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.
Screws holding vertical mode button block
It was discovered that the cause of button not locking was a piece of loose plastic getting in the way so after blowing some compressed air through switches and applying a drop of DeOxit the block was as good as new. Illumination bulb was produced from ordinary eBay variety 5mm white LED using Dremel tool. Pictures below show original and modified LEDs, and LED placement on the board. On a second picture, original bulb can be seen to the right of the LED.
LED shortened to fit
LED replacing light bulb in Tek 7904 vertical mode button block
Overal, I’m very pleased with this modification. The illumination is soft with slight bluish tint which, in my opinion, matches the aluminum faceplate better than “straw yellow” color of original bulbs. It also dims quite nicely with standard control illumination rotary switch of the instrument. I ran out of LEDs so I couldn’t replace all of the bulbs at once, but one day I will surely do this.
Oleg.
Oleg, Thank you for the info on the 7904. I also have one and yesterday turned it back on for the first time in over 6 years (long story). It is in need of your repairs as well as taken to a shop for calibration/repair.
Would you have a source for a manual/manuals for the unit?
I have the 7904 with the following sub units:
7A26/7A24/7B85/7B92A
I will definitely look into doing you mods once I get my hands on a manual.
Merle
Merle,
The manual for 7904 is available at BAMA -> http://bama.edebris.com/manuals/tek/7904/ . You may also find manuals for some of your plugins there. If not, Artek Media sells scans for cheap, you can get a CD with almost everything from 7000 series for like $10.
I am going to mod lights behind small acrylic buttons, like trigger source selectors. It seems that 3mm LED would fit there. I ordered a bunch of LEDs from eBay, will post results when done.
Oleg.
Nice project. I did this to my 7000 series scope but what i did was to use smaller LEDS that were the same size as the bulbs and since all the bulbs use the same supply voltage and only one is on at a time i severed the supply circuit trace and added the limiting resistor on the trace over the break. that way you just solder in the leds without the need for a resistor for each led.