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Re: Some weirdness about a 485
If you don't have freeze spray, but do have one of those "dust blower" air cans, turn it upside down. That often works well enough as a freeze spray.
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--Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 3/26/2022 15:13, Zentronics42@... wrote:
Don¡¯t have freeze spray in the lab. I need to correct that. However I have good edges at the capacitor. So I think that is ok. I am in the process of a full calibration of the scope but for the 485 the trigger section is last. I have found something interesting though. R755 is at its extreme end of adjustment at the bottom so least sensitive I believe. I am running some more testing now, but it looks like it might have been a sensitivity adjustment. I will know in about 30 minutes. After I get through the vertical section it will be on to the trigger section so I will know If there are more issues down there. So random adjustments are ok at least in this case. As they will be corrected later. |
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Re: Some weirdness about a 485
Don¡¯t have freeze spray in the lab. I need to correct that. However I have good edges at the capacitor. So I think that is ok. I am in the process of a full calibration of the scope but for the 485 the trigger section is last. I have found something interesting though. R755 is at its extreme end of adjustment at the bottom so least sensitive I believe. I am running some more testing now, but it looks like it might have been a sensitivity adjustment. I will know in about 30 minutes. After I get through the vertical section it will be on to the trigger section so I will know If there are more issues down there. So random adjustments are ok at least in this case. As they will be corrected later.
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Zen -----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Tom Lee Sent: Saturday, March 26, 2022 6:04 PM To: [email protected] Subject: Re: [TekScopes] Some weirdness about a 485 Hi Zen, TDs are quite thermally sensitive, so try cooling CR751 alone and see what happens. If that proves to be the sensitive element, first take a screenshot of initial conditions, then give R755 (the trig sensitivity trimmer) a little tweak. It may very well be that CR751 has drifted in characteristics. If it hasn't drifted too far, adjustment of R755 will restore functionality. I generally don't advise random tweaks, but here it might be warranted. Just a thought. -- Cheers, Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 3/26/2022 14:03, Zentronics42@... wrote: Some new observations of the fault. The fault is thermally dependent. |
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Re: Some weirdness about a 485
Hi Zen,
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TDs are quite thermally sensitive, so try cooling CR751 alone and see what happens. If that proves to be the sensitive element, first take a screenshot of initial conditions, then give R755 (the trig sensitivity trimmer) a little tweak. It may very well be that CR751 has drifted in characteristics. If it hasn't drifted too far, adjustment of R755 will restore functionality. I generally don't advise random tweaks, but here it might be warranted. Just a thought. -- Cheers, Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 3/26/2022 14:03, Zentronics42@... wrote:
Some new observations of the fault. The fault is thermally dependent. Cold scop triggers just fine warmed up scope triggers miss behave. Fired up the air compressor and cooling down the sweep board Left side of scope pops the trigger back temporally. I have also reseated all the combs on both boards |
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Re: Some weirdness about a 485
Freeze spray on the small value capacitors used to differentiate edges?
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Harvey and if that doesn't work, freeze spray on everything else. H. On 3/26/2022 5:03 PM, Zentronics42@... wrote:
Some new observations of the fault. The fault is thermally dependent. Cold scop triggers just fine warmed up scope triggers miss behave. Fired up the air compressor and cooling down the sweep board Left side of scope pops the trigger back temporally. I have also reseated all the combs on both boards |
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Re: Some weirdness about a 485
Ill check this a well.
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-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Tom Lee Sent: Saturday, March 26, 2022 3:52 PM To: [email protected] Subject: Re: [TekScopes] Some weirdness about a 485 Hi Zen, I've not encountered this particular fault in a 485, but I have seen it in other scopes, where it was caused by a problem with the Schmitt that ultimately feeds the actual trigger circuit. The Schmitt's job is to provide a fast-edged signal, usually through ac-coupling, to the rest of the trigger circuitry. If the edges are lazy, the trigger circuits misbehave. I just looked at the schematics briefly, and the 485 couples into the trigger TDs through C761 (a small 2.5pF, which won't couple low frequencies at all well). Probe the left side of that cap and see if you are getting good Schmitt-like outputs. I'm betting that you won't. Good hunting! --Cheers, Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 3/26/2022 12:24, Zentronics42@... wrote: I am working on doing a restoration on a Tektronix 485 for the lab and I have been arguing with it for a little while. I am curious if anyone has run in to a scope that does not like to trigger at low frequencies? I am currently working on the 50 Ohm calibration and it seems that the scope in 50 Ohm mode REALLY does not like to trigger under 1.2Mhz. So it is struggling with 50 Khz cal signals. Vertical amplitude is good trigger is flakey. I have heard that some of these the TD¡¯s go kind of deaf at the high speed side of things but not the low side. Any ideas what might be causing this. I am continuing to investigate further. |
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Re: Some weirdness about a 485
Some new observations of the fault. The fault is thermally dependent. Cold scop triggers just fine warmed up scope triggers miss behave. Fired up the air compressor and cooling down the sweep board Left side of scope pops the trigger back temporally. I have also reseated all the combs on both boards
