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2467B focus/astigmatism anomaly...
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Chuck Harris
Hi Gang,
I get about one 2467B to work on for every 30, 2465B's, so I haven't got tons of experience with them. I don't have one on my bench, so ditto the problem. They calibrate just like the 2465B, but I don't know much about living with one. So, I got a customer's 2467B that had focus issues where the focus was changing at will. The previous owner really worked over the HV board looking for the problem... my customer inherited those issues... I traced down about a half dozen problems, ranging from bad transistors, lead dress of the focus resistors, wires on the HV module that were down to 1 strand... maybe less?, cut up teflon insulation back to the CRT socket... lots of flux, and other dirt on the board...the usual stupid stuff. But after fixing all of that, there is still an anomaly. If I let the scope warm up, put a cross hair in the center of the screen, do a fine focus/astigmatism adjustment, turn the display, and intensity off, to protect the CRT, and walk away for a couple of hours. When I turn the intensities back up, the screen will be fuzzy at normal viewing intensity levels.. The fuzziness can't be cleared by using the focus knob, only the little screwdriver slotted astigmatism control. I put a DVM on the astigmatism grid, and the value is rock solid. The changes in focus are not the result of changes in astigmatism voltage, but a change in astigmatism voltage is necessary to bring back focus. Sharpen things up, turn off the intensities, walk away, and in another couple of hours, it will be blurry again. Only correctable by adjusting the astigmatism. As an interesting side note, just before the internal timer turns off the CRT beam, the beam and readout will get blurry, but when I move any control to restore the display, it will return to sharpness once again. OK... So, I have a 2465B on my shelves that has a bad CRT, so I thought I would see what it does under similar conditions: It does the same thing! A little bit less so than my customer's scope, but I definitely have to chase the astigmatism around if I leave it for a couple of hours with the beam turned off. Anybody notice any astigmatism weirdness with their 2467B's??? Thanks! -Chuck Harris OBTW, I read in the operator's guide that the 2467B will first turn the display down, after extended inactivity, before it actually goes into shutdown... maybe turning the display down involves blurring the focus??? |
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Follow up:
After about 2h45min and turning up both the intensity and readout, the display and readout were still in the sharpest setting. Adjusting focus and/or astig would make the screen less focussed..... I would suggest to (before and after) check all relevant voltages on the CRT. Geometry, edge focus, y-axis alignment, and the components around Q4422, Q4402 and Q4403. Maybe a cold solder joint in that area? Un saludo, Leo |
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Chuck Harris
Hi Leo,
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Thanks for doing the test. The problem with making measurements in the focus area of the 2467B is the extreme high impedance of the circuitry. Even my 100M HV probe loads the circuits down visibly. However, the low impedance CRT voltages: edge focus, geometry, astigmatism, PDD lens and wall band, anode 1, are all rock solid. As is the cathode of the CRT. So, that points to the actual anode voltage and the focus voltage... neither of which will behave well when measured with any probes I have. It would seem to me that the only way for the astigmatism voltage required for good focus to shift would be if the anode voltage changed. The change in the anode voltage wouldn't necessarily cause a corresponding shift in the focus pot's position for best focus, because the focus electrode's voltage is derived from the cathode supply... Q4402 was found earlier to be shorted. It was replaced with new. I suppose it might be a good idea to figure out how it got to be bad. Perhaps a problem with C4403 did the deed? Puzzling. -Chuck Harris satbeginner wrote: Follow up: |
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Hmmm... maybe use an HPAK 34401A DMM in high resistance mode?? Don't know what your anode voltage is - it could exceed the 34401A's range.? Just a thought.Jim Ford?Sent from my Verizon, Samsung Galaxy smartphone
