Help with fuses blowing at startup

[Ankaret]

I assembled my crack (first DIY kit like this) a few days ago and just can't seem to get it running, the fuse keeps blowing. Im using .5 A 250v slow blow fuses from radio shack. I completed the resistance check and everything checked out as it should. It doesn't make sense to do a voltage check since the fuse is immediately blowing, right?

Here's what Ive done to troubleshoot:

First time I fired it up a fuse blew. Second time I replaced the fuse and the tubes lit up just barely (not a full glow), and something around 13L/14L gave off some smoke. I immediately unplugged. I should also mention that this time the power transformer got very hot very quickly, to the point of beginning to heat shrink the numbered Bottlehead label stuck underneath it. The label is still on, but it's all bubbly now. Do you guys think I need a part replaced given the smoke? How to test and figure out which part needs replacing?

Some more info of what happened...

AFTER THIS...

1) I went through carefully to make sure things were all soldered solidly and no excess wires were touching anything they weren't supposed to. I found that I had forgotten to solder one end of the 270 5w resistors (15L) and a couple hooked wires on B8 and B7, so I soldered those.

2) As I went through the manual again point by point to see if Id missed anything, I noticed I had one of the bands going the wrong way on a rectifier diode (specifically, 20L and 18L were reversed), so I flipped that diode around to correct it.

3) Nothing else I could find, I did the resistance check again and everything is reading as it should be, but trying to fire it up the fuse blows on me immediately as I plug it in still...

Thank you in advance for any troubleshooting help!

 

[STURMJ]

switch to 1A fast blow, I believe this is a bottlehead approved fuse. I had this problem too and the 1A fixed it.

 

[Ankaret]

Ok Ill give this a shot tomorrow. Thanks!

 

[mcandmar]

Do NOT do that, if you have a short the last thing you want to do is put a higher rated fuse in there until you fix the problem.  Its possible the capacitor or transformer could be damaged so i would wait for a bit of guidance from the BH guys before doing anything.

 

[Paul Birkeland]

It's entirely possible that you have one of the 220uF caps in backwards. 

Often times, posting photos of your build can help us pick out what might be amiss. 

 

[Ankaret]

Ok I tried the 1A fuse today (before reading the above post about NOT doing so...) and still blowing. I also re-soldered/checked the direction of the caps and as far as I can tell it looks right.

Have no idea what's up, what to do? Below are some pics. I know the build is not very clean, but I was careful with solder joints and took my time and made sure things went where they were supposed to.

 

[Grainger49]

Sorry, it might just be me but I don't see any pictures.  There is a large blank space there.

 

[Chris65]

Can't see them either.

 

[Ankaret]

Sorry about that, should be fixed now.

 

[2wo]

You mentioned that you had one of the filter caps reversed, might consider replacing it...John

 

[Ankaret]

Would that have been the most likely thing to burn up? If I recall correctly that wasn't where smoke was coming from. Is there a good way to test whether one of the particular components was fried?

 

[Grainger49]

The caps are all correct.  I see the diodes on one side are right but can't clearly see the diodes on the transformer side of the terminal strip.  They should have the banded end away from Terminal 19.

The two tube sockets are correctly oriented.  This trips some folks up.

Blowing the fuse is most often, but not always, a problem back at the power supply.  And I am looking as closely as possible at the incoming wiring.  I'll save the pictures and take a closer look.

Other times something is shorted together or to ground in the circuit.  Since you have looked at this for a while, get a buddy to look at it while you call out what should be.

 

[Paul Birkeland]

This is a potential hazard spot.  Long leads like this really need to be trimmed. There are also many, many wires in your Crack that called for a 1/4" of insulation to be stripped, but have enough insulation removed that they can touch nearby components.  Just check to be sure that a wire soldered to a given terminal isn't touching something else.   

I agree about the backwards cap, toss it out and replace it. 

 

[Ankaret]

Hi, thanks for looking at this and for the suggestions.

That spot with the write is actually the excess from a rectifier diode. I mistakenly above mentioned that I had reversed a capacitor, but that was not right, it was one of the rectifier diodes (little black things) that was reversed the first time I fired it up. Should I replace this rectifier diode?? Where can I buy a quality one like that which came in the kit?

Also, given that this was what was reversed, might it have fried anything else? Still looking for any other help or ideas.

I did look over any long stripped lead areas carefully, and as far as I can tell did my best to make sure they're isolated and nowhere near making contact with anything else. That weird long diode lead that you pointed out (thanks for catching that!) is the worst it gets from what I can see.

 

[Ankaret]

The caps are all correct.  I see the diodes on one side are right but can't clearly see the diodes on the transformer side of the terminal strip.  They should have the banded end away from Terminal 19.

The diodes on the power transformer side have one band facing into 21L and the other band facing the other direction into 18L as per the manual. Hope that's where it should be. But the first time I fired it up I had the one that should be facing 21L facing the wrong direction.

 

[Paul Birkeland]

Yeah, a backwards diode is a dead diode. 

The part is specifically a UF-4007.  A 1N4007 would also work, but not quite as well.

An NTE-575 would likely be available locally to you at a retail outlet selling electronic components.  (IE Fry's or your favorite local joint)

 

[Ankaret]

Im in Seattle, anybody have good recommendations of places to look for a replacement diode?

Seems like Radioshack is about all there is in town...

If I need to order the UF4007, is there anything else I should grab while Im at it that you think would have a high likelihood of needing replacing in my situation?

Thanks for the help as always.

 

[kgoss]

Since you are in Seattle getting them directly from Bottlehead should be quick. That's what I would do if I lived that close to BH headquarters.

 

[caffeinator]

Another thing comes to mind, particularly after seeing the long lead in one of the photos.

I had a very similar problem while building my FPIII.  My build was pretty sanitary, good-looking wiring, resistance checks all normal, but it would instantaneously blow fuses as soon as the power was turned on.  I checked it over and over, tried some higher amperage fuses and still they'd instantaneously blow.

I was starting to tear it down when I found a possible problem - a solder drip that had left a kind of solder stalactite (or stalagmite for those in Australia) that came within about 1mm of the the chassis plate.  As I verified by trying once more with the lights out, the solderlactite was close enough to allow an arc that blew the fuse.  Since it wasn't contacting, it didn't throw the resistance check, making it a pretty confounding problem.

So, if it's not bad wiring or blown components, look all over very carefully for an errant lead, solder blob or any other cause of intermittent shorting contact.

 

[rlyach]

One more thing to consider. If you did have one of the diodes in backwards, chances are that two diodes were damaged. The full wave rectifier uses two diodes to steer the current in the right direction through the rest of the circuit. Then during the other phase the other two diodes steer the current. If one of the diodes is in backwards, it provides a direct current path back to the transformer without going through the rest of the circuit. Consequently, all the high current would have gone through two diodes, and you should replace two or even all four of them. Since your transformer got very hot, it would confirm a direct short across the secondary winding, and a backwards diode is a reasonable explanation for this. A metal short from long leads is another.

[Attached is the diagram of the current flow through the full wave bridge rectifier with proper diode orientation (Good). The positive current flows in the direction that the diode symbol points. During the positive cycle, the current follows the red path. During the negative cycle, the current follows the green path. The bad circuit with the reversed diode shows that during the positive cycle the current simply flows through the two diodes and back to the transformer, creating a high current short. Both of these diodes should be replaced. You don't have to replace the other two if you know which one is reversed and compare it to the circuit. If you just want to be safe, replace all the diodes.]