7904 trace dimming

David G. Member Profile All Messages By This Member #112411 Hello All, I am noticing that as my 7904 is warming, the trace dims. It does this about 3-5 mins after power up and does so all at once. In other words, it is not gradual dimming. I need to increase the intensity by one mark on the knob scale in order to return the trace to similar intensity as before it dimmed. It does not dim beyond that throughout the day after this initial "dimming." Has anyone else experienced this? Just tinkering in the lab............. Cheers, David
Dennis Tillman W7pF Member Profile All Messages By This Member #112415 Hi David, I have seen that on other 7000 scopes and even on some of the small TM500 scopes (SC501, SC502, SC503, SC504, and MR501). It is subtle. It was the subject of some comments on the forum many, many years ago. I will ask a friend of mine who was in the CRT group if he knows what causes it. Dennis Tillman W7PF toggle quoted message Show quoted text -----Original Message----- From: TekScopes@... [mailto:TekScopes@...] Sent: Friday, December 19, 2014 8:55 AM To: TekScopes@... Subject: [TekScopes] 7904 trace dimming Hello All, I am noticing that as my 7904 is warming, the trace dims. It does this about 3-5 mins after power up and does so all at once. In other words, it is not gradual dimming. I need to increase the intensity by one mark on the knob scale in order to return the trace to similar intensity as before it dimmed. It does not dim beyond that throughout the day after this initial "dimming." Has anyone else experienced this? Just tinkering in the lab............. Cheers, David ------------------------------------ Posted by: d.garrido@... ------------------------------------
David Hess Member Profile All Messages By This Member #112417 Many of mine change in brightness as they warm up but it is gradual and stops after a few minutes. If it happens all at once, then I would suspect a high voltage part in the z-axis DC restorer like a diode or capacitor has marginal leakage and may be close to failing. I think it would be easier to just do a general replacement of those high voltage parts than trying to find which one is causing the problem. Before doing that I would test with a different timebase and do the z-axis calibration procedures. If you want to make a measurement, check the low voltage DC level at the output of the z-axis amplifier with a voltmeter and see if it shifts when the brightness changes all at once. I would not use freeze spray on the z-axis circuits because of condensation and high voltage. Another thing I might consider after the above tests is removing the z-axis board and cleaning it thoroughly but if I did, then I would change the high voltage diodes and capacitors anyway. toggle quoted message Show quoted text On 19 Dec 2014 08:55:18 -0800, you wrote: Hello All, I am noticing that as my 7904 is warming, the trace dims. It does this about 3-5 mins after power up and does so all at once. In other words, it is not gradual dimming. I need to increase the intensity by one mark on the knob scale in order to return the trace to similar intensity as before it dimmed. It does not dim beyond that throughout the day after this initial "dimming." Has anyone else experienced this? Just tinkering in the lab............. Cheers, David
Albert Otten All Messages By This Member #112426 A simpler fault would be an unstable trimpot somewhere in the Z-axis amplifier itself. Such was the case in my 7854. Intensity changed suddenly after warming up and often also after switching between display modes (analog or stored waveform). I too would first of all perform a Z-axis calibration and readjust the settings back and forth even when they seem on spec. Albert ---In TekScopes@..., <davidwhess@...> wrote : Many of mine change in brightness as they warm up but it is gradual and stops after a few minutes. If it happens all at once, then I would suspect a high voltage part in the z-axis DC restorer like a diode or capacitor has marginal leakage and may be close to failing. I think it would be easier to just do a general replacement of those high voltage parts than trying to find which one is causing the problem. Before doing that I would test with a different timebase and do the z-axis calibration procedures. If you want to make a measurement, check the low voltage DC level at the output of the z-axis amplifier with a voltmeter and see if it shifts when the brightness changes all at once. ----
Dennis Tillman W7pF Member Profile All Messages By This Member #112429 Hi David, Here is what the head of Tek's Laboratories (he also designed the vertical deflection plates in the 7104 CRT) had to say: "Dennis: I turned on both my 7904 and SC503. In a ten minute span I did not see any change in the trace brightness. So I don't have any evidence that there is such a drop -- at least on these two scopes. There was nothing that we ever recommended from the CRT group that would have led to such a design feature. Maybe it's caused by a noisy intensity control." Dennis Tillman W7PF toggle quoted message Show quoted text -----Original Message----- From: TekScopes@... [mailto:TekScopes@...] Sent: Friday, December 19, 2014 9:56 AM To: TekScopes@... Subject: RE: [TekScopes] 7904 trace dimming Hi David, I have seen that on other 7000 scopes and even on some of the small TM500 scopes (SC501, SC502, SC503, SC504, and MR501). It is subtle. It was the subject of some comments on the forum many, many years ago. I will ask a friend of mine who was in the CRT group if he knows what causes it. Dennis Tillman W7PF -----Original Message----- From: TekScopes@... [mailto:TekScopes@...] Sent: Friday, December 19, 2014 8:55 AM To: TekScopes@... Subject: [TekScopes] 7904 trace dimming Hello All, I am noticing that as my 7904 is warming, the trace dims. It does this about 3-5 mins after power up and does so all at once. In other words, it is not gradual dimming. I need to increase the intensity by one mark on the knob scale in order to return the trace to similar intensity as before it dimmed. It does not dim beyond that throughout the day after this initial "dimming." Has anyone else experienced this? Just tinkering in the lab............. Cheers, David ------------------------------------ Posted by: d.garrido@... ------------------------------------

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7904 trace dimming