When you tear into an old piece of test equipment, you’re probably going to come up against some surprises. That’s especially true of high-precision gear like oscilloscopes from the time before ASICs and ADCs, which had to accomplish so much with discrete components and a lot of engineering ingenuity.
Unfortunately, though, those clever hacks that made everything work sometimes come back to bite you, as [Void Electronics] learned while bringing this classic Tektronix 466 scope back to life. A previous video revealed that the “Works fine, powers up” eBay listing for this scope wasn’t entirely accurate, as it was DOA. That ended up being a bad op-amp in the power supply, which was easily fixed. Once powered up, though, another, more insidious problem cropped up with the vertical attenuator, which failed with any setting divisible by two.
With this curious symptom in mind, [Void] got to work on the scope. Old analog Tek scopes like this use a bank of attenuator modules switched in and out of the signal path by a complex mechanical system of cams. It seemed like one of the modules, specifically the 4x attenuator, was the culprit. [Void] did the obvious first test and compared the module against the known good 4x module in the other channel of the dual-channel scope, but surprisingly, the module worked fine. That meant the problem had to be on the PCB that the module lives on. Close examination with the help of some magnification revealed the culprit — tin whiskers had formed, stretching out from a pad to chassis ground. The tiny metal threads were shorting the signal to ground whenever the 4x module was switched into the signal path. The solution? A quick flick with a sticky note to remove the whiskers!
This was a great fix and a fantastic lesson in looking past the obvious and being observant. It puts us in the mood for breaking out our old Tek scope and seeing what wonders — and challenges — it holds.
An extremely unlikely find, however not impossible. One would never think to look for tin whiskers within an assembly produced prior to the widespread use of ROHS solder, but ROHS solder may have been used by someone making a repair using the closest, most available solder at hand. Tin whiskers remain to be the reason ROHS is not suitable for aerospace applications.
PS for all who are interested in the likes of tin whiskers and dendrites and wish to understand them(as much as possible, as we still have no definitive agreement on the exact mechanism of action), here’s a link: https://nepp.nasa.gov/whisker/background/
Whiskers are also a problem with zinc and silver coatings, even gold in some environments, so it’s not solely a RoHS problem.
You don’t want to repair old electronics with 99%+ tin solder because it won’t react well with whatever there is already, and it will turn to dust if you keep the board in cold storage for a year or more, but other things like SAC305 keep fairly well. They have other stuff that keeps the tin whiskers and tin pest down reasonably well.
Not sure about the 466, but earlier Tek scopes used ceramic terminal strips with silver plating for the actual terminals. Using regular tin-lead solder on those would dissolve the silver and destroy the terminal. Somewhere in those scopes would be a small coil of silver-bearing solder (NOT silver solder) which was specifically for use with those terminals.
also inside germanium transistors in vintage radios
A sticky note! Definitely not a function I was aware they were suited for! Very handy!
Just yesterday I found some dead c***kroaches in my bf’s Amiga 1200.
wasp nest in an old radio i was restoring 😂
I found rat turds in a Fender amp once…
I recall an old Tektronix film showing their repair process in the era around the time that these scopes were being manufactured. One of their first steps was to disassemble the scope and put all the boards in a dishwasher. I’ve done it myself with dirty old electronics and it works well if you allow sufficient time to completely dry the boards (several days in a warm location). I can’t say, but probably would have cleaned up the whisker causing this fault.
Back in the day I toured Tektronix in Beaverton, Oregon and saw that dishwasher. It was a KitchenAid.
Guess what will cause a new short now that is free floating inside?
Got the same issue 20 some years ago in an HDD external rack with 8 disks.