Hi all,
I try to fix this oscilloscope, found in the trash bin of a high school (together with two power supplies, and a HeNe lase, all since repaired).
He had a power problem: no trace on power on, due to a bad solder joint in the SMPS. Once repaired, the trace reappears.
The time base is good, as well as the two channels : each one is able to detect a 1kHz 2V square wave, with good accuracy.
But the trace is blurred : at power on, the trace appears neat, and after a few seconds, it becomes blurred.
The intensity setting does not work (the potentiometer is tested good): no action and no cut-off. The focus adjustment works, but doesn't solve the problem.
Gradually as the trace becomes blurred, the high voltage is reduced : at startup, -2000V on the cathode and Wenhelt, -1890V on focus. Then all these tensions are reduced, starting with the focus. The transformer T3 quickly becomes hot: there must be a short somewhere.
Whitout schematics, it's not easy to identify the problem.
All resistors and diodes near C1108...C1115, seems to be good on test. There is a pulse signal going through C1106, C1107 and C1115, but nothing, except some switching noise I think (something like 5mV, high frequency), is going through C1108 from the motherboard (one side of C1108 : -2000V, the other is at 0V).
I've read this thread, which was very usefull, and this one too.
Despite this, I need help to repair it : schematics, or ideas.
I try to fix this oscilloscope, found in the trash bin of a high school (together with two power supplies, and a HeNe lase, all since repaired).
He had a power problem: no trace on power on, due to a bad solder joint in the SMPS. Once repaired, the trace reappears.
The time base is good, as well as the two channels : each one is able to detect a 1kHz 2V square wave, with good accuracy.
But the trace is blurred : at power on, the trace appears neat, and after a few seconds, it becomes blurred.
![upload_2015-5-2_9-34-40.jpeg upload_2015-5-2_9-34-40.jpeg](https://www.electro-tech-online.com/data/attachments/81/81252-1e916f634d6c390e03207fe68ec078d1.jpg)
![upload_2015-5-2_9-35-40.jpeg upload_2015-5-2_9-35-40.jpeg](https://www.electro-tech-online.com/data/attachments/81/81254-923cd51fbb28b0c091041dd416e78022.jpg)
The intensity setting does not work (the potentiometer is tested good): no action and no cut-off. The focus adjustment works, but doesn't solve the problem.
Gradually as the trace becomes blurred, the high voltage is reduced : at startup, -2000V on the cathode and Wenhelt, -1890V on focus. Then all these tensions are reduced, starting with the focus. The transformer T3 quickly becomes hot: there must be a short somewhere.
Whitout schematics, it's not easy to identify the problem.
All resistors and diodes near C1108...C1115, seems to be good on test. There is a pulse signal going through C1106, C1107 and C1115, but nothing, except some switching noise I think (something like 5mV, high frequency), is going through C1108 from the motherboard (one side of C1108 : -2000V, the other is at 0V).
![DSC_0407.JPG DSC_0407.JPG](https://www.electro-tech-online.com/data/attachments/81/81255-db48f0d72a0504ee7b5e73ec155d13dd.jpg)
![DSC_0408.JPG DSC_0408.JPG](https://www.electro-tech-online.com/data/attachments/81/81256-c2bb7fd1873ac47dacc1b3292e83c370.jpg)
![DSC_0409.JPG DSC_0409.JPG](https://www.electro-tech-online.com/data/attachments/81/81257-2465c06ea35156675c07f84354a1f3ea.jpg)
![DSC_0411.JPG DSC_0411.JPG](https://www.electro-tech-online.com/data/attachments/81/81258-0b04d898d084fc2cf4ff1d8479ffd330.jpg)
I've read this thread, which was very usefull, and this one too.
Despite this, I need help to repair it : schematics, or ideas.