Bram Nauta reunites with a former flame in need of some repairs.
It must have been around 1988 when we met for the first time. Sitting on my desk, you were big, impressive. When I switched you on, I could hear the heavy sound of your fan and all your yellow LEDs started to blink. You had an impressive number of keys on your front side. After the picture tube with only two colors had warmed up, I could see your noisy trace appearing.
You are an HP 4195, a DC-500 MHz vector network analyzer that the university bought, and I was the first to actually use you. During my PhD studies, you were the one that I could rely on. You showed me that my first integrated 100 MHz filter in 3-micron technology worked exactly as expected.
About 30 years later, we met again. Broken, you had been stored away in a storage room for over 10 years. The lab has full of much better, faster, lighter and cheaper machines. Your floppy drive is totally outdated now. Looking at your backside, I couldn’t find any communication connector that’s in use today. Our technician wanted to take you to the recycling station.
But since I had so many fond memories of you, I asked if I could adopt you. With two guys, we had to lift you, and it was a hell of a job to get you into the attic above my garage at home. But at least we were together again, and you were saved from demolition. My plan was to try and repair you after my retirement.
Then came Covid, I was locked down at home and not allowed to go to my lab at the university. I had time to build some electronic circuits and I invested in a modern oscilloscope and network analyzer. I got so enthusiastic again by this tinkering that I said to myself: why wait until I’m retired? I can try to repair my HP 4195 right now. Plenty of time!
I managed to find your service manual and complete schematics online. After I opened you up, I found you packed with stacked printed circuit boards, full of components soldered to the holes of the boards. You were still shiny and looked quite good! Your computer was built with logic ICs and even your tiny memory looked impressive with all the big black ICs with rows of pins on each side. And your RF part was magically shining after all these years. You were a beautiful oldtimer!
After replacing a fuse and a nickel-cadmium backup battery, you came to life. All the LEDs still blinked and the fan buzzed. I ordered a white-noise generator at Aliexpress and tested your receiver channel separately. The white noise looked white indeed on your screen, but there was a gap in the curve between 225 and 340 MHz. Looking at your schematics, I could see you had many filter banks, which were switched in and out by diodes. One bank matched the failing frequency range. There was still a bit of a high-pass characteristic visible at the bottom of the gap, indicating there was still some capacitive crosstalk. This way, I reasoned that a specific diode had to be broken. I located it, replaced it and you worked again!
Technology has advanced a lot and thanks to chips, we have fantastic measurement equipment these days. However, these new machines will be much harder to repair and with the chip shortage, it’s even hard to buy a new machine. That’s why I like you, oldtimer, full of discrete components, but at least I can repair you!