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-------- Original message --------From: Chuck Harris <cfharris@...> Date: 8/16/19 11:54 AM (GMT-08:00) To: [email protected] Subject: Re: [TekScopes] 2467B focus/astigmatism anomaly... Hi Leo,Thanks for doing the test.? The problem with makingmeasurements in the focus area of the 2467B is theextreme high impedance of the circuitry.? Even my100M HV probe loads the circuits down visibly.However, the low impedance CRT voltages:edge focus, geometry, astigmatism, PDD lens and wall band,anode 1, are all rock solid.? As is the cathode of theCRT.So, that points to the actual anode voltage and thefocus voltage... neither of which will behave well whenmeasured with any probes I have.It would seem to me that the only way for the astigmatismvoltage required for good focus to shift would be if theanode voltage changed.The change in the anode voltage wouldn't necessarily causea corresponding shift in the focus pot's position for bestfocus, because the focus electrode's voltage is derived fromthe cathode supply...Q4402 was found earlier to be shorted.? It was replaced withnew.? I suppose it might be a good idea to figure out how itgot to be bad.? Perhaps a problem with C4403 did the deed?Puzzling.-Chuck Harrissatbeginner wrote:> Follow up:> > After about 2h45min and turning up both the intensity and readout,? the display and readout were still in the sharpest setting.> > Adjusting focus and/or astig would make the screen less focussed.....> > I would suggest to (before and after) check all relevant voltages on the CRT.> > Geometry, edge focus, y-axis alignment, and the components around Q4422, Q4402 and Q4403.> > Maybe a cold solder joint in that area?> > Un saludo,> > Leo> > > >
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Chuck Harris
The anode voltage is 15KV... I the 34401 tops out at 1KV with 10M input
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resistance. Not even close. -Chuck Harris Jim Ford wrote: Hmmm... maybe use an HPAK 34401A DMM in high resistance mode? Don't know what your anode voltage is - it could exceed the 34401A's range. Just a thought.Jim Ford Sent |
On Aug 16, 2019, at 10:08 AM, Chuck Harris <cfharris@...> wrote:Yes, I¡¯m have a focus problem. I¡¯ve not tackled it yet. I have a thousand other things to do first. Besides, when I do tackle it, I most likely be hoping you have advise for me. larry |
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Yep, I figured it was a long shot.Thanks, Chuck.?JimSent from my Verizon, Samsung Galaxy smartphone
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-------- Original message --------From: Chuck Harris <cfharris@...> Date: 8/16/19 12:48 PM (GMT-08:00) To: [email protected] Subject: Re: [TekScopes] 2467B focus/astigmatism anomaly... The anode voltage is 15KV... I the 34401 tops out at 1KV with 10M inputresistance.? Not even close.-Chuck HarrisJim Ford wrote:> Hmmm... maybe use an HPAK 34401A DMM in high resistance mode?? Don't know what your anode voltage is - it could exceed the 34401A's range.? Just a thought.Jim Ford Sent
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Chuck Harris
I thought of using a Fluke differential voltmeter that does have virtually
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infinite input impedance when it is in balance, but it too is only 1KV. I am going to have to figure out a way to sneak sideways into this problem. -Chuck Harris Jim Ford wrote: Yep, I figured it was a long shot.Thanks, Chuck. JimSent from my Verizon, Samsung Galaxy smartphone |
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Why don't you use a Fluke 40KV probe these are designed to work with a 10M Ohm input DVM
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George On Friday, August 16, 2019, 9:10:30 PM GMT+1, Chuck Harris <cfharris@...> wrote:
I thought of using a Fluke differential voltmeter that does have virtually infinite input impedance when it is in balance, but it too is only 1KV. I am going to have to figure out a way to sneak sideways into this problem. -Chuck Harris Jim Ford wrote: Yep, I figured it was a long shot.Thanks, Chuck. JimSent from my Verizon, Samsung Galaxy smartphone |
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If you have another power supply that can make 15KV, you could assemble a differential voltmeter using the supply, a battery-powered voltmeter as null detector, and another voltmeter to measure the supply output once you have it nulled. The detector should have autoranging turned off, and have protection diodes across its input.
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I'm feeling a mad-scientist vibe, makes me wish I had a 15KV supply :) Dave Wise Let my creation LIIIIIVE!!!!! -----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Chuck Harris Sent: Friday, August 16, 2019 1:10 PM To: [email protected] Subject: Re: [TekScopes] 2467B focus/astigmatism anomaly... I thought of using a Fluke differential voltmeter that does have virtually infinite input impedance when it is in balance, but it too is only 1KV. I am going to have to figure out a way to sneak sideways into this problem. -Chuck Harris Jim Ford wrote: Yep, I figured it was a long shot.Thanks, Chuck. JimSent from my Verizon, Samsung Galaxy smartphone |
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Chuck Harris
That is where I went first, but when my 100M Keithley probe
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is applied to the focus grid, it pulls the scope out of focus. It is a problem when the measurement equipment adversely affects the measured circuit. -Chuck Harris george edmonds via Groups.Io wrote: Why don't you use a Fluke 40KV probe these are designed to work with a 10M Ohm input DVM |
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Chuck Harris
You're scaring me Dave....
