suggested by other commenters. – Johann Höpfner Aug 20 '23 at 19:15
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    Operating this device in XY-mode requires the following settings: Triggering mode = AMPL; Trigger slope = EXT; X input = Your H-sweep signal (try that 100 Hz square); Sync = Desired amplification of X input (set to '0' in your photo?). – Igor G Aug 21 '23 at 02:06