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As already mentioned, I have a new-to-me 5S14N. Yay.

So far it seems that the horizontal section and triggering work as expected but it has a few problems in the vertical section.

I certainly haven't done all my troubleshooting homework yet, but I thought I'd post up some early findings to see if anyone already knows what they indicate.

As received, it had good bias cells in channel 2 and dead cells in channel 1. Not 0.0V dead but sub-0.1V, iirc. Pretty sure they were under 0.5V. I didn't record the numbers.

Channel 2 looked imperfect but basically functional -- square wave tops not flat but low noise, about expected amplitude allowing for feeding both ch2 & ext trigger, and <500ps rise/fall from a source that's spec'd not quite that fast. Using a 1x probe to snag the ext trigger input probably contributed to distortion.

Channel 1 was a flat line, which made sense for dead bias cells, but responsive to DC offset control, and close to display center with the control centered.

Intermittently the inputs appear to self-oscillate, even with nothing connected. The last time that happened a power cycle fixed it. About 0.5-1V amplitude iirc - similar but not the same amplitude between channels - looks like it might be a sine but not stable enough to distinguish a clear waveform vs aliasing - suggesting a frequency close enough to some harmonic of the scan rate to drift in and out of looking confusingly almost like signal. While typing this, I suppose that indicates that it's happening after the trigger pick-off -- which I suppose couldn't be otherwise because it's all passive until after the trigger pick-off. ?. Anyhow, I mention it because it's there, but seems like a secondary issue at this point because most of the time it doesn't do that.

I tried swapping the sampler boards to put the good cells on channel 1. Internal triggering worked fine. Square waves looked more distorted than before, even without the 1x probe in play, but I figured that could be due to mismatch between the ch2 sampler and the rest of the ch1 circuit until re-adjusted. ?. I've since un-swapped the samplers.

On to more disabling faults:

As described elsewhere[1], I replaced the bias cells in ch1 with silver oxide cells. Nominally 1.55V but measuring 1.59V fresh from the blister pack. That did not magically fix ch1, and now ch1 behaves differently. With the cells removed, which I'd expect to be like it was with dead cells, the ch1 trace is pinned high off screen and unresponsive to the offset control -- as revealed by the beam-finder button. With the cells in place, the trace shifts off the bottom of the display, but responds to the upper range of DC offset and can be shifted up to about 1/3 up the display at maximum DC offset and 0.5V/div. At higher sensitivity (0.2V, higher?) the beam-finder shows the trace responding to the upper range of DC offset but not enough to bring it into the display area. When the trace is visible (0.5V/div, max offset), the unit can trigger and show a stable but noisy and attenuated signal.

I've read that vertical trace shift can indicate fault with the sampling diode pair. If indeed operation with no cells is like operation with dead cells, then this wasn't a fault before I replaced the cells. I'll be a little disappointed if I killed the diode(s) myself, after trying to not do that.

And then a couple things seem kind of strange and maybe specific enough to suggest specific faults.

When ch1 trace is visible (0.5V/div, max offset) and no input connected, the trace shows ~50mV (1/2 minor division) "noise". But not really noise. It appears to alternate between two distinct levels about 50mV apart, at irregular intervals that look like a few 10s of transitions/sec i.e. a random-ish distribution of intervals clustering around a few 10s of msec. Noise on ch2, visible at high sensitivities, looks like ordinary noise.

The next strange-seeming thing is that the "LO NOISE" function shifts the ch1 trace up the display and amplifies it about 2x, in addition to reducing noise. The up-shift looks like less than doubling the large DC offset needed to shift the trace into view. The ch2 trace does not get shifted or amplified. (for completeness: at high sensitivity such that the no-input trace shows noise, the ch2 trace shifts up a fraction of the width of the noisy trace, i.e. noise-reduced trace runs above the center of the normal trace blur, but I assume that's within normal adjustment)

The ch1 up-shift brings the top of the offset range for a no-input trace at 0.2V/div into the bottom half of the screen and the 0.1V/div trace into the bottom division at max offset. Shifting the higher sensitivities into view, together with the slow sweep and mysterious ~2x amplification the LO NOISE mode, gives a better look at the mysterious bi-level "noise" that isn't noise.
Single-sweep traces clearly show irregular alternation between two distinct levels that appear about 100mV apart (about 2x the apparent difference in non-lo-noise view). In LO NOISE operation, it looks like sweeps run about 3/sec, making each horizontal division represent about 1sec/30. The distribution of irregular intervals of alternation appear to cluster around 1 div (i.e. shortest intervals probably not visible and longest intervals <10div), which would be consistent with the appearance of ~tens of level-switches/second. I've posted a photo of three single-sweep LO NOISE traces at 0.2V/div separated by different DC offsets: /g/TekScopes/photo/270747/3351116. It looks like level-switches happen at ~random intervals with a distribution around ~1/div. Assuming the pictured sweeps show about ~1s/30 per horizontal division, that would be consistent with the subjective impression of a few 10s of alternations/second. At the same time, e.g. dual trace display, ch2 at high sensitivity shows what looks like actual random noise.

Having only one scope complicates troubleshooting. I have access to other scopes but I have to take myself and stuff to them with some inconvenience. I'm thinking of making up a partial Y extender (I have edge connectors) to power to the 5S14N from a single vertical slot while using the scope in normal Yt operation with two plugins. Assuming it won't be hard to persuade the unit to operate like it had normal signal connections also. But I don't know how much power the plugins draw vs what the mainframe or a single slot connector can deliver. Seeing that the unit uses the HV supply also ups the ante for build quality if I attempt that :-/

I've written this from memory, so I might end up correcting details later.

Any insights appreciated!

[1] /g/TekScopes/topic/7s14_5s14n_bias_supplies/87551264 ; /g/TekScopes/album?id=270597

keyword: 7S14