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-Chuck Harris Dave Wise wrote: If you have another power supply that can make 15KV, you could assemble a differential voltmeter using the supply, a battery-powered voltmeter as null detector, and another voltmeter to measure the supply output once you have it nulled. The detector should have autoranging turned off, and have protection diodes across its input. |
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Hi Chuck
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The Fluke 40KV probe is 1,000M input, The only other quick solution is an electrostatic meter, but I doubt it would have the resolution that you require. George. On Friday, August 16, 2019, 10:30:26 PM GMT+1, Chuck Harris <cfharris@...> wrote:
That is where I went first, but when my 100M Keithley probe is applied to the focus grid, it pulls the scope out of focus. It is a problem when the measurement equipment adversely affects the measured circuit. -Chuck Harris george edmonds via Groups.Io wrote: ? Why don't you use a Fluke 40KV probe these are designed to work with a 10M Ohm input DVM |
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Who was it that was building the "frankenscope" about 13 years ago?
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-Dave (another one) On Friday, August 16, 2019, 02:36:18 PM PDT, Chuck Harris <cfharris@...> wrote:
You're scaring me Dave.... -Chuck Harris Dave Wise wrote: If you have another power supply that can make 15KV, you could assemble a differential voltmeter using the supply, a battery-powered voltmeter as null detector, and another voltmeter to measure the supply output once you have it nulled.? The detector should have autoranging turned off, and have protection diodes across its input. |
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Chuck Harris
Hi George,
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Yes, you are right... but then, my keithley 1600A is also 1000M input impedance... I just tried the keithley on the focus grid, and it did *not* load it down noticeably. Clearly, I must have tried some other scheme the last time I measured it. I am not sure how I can leave this huge probe connected given that it isn't amenable to a gripper clip. A skyhook, I guess... -Chuck Harris george edmonds via Groups.Io wrote: Hi Chuck |
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Chuck Harris
Sounds like someone is making him a Farnsworth Fusor.
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-Chuck Harris John Griessen wrote: On 8/16/19 3:49 PM, Dave Wise wrote:I'm feeling a mad-scientist vibe, makes me wish I had a 15KV supplyI've got a 40kV Glassman 2kW I'm in the middle of converting from positive to negative. |
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Hi Chuck
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I am quite amused by the 40KV probe as rule one is that you do not hand hold such an item when applied to power. Best of luck George On Friday, August 16, 2019, 11:02:22 PM GMT+1, Chuck Harris <cfharris@...> wrote:
Hi George, Yes, you are right... but then, my keithley 1600A is also 1000M input impedance... I just tried the keithley on the focus grid, and it did *not* load it down noticeably.? Clearly, I must have tried some other scheme the last time I measured it. I am not sure how I can leave this huge probe connected given that it isn't amenable to a gripper clip.? A skyhook, I guess... -Chuck Harris george edmonds via Groups.Io wrote: ? Hi Chuck |
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Chuck,
I assume that after fiddling with the astig setting, you always end up at the setting before turning down intensity? Did you try not touching the astig setting after turning intensity back up? Will focus eventually return even when not touching astig? I'm thinking about leakage somewhere or lack of DC coupling (almost the same here). With intensity off, beam current goes to almost zero. I have seen setups where the astig electrode is closer to the cathode than the focus electrode. That does not seem to be so in this CRT but could it be that the dot distortion caused by disturbing the round shape of the beam (by operating the astig control) after upping the intensity causes a slight charge leak to/from the focus hole in the focus electrode, slightly adjusting the potential on the focus electrode? This raises the question of whether the "dark" situation might actually be the correct situation, which is then *disturbed* after upping intensity or the other way around? I would expect the currents on the focus and astig electrodes to be virtually zero during normal operation but the beam could approach the edge of the focus hole when it's not round, causing some leakage. Perhaps replacing C4403 (since it carries a substantial DC voltage during operation, a slight leakage may disturb things) would be a good idea without having to resort to more extreme measures to see the high impedance HV's. As you say, regular HV probes (like a Fluke 40K8, for DC) or a Tek P6015A or the like won't be of much help in many places here. Raymond |
satbeginner
Raymond Domp Frank
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