Fried pedal with wrong power supply

[carlvan]

Hi everyone, i fried a EWS Brute Drive using a 12V supply with wrong polarity.  I know     The diode in parallel with a 25V 100mF cap in the power section is shorted.  Does anyone know what is a good replacement diode? It's marked F 91 or F 9 i think. Is it a zener? I hope it didn't zapped anything else.
thx

[patrick398]

Welcome!

That looks painful. If it's indeed in paralell with supply you could replace it with a 1N4001/1N4007 but there's no guarantee there isn't some damage done elsewhere

[antonis]

I would suspect more Q1 (2N5088) input CE amp BJT than 4558 op-amp..
(although it appears to hane more than 400k resistance from Base to (reverse polarity) ground, its Emitter-Base reverse voltage rating might be exceeded..)   

[duck_arse]

also welcome.

1N914A. equivalent to 1N4148, and many other small signal diodes.

[carlvan]

Thanks a lot everyone for the excellent advice. I replaced it with a 1N4148 diode. It's like new   

[ElectricDruid]

Thanks a lot everyone for the excellent advice. I replaced it with a 1N4148 diode. It's like new   

Well done! Isn't it nice when it works?!

[R.G.]

I replaced it with a 1N4148 diode. It's like new   

The 1N914 and 1N4148 are both rated for 100ma forward current. Since the diode is reverse biased all the time except when it's protecting the pedal, why not put in a 1N4000 series diode, good for 1A continuously, and about 35A pulses?

[antonis]

The 1N914 and 1N4148 are both rated for 100ma forward current. Since the diode is reverse biased all the time except when it's protecting the pedal, why not put in a 1N4000 series diode, good for 1A continuously, and about 35A pulses?

For also protecting Power Supply, perhaps..?? 
(in the form of 100mA slow blow fuse..)

[R.G.]

Could be. But that design idea counts on the diode shorting reliably, and then on the power supply not being able to burn open the short.
I'd go for a fuse or current limiter ahead of the diode, or a diode that will simply take the strain. But that's just me.

[antonis]

But that's just me.

Could I presume unmitigated igorance about power supply basics for you, R.G. ..?? 

BTW, there were some older pedal designs with a resistor in series with reverse polarity protection diode as current limiter device..
(obsolete now, for developing across them dangerous for powered circuit reverse polarity voltage reason, I suppose..)

[ashcat_lt]

Yeah you basically got lucky that the diode shorted.  In every case I've ever seen, that diode just explodes and unless you get "lucky" enough that it also destroys traces on the PCB, the reverse polarity is then free to just blow up everything else in the pedal that cares about PSU polarity.  I call this a "warranty protection diode" because it doesn't protect the circuit so much as prove to the manufacturer that you used the wrong polarity so they don't have to fix it for free.

Series diode (rather than parallel) will actually protect the circtuit by just not passing wrong polarity current.  Yes, there's a voltage drop.  Sometimes that doesn't matter.  Sometimes that makes it sound better.  Usually you could just use a Schottky and not be able to tell the difference.

[PRR]

> a diode that will simply take the strain

[ElectricDruid]

> a diode that will simply take the strain

Nar, I'm using those for clippers. Need something better for power supply protection.