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468 With Odd Trace Timing issue when in Vertical ALT Mode Only.

DaveH52
 

It was just a guess. I saw a cable going to the trigger input.

Mlynch001
 

On Sat, Nov 2, 2019 at 12:40 AM, Mlynch001 wrote:


And it must be something which does not light the TRIGGER LED when this
happens.
Should have said: "And it must be something which does not ALWAYS light the TRIGGER LED when this happens."

It seems to happen regardless of the trigger control position or whether the trigger LED is on or off.

I'm still going to take a look at the trigger circuitry.
--
Michael Lynch
Dardanelle, AR

Mlynch001
 

On Fri, Nov 1, 2019 at 11:48 PM, DaveH52 wrote:


I'm thinking that it looks like it's not syncing to the trigger signal you're
feeding it, and that it's showing Ch-A, CH-B, and the Trigger input
alternately. Just a guess, take it with a grain of salt-substitute.
Dave,

That is something to consider. Possibly an "inadvertent" trigger from somewhere? In these pictures I am not feeding any signals to the scope. Nothing whatsoever going in. So are you thinking that there might be a problem with the trigger circuitry? If it is triggering, it is doing so from a source within the instrument. So maybe from the "LINE" trigger pick off? And it must be something which does not light the TRIGGER LED when this happens. When I do feed a signal, I get the same sort of reaction on the resulting waveform(s). I will check the trigger circuitry, per the manual. Who knows, I might find something that I did not think of. I appreciate your reply and the advice.

Thanks!

--
Michael Lynch
Dardanelle, AR

DaveH52
 

I'm thinking that it looks like it's not syncing to the trigger signal you're feeding it, and that it's showing Ch-A, CH-B, and the Trigger input alternately. Just a guess, take it with a grain of salt-substitute.

Mlynch001
 

Link for the Photos of the traces in question.

https://groups.io/g/TekScopes/album?id=127536

Anyone with Ideas or suggestions would be appreciated.

Feel free to question me about this issue.

THANKS!

--
Michael Lynch
Dardanelle, AR

Mlynch001
 

All;

I recently bought a couple of 468 Scopes, I have very little invested, with the exception of shipping costs. I wanted to get at least one good working scope out of the two, with at least a pile of extra parts. Two working units would be awesome. I was able to get this one to work after a lot of cleaning of many dirty cam switches. These are my first 468's, so they are new to me. I have done some repairs on 465B and 475A's in the past, but have never encountered this before. LV Power supplies are spot on. Ripple on all supplies is within limits. HV Supplies check good. I have the Original Service Manuals. My issue now is that when in "Non-Storage" Mode, the A trace has a small gap or black spot in the A trace at low speeds and at higher TIME/DIV sweep speeds, it becomes a short trace on the left then what appears to be the beam switching from A to B and back again. In the A Trigger view, there is the same symptom This only happens with the VERT/ Alt button released (ALTERNATE Mode Only). When it is in "VERT Chop" (ALT Button Depressed) the anomaly disappears. This is not apparent in the lower TIME/DIV settings, but becomes more visible as the scope is set to fasted and faster speeds. The scope is pretty much useless at high speeds unless you switch to "CHOP". The problem is only apparent in the analog mode, it does not appear to affect the digital traces in any fashion. Storage seems to work quite well, even the cursors work and agree with the readout. I am thinking this is a "Pre-Digital" sweep timing or Channel blanking timing issue. My thought is that the trace is not beginning at the right time or the blanking circuit is out of time with the trace. I beleive that this fault is allowing me to actually witness the transition of the electron beam between the two alternating traces. Of course, when it is in "CHOP" the traces both appear normal. Since this is basically a 465B with digital storage added; and the DIGITAL portion does not seem to be affected, I thought some of you more experienced 465B people might have an idea about this. I have posted photos of the traces in the photo section.
--
Michael Lynch
Dardanelle, AR