Tektronix TDS340 with noise on channel 1 only, where to start? Schematics?

[cincin]

Hi, noob here,

I have an old Tektronix TDS 340 (w/ Option 14) in great condition. It's clean in and out has obviously been well taken care of, was calibrated in 2018, and passes all diagnostic self tests and calibration.

The one flaw is that channel 1 is a bit noisy compared to channel 2.

I know that these can often use new caps. While the noise is not *necessarily* caused by bad cap(s), I think it's a good place to start.

My question is: Since the noise is only on channel 1, I'm guessing the problem probably isn't on the power supply board, that would affect both channels, but which area, or which caps, are past the channel separation? Or generally which caps should I suspect first?

Also, if anyone has the schematics for this unit, that would be great. All I can find is the TDS340A Technical Reference manual, and I don't know how different those 2 units are.

Cheers!

[bdunham7]

Throwing caps at it is not an efficient way of solving the issue, even if it turns out that a defective capacitor is the issue.

The place to start would be to characterize the noise.  Compared to CH2, how much noise, at what frequencies, periodic or random, etc.  Maybe a screenshot at the lowest V/div and timebases 10ms/div, 1ms/div, 1us/div and 50ns/div?

I don't know the differences between the TDS340 and TDS340A.  There's probably enough similarity that you can start with the TDS340A manual.

[Wallace Gasiewicz]

Does your scope have relays in the attenuator? I think it does and the attenuators are a source of noise. You can see if the noise is on all ranges to see if there is a dirty attenuator on one range.

[cincin]

After closer inspection (by which I mean the elimination of noob-level user error), the noise is the same amplitude on both channels. Now, wether or not this noise is within acceptable range is my next question. I'm looking at about 800uV of random noise on both channels. The noise signals on the 2 channels don't cancel themselves out (are not the same, or reciprocal). The detected frequency varies quite a bit, and the wave can't be pinned by the trigger, which I imagine would be the case trying to detect a frq in noise.

The middle line is the math result of Ch1 + Ch2. I should add that this is all done with no probes attached.

It's present on all vertical and horizontal scales, obv looking smaller on larger V/div:

2mV & 1uS


2mV & 100uS


2mV & 10ms

[bdunham7]

My first impression is that nothing looks abnormal.  You might try it with a short (a 50 ohm load will do) across the inputs as high impedance inputs are naturally noisy when open circuit. 

[james_s]

That looks pretty normal to me. Oscilloscopes aren't normally used to measure microvolts and the TDS300 was an entry level model by Tek standards. I think your scope is working fine.

[cincin]

Thank you!