Date
1 - 19 of 19
A possibly sick 7904A
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Eric
I was working on some 7000 series plugins and I noticed some strange behavior out of my 7904A. during the high frequency calibration I found that the scope was a little dim readable for the calibration of the plugin but not bright. To be fair this was at 500 pS/Div sweep speed. But when I turned the brightness all the way up the beam de-focused. I could bring it back in to focus with the front panel however the readout de-focused at that point. Does any one have an idea on where to start looking. If I have to do a full Cal on the frame that is ok as I have one of the -01 calibrators for it. Just trying to get some plugins fixed for the 7854 that is on the way.
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Typically you need a faster repetition rate on your test signal for it to be bright at 500pS/div.
I'm guessing that you were using a fast rise pulse from a CG551AP or CG5001 or PG506 for calibration. If so this has a pretty low repetition rate compared to the time for a sweep at 500pS/div so it will be dim. HtH David
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Eric
Hey David
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that is exactly what the signal was it was a fast rise at 1Khz. Eric
On 8/17/2020 8:35 PM, David C. Partridge wrote:
Typically you need a faster repetition rate on your test signal for it to be bright at 500pS/div.
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Roger Evans
One of the nice features on the 485 is that you can switch the calibrator from 1kHz for the standard compensation adjustments to 1MHz for the rise time checks when otherwise the trace would be very faint. The PG506 has intermediate rep rates as well.
Regards, Roger
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Eric
Roger,
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I am also worried about the focus issue and I am hoping to rule out the tube. Set to a high brightness I have to refocus the tube. But this then causes the readout to be unfocused. Am I being unrealistic to expect a trace to be in focus at any brightness as well as the readout?
On 8/18/2020 5:01 AM, Roger Evans via groups.io wrote:
One of the nice features on the 485 is that you can switch the calibrator from 1kHz for the standard compensation adjustments to 1MHz for the rise time checks when otherwise the trace would be very faint. The PG506 has intermediate rep rates as well.
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Roger Evans
I am not an expert on this, trained in physics not electronics. When you look at a low rep rate signal at 500ps/div the beam is on a very small fraction of the time, about 5ns every msec so one part in 200000. The readout is being triggered automatically and runs around 1kHz or so, I don't remember the actual clock speeds. So for the same brightness the signal trace has to carry maybe 1000 times the current compared with the readout, this means a higher density of electrons and the space charge can affect the beam focussing compared with a low current beam.
I tried putting a fast rise square wave into my 7904 at 1kHz and qualitatively I see what you see. I have to turn up the brightness a lot to see the trace at 1ns/div and it does defocus compared with the readout. I can improve the focus slightly and that worsens the focus of the readout significantly. Maybe if we get some more comments we can decide who is unrealistic and who is easily satisfied. I have no idea what the factory specifications were. Regards, Roger
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Chuck Harris
Your description is pretty accurate.
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The more modern scopes have compensation circuitry that changes the focus voltage with the instantaneous intensity setting.... and changes the instantaneous intensity settings for the different beam uses... So, the intensity is separately controllable for the A horizontal plugin, the B horizontal plugin, the readout, and in some cases, the "intensified" horizontal A vs B settings... but FOCUS, and ASTIGMATISM, each have only one user control. Each of these settings result in a different beam current, which require a different focus/astigmatism voltage. For the 7904A, which was one of the earlier adopters of this enhancement, they have a circuit called Auto Focus, and in the Performance check and Adjustment procedure, page 5-12 of my manual, they explain how it gets adjusted. The do the main focus adjustment with the beam very dim, and then they adjust the Auto Focus circuit with the beam at full intensity control position. -Chuck Harris Roger Evans via groups.io wrote:
I am not an expert on this, trained in physics not electronics. When you look at a low rep rate signal at 500ps/div the beam is on a very small fraction of the time, about 5ns every msec so one part in 200000. The readout is being triggered automatically and runs around 1kHz or so, I don't remember the actual clock speeds. So for the same brightness the signal trace has to carry maybe 1000 times the current compared with the readout, this means a higher density of electrons and the space charge can affect the beam focussing compared with a low current beam.
