When I was kid of about 10 years old I learned a lot about things by taking them apart. Quite often what I learned was that the owner of the now dismantled thing might not be very happy with me since I seemed to have broken some vital part during the dismantling process. None the less many lessons were learned in the theme “What one man can build, I can take apart.”
In the modern world “man” has been replaced by machines, and what man’s machines can build is too damn small for my old eyes to see or for my fingers to grab – and so is ‘Nigel proof’. However, despite the modern need to make everyone much smaller, some things can still be dismantled and even repaired.
A few days ago I was using my ancient Tektronix AS 475 four channel 100 MHz scope to diagnose an ailment in a tube amp. In a moment of weakness I used one of my very nice Agilent 300 MHz X10 probes to look at a phase splitter anode and the anode voltage on a couple of EL34 output tubes.
A while later I could understand why one channel was showing a huge signal when the next channel showed a small one. Same probes on both. Same vertical settings. I put a meter on the scope probe from tip to BNC center and found that what should have been 9.1 Mohm was only about 475 kohm (which as actually the resistance of the cable – really). Bugger – I’d fried the tiny adjustable capacitor in the probe tip and shorted out the little resistors in there.
After a few minutes online I decided that $50 or more for a new probe was silly if I could fix this one. So here is the saga of pulling my probe apart and ….. yes, more or less fixing it. The end product no longer has an adjustable capacitor in the tip and hence not very accurate high end performance but as I only ever do audio work it’s fine.
Getting the probe apart involved slitting the plastic case lengthwise. Once that was off it was fairly easy to pull most of it apart. I found that I didn’t need to pull the cable out of the tip as it’s actually connected to the probe with a nice little connector. Some super glue and heatshrink tube fixed that.
There was no way I could find a little variable capacitor so I experimented with assorted small caps (10 pF) that I had to see when the scope’s test squarewave looked reasonable. I used two 10 pF caps in series to get a small enough capacitance. It’s really still too big but there wasn’t room to squish three into the tube and those were the smallest ones I had.
This was probably a waste of time as the caps I used are maybe 50 V rated so I now have a maybe 100 V probe. But it’ll do for low voltage stuff and stands as a reminder to use the x100 high voltage probe I normally use on tube amps.
Here for your entertainment are the pictures. Probably best to buy a new probe and not waste your time!
