Interesting diode failure mode

[Prehistoricman]

Story time.
I was fiddling around with my guitar pedals when I realised that my vintage PSK DST-2 distortion pedal (schematic here) was really warm. It was so warm, in fact, that the sticky-back Velcro had detached from the pedal. I had to investigate.



Under the highlighted brown patch was a diode. A reverse-polarity protection diode intended to short the supply if it's coming in in the wrong direction. I first spotted the diode not by the brown patch, but an intense burning sensation.



This generic tester did not recognise the part. Let's do some analysis of our own...



On the right you can see the normal diode characteristic but with a super strange reverse-bias one.
This puppy was dissipating about 2 watts inside the pedal. I thought diodes failed short...?

[T3sl4co1l]

Cool, a partial failure.  Probably burned through in a spot, causing a ~resistive characteristic?

Would guess replacing it with a TVS or zener would be recommended.

Tim

[duak]

I remember finding a Zener diode that did something like this once.  It was in a power supply and was supposed to regulate a voltage for a low level stage.  I sort of recall it running hot so I replaced it with one with greater dissipation.

If you look up "Zener zapping" you can see that with enough overcurrent, the junction can be completely shorted out. Apparently, the aluminum in the bonding pads diffuses into the silicon.  I suppose if the Zener was operated just above its limit, over time enough aluminum atoms migrate and cause a parallel resistance while leaving the diode junction somewhat functional.

Cheers,

[Prehistoricman]

Interesting!
I suppose it could have been a Zener. I don't have any official schematic and the number on the package doesn't bring up any results (011AA).

Replacing the diode has fixed a few odd behaviours of the pedal (sorry, Tesla, I just used a random diode that was lying on my desk). It used to turn on (engage the effect) after unplugging another pedal earlier in the chain. That's been happening for a very long time so I suppose it's been faulty (to a lesser extent) for an even longer time.