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On Wed, Aug 19, 2020 at 12:12 PM, Roger Evans wrote:
I remember my disappointment when I started collecting 'scopes in 2012/13: I fed the < 1ns rise time signal from a PG506 into my newly acquired 7904A at a fast time base setting, expecting to clearly see the wave shape. The brightness was very disappointing, even at 1 MHz repetition rate. It turned out that the CRT had seen better days and had grown a bit tired. At normal repetition rates, as encountered by the ham that had sold me this instrument, all was fine. The 7904's and 7904A that I acquired later were much better but the limitations in writing rate were obvious. In the analog 'scope realm, apart from the 7104 or the 2467 with their MCP CRT's, nothing beats a 7834 or 7934 analog storage 'scope in this respect: Even single-shot 1ns edge speeds can be seen clearly. Raymond
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teamlarryohio
If focus stayed constant irrespective of beam intensity, there would have been no need for a front panel focus control.
-ls-
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Chuck Harris
Indeed, but that didn't stop the tek engineers from trying
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to make FOCUS a universal control that worked equally well on both beams, and the readout. They failed, of course, but got a pretty good compromise that worked on most normal intensity levels. You have to get the internal controls adjusted right, though... -Chuck Harris teamlarryohio wrote:
If focus stayed constant irrespective of beam intensity, there would have been no need for a front panel focus control.
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Eric
Looks like my frame needs a bit of a tune up then. Ill be working on this once I get the current plugin done. I have shied away from the MCP CRT as read some where that those tubes were all on borrowed time as even at very low levels on intensity there was some slight phosphor damage. Not sure if this turned out to be the case.
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Eric
On 8/19/2020 8:48 AM, Chuck Harris wrote:
Your description is pretty accurate.
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Dave Daniel
AFAIK, the 7904A does *not* use and MCP CRT. The 7104 and 2467 do use MCP CRTs.
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DaveD
On Aug 19, 2020, at 15:24, Eric <ericsp@gmail.com> wrote:
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On Wed, Aug 19, 2020 at 10:03 PM, Dave Daniel wrote:
That's correct or you wouldn't experience the writing rate being as "limited" as with all the other regular models. Raymond
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Eric
The lack of an MCP is why I went with the 7904 as apposed to the 7104. Due to the difficulty of being able to source a tube if I needed one.
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On 8/19/2020 4:03 PM, Dave Daniel wrote:
AFAIK, the 7904A does *not* use and MCP CRT. The 7104 and 2467 do use MCP CRTs.
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On Wed, Aug 19, 2020 at 09:24 PM, Eric wrote:
I think that's generally true, though it shouldn't be exaggerated. Acceptable brightness levels are fine and the 'scope does have some protections. OTOH, some people like to see how bright the image of their 'scope will go. That's not what these 'scopes were designed for: A few years ago, I had a friend who proudly showed me the screen brightness of his newly acquired 7104, the only problem being that brightness couldn't be turned down. Obviously a DC restorer problem or the like. After repair there was very little permanent (local) loss of brightness and no burned spots were visible. Quite different from the HP 1727A analog storage 'scope that turned out to have burnt its target. The CRT in the 2467 appears to be far more resilient than the much older design in the 7104. It has better protection as well I guess. At low/normal brightness levels, life should be fine. I only use a 7104 when I especially feel like it. It's not the best choice as a regular 'scope. Its screen graticule size is a bit smaller than regular models' as well (8mm/div vs. 10mm/div). AFAIK, the 2467BHD, with its MCP CRT, was intended for regular HDTV-work. FWIW, the CRT of my 7104 is as new, as are the CRT's of my 2467B and 2467BHD. Raymond
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On Wed, Aug 19, 2020 at 10:47 PM, Eric wrote:
Not having an MCP CRT hardly ever is (or rather: was) a problem. It's not like "You don't have a good 'analog 'scope unless it has an MCP CRT"... Raymond
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On Wed, Aug 19, 2020 at 10:47 PM, Eric wrote:
That's been my excuse for having two or, better even, three copies of all models for a few years now. But we're treading on very different ground here... Raymond
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Eric
I will blaspheme here for a little if I will be indulged. Don't get me wrong I do love the 7904A but for the VERY slow and the very fast above 1Ghz I will take the screen of a digital scope. Seeing 1hZ sign waves on a CRO is not easy with out a camera. of course then again.. I might be a noob
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Eric
On 8/19/2020 5:00 PM, Raymond Domp Frank wrote:
On Wed, Aug 19, 2020 at 10:47 PM, Eric wrote:Due to the difficulty of being able to source a tube if I needed one.That's been my excuse for having two or, better even, three copies of all models for a few years now. But we're treading on very different ground here...
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On Wed, Aug 19, 2020 at 11:04 PM, Eric wrote:
Very few of us use veteran analog 'scopes because they are better suited to the job. That's only true in some situations, if compared with fast-refreshing digital 'scopes. Raymond
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