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Hi,
with the time-base equal or slower than 1ms/scale I noticed "blank outs" on the trace when I turned on the readout. The blank out are about 1mm in length at the same horizontal position where the text would appear. I can fix this issue when I replace the A5 board from my second scope. So the issue must come from the A5 board. I am requesting help to repair the A5 board - which I had fixed for leaky capacitor. PCB traced looks Ok and I replaced corroded resistor etc. Calibration was done as well. I read the theory of operation but I could not come up with a good trouble shooting plan. Maybe some does not switch fast enough. Does anyone has a good idea to look for? Bernd |
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Are there dual modes for readout refresh on the board, and is the old board set up the same way as the new one?
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Harvey On 3/25/2022 6:58 PM, Dr. Bernd Burfeindt via groups.io wrote:
Hi, |
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I¡¯ve never looked at these signals, but to my understanding, the readout
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board asserts /ROR when it wants to display dots. The display sequencer, however, gates access to the trace through the /ROA signal. I¡¯d start by looking at those two, comparing between good and bad boards. Note that the readout is stealing the trace, and if this is not random to the sweep, you may be able to observe the stealing as it occurs at the same spot during consecutive sweeps. If this is happening in auto trigger mode without trigger, try whether you still see it during triggered sweeps. See if holdoff changes the problem at all. ?ann f?s., 25. mar. 2022 kl. 18:58 skrifa?i Dr. Bernd Burfeindt via groups.io <femtocam@...>: Hi, |
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As others have said, the readout works in two modes.? The beam (a single one) is always shared.? The readout circuitry can either wait for a retrace (useful for high sweep speeds), or chop the beam at a given rate.? Chopping the beam produces those little blanks in the trace.? A little extra switching would have fixed that, but instead, Tek used a slide switch on the readout board so you could pick the most convenient setting.? That's the "dual mode" I was talking about.
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Harvey On 3/25/2022 8:01 PM, Harvey White wrote:
Are there dual modes for readout refresh on the board, and is the old board set up the same way as the new one? |
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On Sat, Mar 26, 2022 at 10:22 AM Harvey White <madyn@...>
wrote: As others have said, the readout works in two modes. The beam (a singleI've been through the readout schematics in a fair amount of detail before, but I don't remember seeing a switch anywhere. I went back to look at the readout schematics just now, and I still can't find a switch. Is it possible you're confusing the 2465B with some other readout implementation? According to the Theory of Operation, the 2465en have fairly elaborate machinery that strives to maintain a flicker-free, uniform brightness readout, no matter the sweep speed or trigger frequency. |
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They do it differently here?? I have a 2430A, and never noticed a problem with mine.
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Harvey On 3/26/2022 10:55 AM, Raymond Domp Frank wrote:
On Sat, Mar 26, 2022 at 03:22 PM, Harvey White wrote:Tek used a slide switch on the readout board so you could pick the mostThat's the way it was done in the 7000 series. |
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If there's no switch, then they did it in circuitry.? I'm used to the 7000 series.
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Harvey On 3/26/2022 10:47 AM, Siggi via groups.io wrote:
On Sat, Mar 26, 2022 at 10:22 AM Harvey White <madyn@...> |
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On Sat, Mar 26, 2022 at 04:03 PM, Harvey White wrote:
I wasn't implying that: AFAIK, it's the same (chopping within or alternating between trace to access the readout) in the 24X5/7 'scopes, except work has been done to avoid needing a switch, like in the 7000's. The 24X0's are very different beasts; being digital 'scopes, their display update rate is mostly independent from the signal (except maybe at very low frequencies, because the data just isn't there yet). Raymond |
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On Fri, Mar 25, 2022 at 11:58 PM, Dr. Bernd Burfeindt wrote:
I find this intriguing: Are the readout excursions timed at constant intervals from the trigger event and at the exact moment the horizontal dot position is just beneath the readout? I don't think that would be desirable; a random interruption largely hides the gap, so if I interpret Bernd's description correctly, it would indicate a malfunction. Raymond |
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So true.? Sorry.
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But they did manage to incorporate some more smarts. I only have 3 "true" digital scopes, and generally (because of the CRT) treat the 2430A almost as if it were analog. almost......... maybe...... Thanks Harvey On 3/26/2022 11:11 AM, Raymond Domp Frank wrote:
On Sat, Mar 26, 2022 at 04:03 PM, Harvey White wrote:They do it differently here?? I have a 2430A, and never noticed a problemI wasn't implying that: AFAIK, it's the same (chopping within or alternating between trace to access the readout) in the 24X5/7 'scopes, except work has been done to avoid needing a switch, like in the 7000's. |
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Not sure that this is much of a contribution, but the 24x5 scopes seem to use a more complex method of determining when a readout sweep occurs (than the 7k series). There are four "priorities" for the readout, depending on sweep speed (and other factors?). Summarizing the 2445A service manual (from pages 3-30 and 3-31):
Priority 1: ideally readout display should happen between traces, when the scope is not trying to display a waveform - before a trace starts, after a trace finishes, or between consecutive traces. Readout display in this mode causes no interference with the display of a waveform. This mode is only used when the sweep time is fast enough to display flicker-free readout, and slow enough to complete drawing the readout during the holdoff time. Priority 2: the second most desirable time to display readout is between sweeps when the scope is waiting for a trigger event. This mode may cause slight interference with the leading edge of waveform display. Priorities 3 and 4: the least desirable times are during a sweep. Here readout dots are "chopped" into the waveform display. The manual claims that "the waveform blanking associated with these displays is relatively random" and "is usually not noticeable."The manual says that priority 4 indicates a higher demand for display time. There is specific circuitry (the "Refresh Pioritizer") that determines which mode is used and when readout dots are displayed. In the 2445A these are labeled U2850(A/B), U2950A, U2990A, and U2985. It's complicated, and it sounds like it can display fragments of readout at a time, maybe even just one dot at a time, all in an attempt to avoid blanking parts of a displayed waveform while maintaining a flicker-free readout image with proper intensity, regardless of sweep speed. -- Jeff Dutky |
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Hi
I think I found the issue. It looks like the U2890 is bad. I found this by touching all IC with a "grounded" finger and the issue got worse when I touched U2890. When using ice spray the trace gap disappeared. Re-soldering did not fixed it- so I ordered a new IC from Mouser. Will let you know on two days it this fix this issue. Bernd |
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Hi
I solved the issue. I was not the U2890. I was a broken trace underneath U2965- probably a result of the leaky capacitor and at that time for repair I did not notice the broken trace because it was underneath the IC. It was the connection from U2965 (pin8) to to U2890 (pin 10/9). - (working as an invertor). It was partially working because the inputs were coupling capacitively- that why I could it make worse by touching the IC with my finger. BTW the A5 schematic is totally wrong and does not show that connection ( show to a different gate of U2890). Fortunately I had working A5 board and I could measure the traces.. Maybe there is an update schematic - please let me know. Bernd |
Raymond Domp Frank