Front panel interconnect board of Tektronix 7904
I am a proud owner of several Tektronix 7000-series mainframes. Among them, 7904 500MHz 4-compartment oscilloscope is my bench workhorse. The screen is big enough, the bandwidth is adequate for majority of tasks I do, and absence of cooling fan makes it pleasantly quiet. In addition, the instrument is lightweight comparing to other 7000 mainframes and has legs mounted on the rear panel making it possible to put scope on the floor in a vertical position.
I became annoyed by malfunction of right buttons of both vertical and horizontal mode selectors (they failed to lock in place) and decided that my scope deserves some TLC. Besides fixing the buttons I also wanted to replace some dead illumination bulbs with LEDs. Mode switches are dual, with one switch in the pair dedicated to turning light bulb on/off. Bulbs are powered by 5V and dimming is implemented by means of two diode drops. So the 3V LED restricted to 10ma by a resistor shall work fine and even be able to dim a little. In addition, LED power is comsumption about 8% of a bulb.
The front panel buttons are mounted on a narrow PCB (called “A3- Front Panel Interconnect Board” in the service manual) running across the middle of the front panel. It can be easily accessed after removing side panels. There are several cables connected to the board on both sides, it is good idea to mark them before disconnecting. The title picture shows left side of the board (one close to horizontal bay “B”) with my marks on it. After pulling out cables I moved the PCB carefully towards the rear and to the side. The board bends easily and is somewhat accessible form the top of the instrument.
The button assembly is held in place by screws also used to mount upper plug-in guide bars in the plug-in compartment. Picture below shows location of the left ones (holding vertical mode buttons). After unscrewing four Phillips-head screws I pulled the vertical mode buttons out of the chassis, carefully guiding it around cables, structural members and dead mice.
Continue reading Repairing front panel buttons of Tektronix 7904 oscilloscope.