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(Note : This page was converted from my twitter posts for archiving purpose.)
The Tektronix TDS 220 CCFL lamp was at its end of life. I replaced it by a LED strip. Working great, less power usage and less heat :)
The power supply deliver these voltages : +5V, -5V and -23,5V
The +5V is used by the digital and analog parts, the -5V for the analog part and the CCFL driver, and the -23,5V for the LCD screen.
Power supply pinout :
I used a "CA-490-480MM-22W-87LED" strip. (i used many of them to fix various PC and TV screens...).
This one is arranged in groups of 3 LEDs in-series. To match the original CCFL length, i have cut the strip to have 3 * 6 LEDs.
The "CA-490-480MM-22W-87LED" 3 LEDs in-series appears to have a Vf voltage of ~7.5V, i mean : the LEDs start to shine at this voltage.
So a higher voltage is needed, but the 23.5V appears too high for this to me.
I choose to use the +5V and -5V power lines to supply the LEDs strip (The LEDs strip "see" 10V).
I made some tests to know much current i need to have the right brightness.
It appears that a current between 20mA and 50mA is well enough for this job :)
I used 100 Ohms and 75 Ohms resistors in parallel (equivalent to ~42.85 Ohms) as current limiter for the strip.
I have also disconnected/disabled the CCFL driver stage since it is not used anymore. For this i removed L6 on the power supply board.
With this 43 ohms resistor i measured ~36mA to supply the LEDs strip. Only ~56mW is dissipated by the resistors and 360mW used by the whole backlight. Less power consumption, less heat, less noise 😀. This nice 22 years-old oscilloscope is up and running again.
4 April 2021
Power supply output current (with the above backlight mod):
Power supply chemical capacitors (all radial - Most NCC SXE):
10 September 2023
30 January 2024
(C) HxC2001 / Jean-François DEL NERO