Strange artifacts on Tek 2445

[rodcastler]

Hi,

My Tektronix 2445 works well overall but shows this strange behavior. Once the swipe finishes each round, these artifacts (dots) flash for a split second while there's no trace and they are gone for (and during) the next sweep. Photos attached.

- If the time base is high enough to produce a line trace, these dots are never visible (or if they are produced, they don't get enough screen time to become perceivable).
- If the time base is low enough, they are very clear, and appear as soon as the trace leaves the screen, always in the exact same position.

- Adjusting trigger modes / levels doesn't make a difference.

- If I reduce the holdoff to zero, they flash briefly and then gone. If I max out the hold off, they remain there steadily for the entire holdoff time.

- The artifacts don't move in any direction. And their position seem to describe a curve if I were to join the dots, but that may be coincidental.


They seem to be harmless, but I'm wondering what the cause may be, and if there is a potential repair.


- This scope went through a complete cap replacement on the power supply section earlier this year. These artifacts were present before and after the recap.


Any thoughts?

[Kean]

It seems to me that it is a display blanking issue during readout display, as all the dot artifacts seem to be in the readout regions.  If so, then I am not sure if that would be a fault in the blanking signals, or just a side effect of the implementation.

There is a a "refresh prioritizer" mentioned in the service manual, which controls the regular updating of the readout display, and that could be giving more time to the readout under the conditions you mention and making it more noticeable.

While I've used one of these scopes many years ago, I've never owned one myself so I am not too familiar with them to know whether this is normal.

[tggzzz]

Dots aren't visible on my 2465.

What is the result of running all the self tests?

[MarkL]

A couple of suggestions:

- Check that all the power supply voltages are in spec according to Table 5-1 in the service manual.

- Try adjusting the Grid Bias (R1878) as described in the CRT Adjustments section.  Mark its position with a fine Sharpie before moving it.  If it's not having the desired effect, put it back where it was.

[rodcastler]

Seems like all of the automatic self-tests on boot time pass, I don't see any errors. I am unaware of how to trigger other self-tests.

[tggzzz]

Seems like all of the automatic self-tests on boot time pass, I don't see any errors. I am unaware of how to trigger other self-tests.

RTFSM, of course It is on the tekwiki. https://w140.com/tekwiki/wiki/2465

At least you don't have to worry about the A5 electrolytic and the NVRAM, but the RIFA delayed action smoke bombs may make themselves known. If you do replace the RIFAs, then beware of the fan collect and consider replacing the PSU electrolytics.

[rodcastler]

RTFSM, of course It is on the tekwiki. https://w140.com/tekwiki/wiki/2465

At least you don't have to worry about the A5 electrolytic and the NVRAM, but the RIFA delayed action smoke bombs may make themselves known. If you do replace the RIFAs, then beware of the fan collect and consider replacing the PSU electrolytics.

That key combination was really hidden in TFSM. I did not catch it, so thanks.

Interestingly as Kean mentioned earlier, it seems to be related to the readouts. While in the diagnostics menu, the dots are always present, which may indeed correlate to character generation or something.

So i did another test and disabled the readouts using the proper knob: Only one of all the dots appear in that case.

Photo shows how the dots are present during the diagnostic monitor. The self diagnostics show ALL PASS. I wonder if the character generator ROM may be at fault.

[tggzzz]

That key combination was really hidden in TFSM. I did not catch it, so thanks.

Yup, and you're welcome

Consider pointing a video camera at the screen while using the test routines to skip through the 256 NVRAM locations. If it does develop a fault, you'll then have a record of what it was.

I wonder if the character generator ROM may be at fault.

I doubt it, since the dots are too regular and not related to a single character spacing.

It might just be due to some signal integrity problem, and the first thing to do in that regard is check the power rails Vdc and noise. There's the ?J14? connector on the main board for that.

[factory]

Have you looked at the readout troublshooting procedure in the manual, there is a mention of "fixed spots (possibly flickering)?" and "check grid bias adjustment" this adjustment has already been mentioned in this thread.

David

[kmoonwalker]

I doubt it, since the dots are too regular and not related to a single character spacing.

there is one interesting thing thou - in first post first top and bottom "dots" from left  are not in level with other dots but below text

If you already made recap then some general resoldering/weak solder spots hunt/connectors cleaning/usual abc + careful visual inspection if some insect did not crawled in and died between two legs of logic element or some capacitor leak then check if problem still persists