Leaky/Intermittent Tantalum Capacitors

[ppppenguin]

Using tants for timing etc is fine. it's where they are exposed to enough power to fry them that gives problems. Since Tek would hardly have knowingly used poor quality parts it seems that this is a common problem as Jeremy has found.

[pamphonica]

I have also used them for timing and they are definitely fine.
The main problem with failing tants on decoupler duty is that they fail short-circuit, not open. that means they often also take out associated pass transistors, fuses etc. Electrolytics seem to go high ESR rather than short, so just become ineffective.
Although I must say I had to take out a big 4700uF/70V cap the other day in a signal generator power supply that had turned itself into a 4 ohm resistor with similar consequences.
The main failure on tiny SMD (surface mount) electrolytics nowadays, especially those from the 80s is that they start to leak, and the leakage can destroy or badly contaminate all the nearby pcb traces. So you just can't win!

[ppppenguin]

Nowadays many designers will avoid electrolytics are far as possible. Still need a few critical ones in switchmode PSUs but the vast majority of coupling and decoupling can be done with ceramics. 100uF low voltage ceramics are cheap and plentiful. Higher values are available. They're not the most wonderful of caps, with wide tolerances and C depending on applied voltage. But then electrolytics are hardly wonderful either.

[Craig]

The Tektronix problem relates to solid tantalum beads used as supply rail decoupling. This mainly affects 500 power rack plugins, 7000 series oscilloscopes and their plugins. Tek, as we all did back then, used tant beads in this application, but with insufficient voltage overhead. So 6V parts on 5V rails and 16V parts on 15V rails - and that is why the voltage ratings are what they are.

The problem is when subjected to start-up transients - either voltage or current - in the fullness of time they randomly go short circuit. And they really are a hard short, either burning or causing other things to burn and/or the power supply to crowbar or go into protect mode.

The fix, as already suggested, is to replace them with aluminium electrolytics, with several times the capacitance to ensure ESR is comparable. In fact in later serial numbers, Tek did precisely that, perhaps because they had identified reliability issues with tant beads for rail decoupling.

I have not seen a failure in a tant bead used at other points in the circuits.

[Refugee]

If you really want to go short hunting for sport just go for an early 1980s mono TV camera and you will not be disappointed.

[Craig]

What a lot of boards!

[Refugee]

There is parts from the camera ans some spares. The complete camera has 6 boards. Three boards do the pictures and the other three are regulators, video over power and PTZ. Engineers hated them when they were still in production and yes those capacitors used to blow up even when they were brand new.