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I should also note that this a now a trigger fault. 50 ohms 1meg and external ALL behave the same way. Unfortunately a thermal camera would help troubleshooting on this one but I don¡¯t have one of those. Letting the scope cool off to reseat some transistors this most DEFFANATLY behaves like a capastitvely coupling issue. Higher frequances making it through to the trigger circuits better. At the moment I think skipping ahead in the calibration document will not help due to the scope not triggering on signals of 8 division of display. And I can watch it fail. As the scope warms up it gets flickery then trigger falls off this is with not changing any controls on the front panel. Signal source is known good and checked so it is not flaking out. Zen -----Original Message-----
From: [email protected] <[email protected]> On Behalf Of David Templeton Sent: Saturday, March 26, 2022 4:29 PM To: [email protected] Subject: Re: [TekScopes] Some weirdness about a 485 I¡¯ve not seen that scope, does it have an LF or HF rejection on the trigger coupling options? David On 26 Mar 2022, at 19:24, Zentronics42@... wrote: |
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Re: Some weirdness about a 485
Hi Zen,
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I've not encountered this particular fault in a 485, but I have seen it in other scopes, where it was caused by a problem with the Schmitt that ultimately feeds the actual trigger circuit. The Schmitt's job is to provide a fast-edged signal, usually through ac-coupling, to the rest of the trigger circuitry. If the edges are lazy, the trigger circuits misbehave. I just looked at the schematics briefly, and the 485 couples into the trigger TDs through C761 (a small 2.5pF, which won't couple low frequencies at all well). Probe the left side of that cap and see if you are getting good Schmitt-like outputs. I'm betting that you won't. Good hunting! --Cheers, Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 3/26/2022 12:24, Zentronics42@... wrote:
I am working on doing a restoration on a Tektronix 485 for the lab and I have been arguing with it for a little while. I am curious if anyone has run in to a scope that does not like to trigger at low frequencies? I am currently working on the 50 Ohm calibration and it seems that the scope in 50 Ohm mode REALLY does not like to trigger under 1.2Mhz. So it is struggling with 50 Khz cal signals. Vertical amplitude is good trigger is flakey. I have heard that some of these the TD¡¯s go kind of deaf at the high speed side of things but not the low side. Any ideas what might be causing this. I am continuing to investigate further. |
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Re: Some weirdness about a 485
On Sat, Mar 26, 2022 at 12:24 PM, <Zentronics42@...> wrote:
-------- Do you see a similar behavior with external trigger? I haven't seen this particular issue but if you look at the trigger circuit around the tunnel diodes there is a delay line that adds high frequency noise immunity by making sure two decisions that are apart by the time delay of the transmission line are the same. If there is enough offset in trigger points and a slow rising waveform two TDs may not trigger at the same point. You may need to trim/calibrate R765 and R755 per step 73 of calibration procedure. Ozan |
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Some weirdness about a 485
I am working on doing a restoration on a Tektronix 485 for the lab and I have been arguing with it for a little while. I am curious if anyone has run in to a scope that does not like to trigger at low frequencies? I am currently working on the 50 Ohm calibration and it seems that the scope in 50 Ohm mode REALLY does not like to trigger under 1.2Mhz. So it is struggling with 50 Khz cal signals. Vertical amplitude is good trigger is flakey. I have heard that some of these the TD¡¯s go kind of deaf at the high speed side of things but not the low side. Any ideas what might be causing this. I am continuing to investigate further.
Zen |
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Re: -2465B - trace show gaps when turn on readout -requesting help
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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Re: -2465B - trace show gaps when turn on readout -requesting help
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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Re: 466 front panel lights, and early / late models
and if the voltage supply is variable, you get dimming LEDS.......
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Harvey On 3/26/2022 11:27 AM, David Templeton wrote:
Thank you, that clears most of it up. |
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Re: 468 A15 Board Removal
The output high-voltage lead from the multiplier goes through a hole in the chassis and snakes under the CRT shield to a connection on the other side. Removing the multiplier, or freeing the A15 board, requires removing the CRT as shown in this video:
Before this can be done, both vertical boards and the attenuator must be removed as shown in this video: Bob -- Bob Haas |
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Re: -2465B - trace show gaps when turn on readout -requesting help
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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Re: -2465B - trace show gaps when turn on readout -requesting help
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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Re: -2465B - trace show gaps when turn on readout -requesting help
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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Re: -2465B - trace show gaps when turn on readout -requesting help
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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Re: -2465B - trace show gaps when turn on readout -requesting help
On Sat, Mar 26, 2022 at 03:22 PM, Harvey White wrote:
That's the way it was done in the 7000 series. Raymond |
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Re: -2465B - trace show gaps when turn on readout -requesting help
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. |
Bob Haas
Raymond Domp Frank
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