What causes a rectifying diode to fail?

[joecool85]

I'm working on a Genz Benz Tranzamp g30 and it has at least two shorted out rectifying diodes.

When I got the amp on my bench it wouldn't power up.  It had a blown fuse.  Not seeing anything obviously burnt, I grabbed a new fuse and plugged it in.  When switched on the speaker hums really loud and when I disconnected the poweramp totally out of the equation I still get quite a bit of hum...but from the transformer.  If it stays on for more than 4-5 seconds, one of the rectifying diodes will start glowing and then start to smoke the PCB.  They are 1n4002 diodes.  I have 1n4004 on hand I am planning on replacing them with.

So my question is, should I be able to just put in the new diodes and be on my merry way?  Or is it more likely something else is wrong and that's what caused the original diodes to short?

[Enzo]

Fix it and find out.  Diodes fail, all by themselves.  DIodes also fail when somethiing else draws too much current through them.  We won't know until we get there.

How many rectifier diodes in that circuit?  Two?  Four?  They cost a nickel, replace them all.  Any remaining "good" ones were stressed by the shorted one/s.

Check your outputs for shorts.  If they show shorted, then get to work on that section too.  If they SEEM OK, they probably are.

A shorted filter cap could damage a diode, but that doesn;t really happen all that often.

You usually won;t find some reason some part fails.

[joecool85]

I checked the output before even opening the amp, it's fine.  I already have a handfull of 1n4004 on hand so I will replace all four rectifying diodes and give it a go.

[JPHeisz]

I learned my lesson with diodes (had 1N5406's short and nearly smoked the transformer) - I only use bridge rectifiers now, for anything that draws serious current.
Might be worthwhile to replace those with a 15-25A bridge.

[joecool85]

I learned my lesson with diodes (had 1N5406's short and nearly smoked the transformer) - I only use bridge rectifiers now, for anything that draws serious current.
Might be worthwhile to replace those with a 15-25A bridge.

If it was mine I would consider it, but I'm doing some repair work for a local guitar shop and need to keep the cost as low as I can while still making a good repair.  If it was mine I would also remove the on-board fuse and put in a chassis fuse holder.

I'll keep you guys posted.  I won't have time to do the repair till Sunday.

[joecool85]

I replaced the blown fuse and the 4 rectifying diodes.  The power supply is good now, reading +27.7v/-27.3v.  But when you power it up it hums like crazy (quite loud) no matter what is plugged into it.  So I decided to pull the daughter board the TDA2050 is on and found that one of the pins (I think it was pin 5, V+) was scorched and the solder was mostly gone and what was there looked like ash.  I cleaned it up, resoldered that pin and it still acts the same.  I'm thinking the TDA2050 is cooked.

What do you guys think?

[J M Fahey]

I think you can serve that TDA2050 with a side order of fries.

[joecool85]

I was wrong on the pin, it was pin 4.  The output pin.

What would cause it to do that?  Would it be that the diode (only one had shorted) died and cooked the TDA2050 or would it be that the TDA2050 cooked and that fried the diode?

The speaker measures 7.2 ohms and it is an 8ohm speaker, so that should be fine.

[Enzo]

We will never know.   Parts fail.  There is often no correlation with some event.  The 2050 is far more likely to fail first over the diode.  A shorted rectifier is likely to just blow a fuse, and not partidularly likely to blow the 2050.  SO 2050 first diode second is more likely, but who knows.  Either way we have to replace those parts, and then the amop will be fine.

The loud hum was probably just DC on your speaker.

[joecool85]

We will never know.   Parts fail.  There is often no correlation with some event.  The 2050 is far more likely to fail first over the diode.  A shorted rectifier is likely to just blow a fuse, and not partidularly likely to blow the 2050.  SO 2050 first diode second is more likely, but who knows.  Either way we have to replace those parts, and then the amop will be fine.

The loud hum was probably just DC on your speaker.

You don't think it would have fried anything else along with the 2050?  I mean, I'm sure it could have, but it doesn't appear to be so.  No hot marks on the top of bottom of the main board.  I was just concerned it might have damaged one or more of the three JRC4558 opamps.  That would be a pain to replace all of those.

Hopefully the music store will want to continue repairs and pay the few extra bucks for the chip and labor.  I'm thinking you're right and a new chip would get it going again.

[J M Fahey]

No hot marks on the top of bottom of the main board.

You can't *visually* repair am amplifier, just by looking for burn marks or bulged capacitors.
Maybe 5% of bad parts "show"it; for the other 95% you'll have to actually measure.

I was just concerned it might have damaged one or more of the three JRC4558 opamps.  That would be a pain to replace all of those.

Don't get why a failed PSU diode or a power amp chip might damage a preamp chip.

[joecool85]

No hot marks on the top of bottom of the main board.

You can't *visually* repair am amplifier, just by looking for burn marks or bulged capacitors.
Maybe 5% of bad parts "show"it; for the other 95% you'll have to actually measure.

I was just concerned it might have damaged one or more of the three JRC4558 opamps.  That would be a pain to replace all of those.

Don't get why a failed PSU diode or a power amp chip might damage a preamp chip.

I was concerned about it throwing AC through the chips - nothing to be worried about?

[J M Fahey]

A shorted diode would let AC current pass freely, yes, but that will be shorted to ground by the filter capacitors, blowing the main fuse (that's what happened on your amplifier).
Besides that, preamp ICs are fed through a couple resistors and Zeners, which would further block any undue voltage that tried to reach them.
That's why I said that it's not impossible but quite unlikely a failure.
If anything, the power amp chip or transistors *might* be in danger because they are fed through a more direct path.
Personally I connect every suspect amp through a series limiting lamp, until I am sure it works properly.

[joecool85]

A shorted diode would let AC current pass freely, yes, but that will be shorted to ground by the filter capacitors, blowing the main fuse (that's what happened on your amplifier).
Besides that, preamp ICs are fed through a couple resistors and Zeners, which would further block any undue voltage that tried to reach them.
That's why I said that it's not impossible but quite unlikely a failure.
If anything, the power amp chip or transistors *might* be in danger because they are fed through a more direct path.
Personally I connect every suspect amp through a series limiting lamp, until I am sure it works properly.

I should start doing that as well (the lamp thing).  Do you simply have a light bulb in series with the amp?  If so, what wattage would suffice?  I think I might make a special bulb holder with outlet on it to plug into when testing amps.

[guitarkitbuilder]

I learned my lesson with diodes (had 1N5406's short and nearly smoked the transformer) - I only use bridge rectifiers now, for anything that draws serious current.
Might be worthwhile to replace those with a 15-25A bridge.

A bridge rectifier is four diodes in a pre-configured package, so it isn't inherently safer.  If you replace lower current diodes with a higher rated bridge rectifier, you might keep the diode/bridge from failing but you'll just be inviting some other problem, since something is still drawing too much current through the rectification circuit.  I just finished repairing a Valvestate 8200 amp that was blowing the PC mount fuses and some series resistors in both the + and - rails of the low voltage power supply that feeds all sub-sections of the amplifier (preamp, reverb, chorus, etc.).  I had to isolate the problem first by disconnecting sections of the amp from the power supply until I could find where the current overdraw was coming from, and then further isolating it to a component.  In my case it turned out to be an opamp in the reverb circuit that was pulling too much from both rails.  I suggest you try a similar approach in your Genz Benz amp.