My Physics teacher recently acquired some old electronics equipment from a company that was throwing them out, and said that I could have some. I ended up with a Hewlett-Packard digitising oscilloscope from the early 90s. The display turned out to be broken, but before taking it apart I wondered whether I could fix it somehow.
The horizontal deflection of the cathode ray tube display was broken, so that only a narrow band at the centre was shown:
To start searching for the cause I opened up the oscilloscope to look for any obviously damaged components such as capacitors that had inflated. Firstly I had to discharge the cathode ray tube (apparently they operate on thousands of volts). I left it for a week and then stuck a grounded screwdriver under the rubber cap protecting the anode to ensure it was fully discharged.
However, there was no obvious issue so I turned to the internet and asked a question on the r/fixit community of Reddit.
They referred me to the EEVBlog forum, where I asked the question again. This time they suggested I look for a large capacitor on the CRT control board to the right of the display. In order to remove this board so I could access it I had to cut through a number of wires:
There was a large, suspicious looking 3.3uF bipolar capacitor on the board as you can see:
I decided to remove and test this one first. Using my £4 Chinese LCR meter I found that it had a capacitance of 0.8 uF when it should be 3.3uF, and an ESR of 150 Ohm when you’d expect around 4.7 Ohm. I decided that this was likely the culprit. I ordered some replacement 3.3uF bipolar capacitors for £1.97 on AliExpress and some heat shrink tubing for £0.38 to reconnect the wires I had to cut.
It’s amazing how much components have miniaturised in only around 30 years:
I soldered the new capacitor to the board:
And soldered the cables back together, covering the joints with heat shrink tubing:
At this point it was time to turn it on. I was half expecting something to explode or go up in flames, so I placed it outside, flipped the switch and watched from a distance. To my delight, the entire screen lit up:
I verified it was working by connecting an Arduino Uno producing a PWM signal with a 25% duty cycle at 5V:
I also enjoyed looking at the rough 50Hz sine wave when I touched the probe. Apparently my body acts as a crude antenna for the AC mains in the walls:
So that proves that there’s no school like the old school!
Brychan Thomas' projects 