Fender blues junior

[Off-Beat]

So I got told smoke was coming from the little blues junior... I opened it up and aside from the smell, right away you could tell this thing was smokin alright! There is a nasty burn on the board and black crap everywhere. I assumed a cap had exploded but turns out a diode (d12) has failed and burnt a hole right through the pcb.

Just wondering if anyone has seen this before? I guess il just have to bridge the burnt parts of the board and replace the bad components. With a quick look at the schematic D12 is connected to the cathode of the el34 to ground.

[52 Bill]

I assumed a cap had exploded but turns out a diode (d12) has failed and burnt a hole right through the pcb.

With a quick look at the schematic D12 is connected to the cathode of the el34 to ground.

If this is a Blues Junior III, then D12 is a flyback diode. There are no cathode protection diodes on the EL84s that I know of.

[km6xz]

May have been operated with an intermittent speaker for high voltage to develop across that diode and arc. Thousands of volts can be generated in the primary of the output transformer if the load is abruptly disconnected when running a high level signal. You will have to cut out every bit of the burned PC fiberglass or better removed connections to the area and wire point to point. Once an arc started it will be hard to stop it from arcing because the board becomes partially carbonized.

[lowell]

Agree that D12 must be from PLATE to ground not cathode. Unless someone has done some kind fixed bias mod.

[Off-Beat]

I will have to check again as I am not 100% sure of the model as its worn off the back plate. It's heated so much that all the tube sockets are now loose and need resoldered. Would the main cause of this be the diode failing? I don't think it's been oporated with a dodgy speaker, it has been used in rehearsal rooms with the original speaker in it which tests fine.

[Brad1]

Would the main cause of this be the diode failing?

The diode obviously failed, and that may have caused other problems.

But, nobody can assume that the diode 'just failed', and was the initial cause of the entire incident.

'Tech-think' goes something like this:

First thing: Use all senses. Sight, smell, sound. You have done that. You smelled burned-up component, and saw one toasted.

Next thing: NEVER assume that the one or more burned-up components are the total cause of the problem, and replacing those will entirely fix it. (Though, with some research and/or previous knowledge, one MAY assume certain models of ceratin products have this inherent, often-observed problem that is ALWAYS the same...like the 5W ceramic resistors on Fender Deville/Deluxe amps, etc.)

Then: Ask yourself the next question. Did this (those) component(s) just go bad, and that is my entire problem? OR, did something else CAUSE that (those) component(s) to fry, and should I proceed carefully, and not just slap in new ones, plug it in, and fire it up?!

Next: Tell yourself that are often problems not easily visible with connections, solder joints, and other components that may have gone bad, but aren't visibly damaged.

Then: Hop on a forum, describe the problem, and don't rule out ANY possibility that someone might have mentioned...because they may have seen this before. Their possibility may NOT be the problem, but never dismiss it until you have verified. Such as "I KNOW it's not that...because it WAS working fine in the practice room." Well...it WAS working fine...until it blew up. And...since it's no longer working, it isn't possible to confirm if that suggestion is or is not a valid one. It COULD have just all of a sudden happened. So...never assume it's fine...just because it WAS working fine. Doesn't take much for an intermittent to create havoc.

Lastly, don't get TOO hopped up with enthusiasm that once you hack up that board, clean things and manage to cobble the thing back together with new components...that it will DEFINITELY work. It could be a few other things that nobody has even mentioned that may just take it all out again, if you just plug it in and flip the switch. it may just work fine. Who knows? it may work for awhile, and there may have been some hard-to-see intermittent thing going on (like an intermittent speaker connection that never got properly tested and cleaned and confirmed reliable in the haste to get'er up and running again.)

The point is...think LOGICALLY...which means NEVER ASSUME. Often, it's something else that failed (or is failing...but does not display any obvious observable damage) that has caused a weaker component to sacrifice itself.

Proceed methodically...and CAREFULLY, without any assumptions.

Good luck.

Brad1

[Off-Beat]

Thanks. I do plan on testing every component to the best of my knowledge before switching it back on. Prob best using a variac before giving it full power again.

[Alex R]

May have been operated with an intermittent speaker for high voltage to develop across that diode and arc. Thousands of volts can be generated in the primary of the output transformer if the load is abruptly disconnected when running a high level signal. You will have to cut out every bit of the burned PC fiberglass or better removed connections to the area and wire point to point. Once an arc started it will be hard to stop it from arcing because the board becomes partially carbonized.

+1. I've had them come in with the speaker jack plugged into the footswitch socket. There should be a label on the back panel identifying those jacks. I do this when I add a line-out down there, otherwise the confusion is too much.

[lowell]

After fixing what we KNOW needs fixing, leave the power tubes out on initial power-up. Will be best to check power tube socket voltages before reinstalling the tubes. When you get to this step let us know and we can detail what figures you'll wanna confirm. Certainly wouldnt hurt to check DC resistance of the OT primaries as well. The flyback diode meltdown means the OT saw a hige spike which could have arced the windings.

My guess is each primary should be 40-80ohms from end to Center Tap. If they aren't in that range and if they aren't withing 5ohms of eachother then the OT is suspect.

[pdf64]

The OT primary on my WEM Dominator (8k for 2 x EL84) is 74.5 R and 84.6 R from CT to either plate terminal; it's good, so the difference may be a little higher than suggested

[g1]

It's heated so much that all the tube sockets are now loose and need resoldered. Would the main cause of this be the diode failing?

The loose solder on the tube sockets is fairly common with this line of amplifiers and is not likely related to the diode issue.

[Off-Beat]

I had replaced the tubes in this amp a few months back and went over all those solder points with fresh solder. So I assumed it was the heat from the burn up as they are VERY loose now.

[g1]

If the solder went bad again so quick, perhaps the power tubes are biased very hot?