7854 readout and stored waveform jitter
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Hi,
during work, all of the sudden my 7854 started to show jitter of the readout in the horizontal and vertical directions. In stored mode the waveform is jittery as well, but in scope mode the waveform is rock solid. I checked the voltages of the main power-supply and aux supply plus the 10V reference on the display board are all are fine without excessive ripple. The amplitude of the jitter is about 1/32". Somehow I can't believe it is a problem in the digital circuitry, first I thought the DAC reference voltage had a problem, but its fine. I love this scope and I use it more frequently than any other. -- Best regards, Fred S. |
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Hi Fred,
I don't remember which circuitry (DAC and so on) is used in common to display stored waveforms, text and Readout. The fault in your 7854 seems to affect everything. Some further ideas. Is the ID text jittery (in scope mode)? If you switch off Readout and then store a waveform, is the stored waveform jittery? If you also have the keyboard, choose Program Entry mode and type something. This mode completely avoids anything analog and switching between stored waveforms and text or readout. Jittery? Albert Hi Albert, thanks for your help. Yes, I have a keyboard. I tested the cases you mentioned and everything is jittery except the non-stored waveform. -- Best regards, Fred S. |
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On Sat, Dec 1, 2018 at 08:08 PM, <fred@...> wrote:
I am at home and have neither a 7854 nor a service manual available. So, just a question and some wild guesses: 1. Can you get an idea about the frequency of the jitter? That could give an indication of its origin. 2. ISTR that the Aux Regulator is in the "Digital Cage", behind the boards. I guess that's the aux supply that you mentioned and checked? 3. Several boards have local RC- or LC-filters in their power supply input lines. If a cap became open or lost (most of) its capacity, the local series impedance would suffer greatly, with large "ripple" as a result. Again, getting an impression of the jitter frequency may point towards its origin on one of the boards, seeping through because of high local PS impedance. 4. Did you try "stirring" (the harmonica) connectors? Especially if the problem is intermittent, even a cable may be at fault. I've read (somewhere in Hakan's docs) that gold plating in some situations tended to delaminate from its base material, causing bad/intermittent contacts in harmonica connectors. Raymond Raymond |
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Hi Fred,
We share a love of the 7854. I have a few troubleshooting related questions and suggestions of my own: It sounds you have isolated the problem to the display board. As Raymond Domp Frank already suggested, remove and reseat ALL contacts to break oxides. Is there any chance this is an intermittent problem? If this is an intermittent problem is it temperature sensitive - you can probably test this with a hair dryer and a can of FreezIt. Is this jitter present the moment the CRT warms up from being powered off overnight? That would be a clue its temp sensitive. At this point my favorite tool is a can of FreezIt with a thin tube to reach into difficult places. My first test is to use FreezIt to freeze everything on the entire board hoping that momentarily cures the problem or alters it's behavior in some way. If that happens I proceed to the next step after the board has a chance to warm up again. Now I spray FreezIt locally in different areas to narrow down the search. Finally I start freezing individual ICs, semiconductors, and tantalum caps hoping to get a reaction from one of them. Dennis Tillman W7PF I replaced all electrolytics on the display board, checked the inductors, but no change. Probed +15V1, -15V1, +15V2 and -15V2, everything is fine. The only components common to horizontal and vertical outputs are U620 and U1720, but I don't have replacements at the moment. The problem is not intermittent, the jitter frequency is a only couple of Hertz. Tried the freeze spray, no sensitivity. Could it be that the start time of the readout/stored waveform is jittery? What's the mechanism that starts the readout? -- Best regards, Fred S. |
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I can believe that if the LF13333N is going flakey that the display might jitter some, it's used to switch between sweep and digital signals.
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Dave -----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of fred@... Sent: 02 December 2018 21:20 To: [email protected] Subject: Re: [TekScopes] 7854 readout and stored waveform jitter Hi Fred, Look up the part numbers for U620 and U1720 in the service manual. Then we can check the Tek Semiconductor Common Parts Catalog and find out what they are. Dennis Tillman W7PF Hi Dennis, I know the part numbers: LF353 and LF13333, I just don't have any of these. -- Best regards, Fred S. |
Fred S.
Raymond Domp Frank
David C. Partridge