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.
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.