Burnt diode...sources of the problem?

liddokun · February 01, 2009, 02:36:39 PM

What are common sources for a diode to burn up in a circuit? I'm trying to help out someone who has a Keeley modded BD-2. He says he plugged in the boss power supply and he saw smoke coming coming out, so he unplugged it. Opened up the pedal, and saw the power diode by the dc jack had burnt up.

kurtlives · February 01, 2009, 02:53:55 PM

Reverse polarity from the adapter, hence the reverse polarity protection diode.

I have had Zeners that internally failed though and shorted to ground, which then caused them to heat up and fry.

Did you get it sorted out though?

liddokun · February 01, 2009, 03:21:11 PM

So I'm assuming I can just replace the diode and it will be fine?

frank_p · February 01, 2009, 03:34:12 PM

I don't know if it's a good idea, but maybe he could replace the diode with one that can handle more Voltage.

R.G. · February 01, 2009, 03:36:47 PM

Replace the diode. But then test the pedal.

Boss DC power supplies generally don't have enough current to burn up the common 1N400x diodes used for polarity protection. These are 1A rated diodes and the regulators in the usual Boss power supplies are rated at 1A, so if everything is as stated, a diode could be left across a Boss power supply for an indefinite period without burning either the power supply or the diode. As a second issue, It's highly unlikely that even Keeley would put in a power jack that is backwards for the standard Boss power supply.

I believe that your friend has accidentally applied the "Power Supply Kiss of Death" to the pedal and doesn't realize it. This is done by plugging in a 9V AC power supply with the same kind of plug as the Boss power supply. A couple of different versions of these exist. This situation will first fry the diode until it shorts, and then if left long enough will burn the diode open, generally burning the PCB under the diode while doing this. When the diode opens, the alternating 9V ac (which is 1.414*9 = 12.7V peak) to the now-unprotected circuits, which generally then die.

If you're lucky, the diode was still shorted and all the ICs and electro caps on the PCB are still OK. If not, they are dead. If you're really, really unlucky, SOME of the parts on the board are dead, and the others are close but will still appear to work and then fail a little later.

As your friend to look at all the power adapters on his pedalboard. If any of them provide 9Vac, think really hard about replacing all the ICs and all the electro caps to avoid future chain failures.

Quote from: frank_p on February 01, 2009, 03:34:12 PM
I don't know if it's a good idea, but maybe he could replace the diode with one that can handle more Voltage.

That won't help. The diode probably didn't fail because it was overvolted; likely it was simply cooked by too high a current as I outlined.

liddokun · February 01, 2009, 05:39:28 PM

Well, I asked him, and he says he's using one of the newer Boss PSA's, which has the green LED on the adaptor, which he says also works fine with his DD3. Of course, he could be mistaken on this one, but for the most case, I'm suspicious because that wouldn't be able to burn the diode. Assuming he is using the PSA, which should be 9vdc, what options do I have? I could replace the diode with higher current rated diode, have a look at the IC's.

R.G. · February 01, 2009, 11:38:26 PM

I don't know about the Boss PSA, but I have personally taken a 1.7A output 1Spot and connected a 1A diode to it the bad (conducting!) way. Let it cook for a couple of hours. No damage to the power supply or the diode.

The older PSAs were transformer/rectifier/filter/7809. The new ones are switching regulators, I think.

liddokun · February 02, 2009, 05:19:32 PM

Interesting. Then I'm at a loss as to what the problem might be. Perhaps he's hiding a part of the story to me.

R.G. · February 02, 2009, 06:51:48 PM

It may well be that he doesn't know about the 9Vac thing. All of the plugs in a pedalboard look alike. It's easy to plug in the wrong one and miss it.

liddokun · February 02, 2009, 07:06:30 PM

Well he did say it works fine with his other Boss pedal, the DD3.

petemoore · February 02, 2009, 07:14:43 PM

http://www.zzounds.com/item--BOSPSA120S
The DD3 can use this one...9vdc.

liddokun · February 03, 2009, 04:19:05 PM

Well, yes, that's the adaptor he's using, which doesn't explain why the diode burned...because it's DC voltage the pedal was getting, not AC.

R.G. · February 03, 2009, 04:51:37 PM

Once more - is that the ONLY adapter he's using?

Even if it is, it is possible that the Boss doesn't have a soft current limit like I'm used to working with. Here's one way to check. Take a 1N4002 through 4007 diode, any one in that series, and connect it to the Boss PSAxxxx he has in the direction which would make current flow. Plug in the Boss adapter. Does the diode smoke? If not, the Boss didn't do it. If yes, it may have.

But is that the ONLY adapter he's using?

liddokun · February 03, 2009, 05:47:04 PM

Yes, he says its the only one he's using, plugged it in with the BD-2, got the problem, but it works fine with his DD3. I'll try the diode test.

MikeH · February 03, 2009, 05:51:02 PM

Is it possible the diode was suspect from the start and just finally failed?

liddokun · February 03, 2009, 06:46:50 PM

Quote from: MikeH on February 03, 2009, 05:51:02 PM
Is it possible the diode was suspect from the start and just finally failed?

That could be it. As it stands, there really isn't much I can do. Nothing else seems burnt or damaged. I'll just have to replace the diode.

R.G. · February 03, 2009, 07:44:53 PM

Sometimes Mother Nature just does something to get your attention.

She's like that.

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Burnt diode...sources of the problem?