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| HP 1741A oscilloscope - restoration project |
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| mayeri:
The result: in CONV mode the spot has disappeared. |
| mayeri:
I noticed a strange phenomenon in Conventional mode: The beam intensity does not function properly. When turning button the brightness increases linearly in the first period, then suddenly becomes brighter and then returns to its previous brightness level. From this point the brightness does not change any further turning of the button. |
| mayeri:
Surely there is a problem in storage circuit. Turning brightness button: |
| tekfan:
--- Quote from: mayeri on December 13, 2012, 12:34:00 am ---I noticed a strange phenomenon in Conventional mode: The beam intensity does not function properly. When turning button the brightness increases linearly in the first period, then suddenly becomes brighter and then returns to its previous brightness level. From this point the brightness does not change any further turning of the button. --- End quote --- This is the screen protection kicking in. There's a maximum current limit trimpot in the HV power supply that sets the max current to the CRT. If this current is exceeded it will slightly reduce it by dimming the beam. If the spot on the screen is too bright it might permanently burn the metal storage meshes behind the screen. When in storage mode there is a very small area where the beam is visible and the entire screen being flooded by the excess electrons if the beam intensity is too great. So you must use the minimum intensity to get a nice, visible display. The brightness control is operating as it should. Somewhere in the middle position you should get a pre-fogged screen state (slight green spots appear). The storage CRTs age quite a bit more noticeably than regular non storage. That's why regular calibration of the storage circuit is very important. You can do a partial adjustment with only a screwdriver, multimeter and good eye. Look at page 43 where the storage adjustment section starts (especially paragraph 5-62). That will bring the brightness control back into the proper range. If you have another scope and a signal generator you can do a complete storage circuit calibration. It seem that in the pictures you took the delayed timebase is turned on. Turn it off by rotating the outer timebase knob fully clockwise before making adjustments or storing images. It really works only in conventional mode since the intensified spot on the trace causes the screen to flood in storage mode. |
| mayeri:
Thank you very much for your continuous support. Even I want to show this… the screwdriver was used enough. |
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