purpose of a signal diode across 9v jack - need to troubleshoot

[DMS]

Hi all,

I just bought a used Trem pedal, and the battery was DOA, so I replaced it.  After putting in a used battery, I still had no operation, and could soon sense the smell of cooking components.  When I opened the back, I noticed 2 things - hot, smelly battery and a diode across the +/- of the 9v jack  - also very warm, and looking a bit distressed.  Can someone explain the function of this diode, and hazard a guess as to its likely type?  To my untrained eye it has the look of a signal diode, like a 1N418 or something similar.

Thanks, DMS

[Seljer]

the diode was there as basic polarity protection.

read the intro part of this article:
http://www.geofex.com/Article_Folders/mosswitch/mosswitch.htm

A cleverer arrangement is to put the diode in parallel with the effect so it's reverse biased by the normal polarity. When the voltage is reversed, the diode conducts heavily and clamps the reverse voltage to no more than one diode drop. You don't even have to pay the diode forward drop in normal operation. Slick, huh?

Of course, when the voltage is reversed and the diode keeps the reverse voltage clamped to 0.7V, the diode and the battery are engaged in a duel to the death; the battery is determined to bring the voltage up to its internal voltage, the diode is determined to hold it down. The current that flows will often heat a 9V battery so hot you can't hold it in your hand, and may burn out and short the diode - now the pedal is "dead" because the diode died and won't let any voltage reach it the right direction, either!

[DMS]

Thanks - this is exactly what I needed! So I guess I need to go check polarities, in case the pedal had been modified or "repaired" prior to my receiving it.

[Peter Snowberg]

Welcome to the forum.

My guess is that your pedal was plugged into an AC adaptor with the wrong polarity. The diode failed in dead-short mode, and it has not made noise since.

You can always cut the diode out and that will almost certainly make the effect spring back to life.

I would remove it and replace it with a power diode like any 1N400x series. You could also use a Schottky like the 1N5818, which has 1/2 of the forward voltage drop or a regular Si diode and will offer better protection for Ge transistor circuits.