resistance check passed, failed at voltage check

[yuanjingxue]

Hi, I just finished my kit over the weekend and then I did the resistance check, all passed without a problem.

Then when I plugged in the power cord, all the voltage value is pretty low, checked terminal 1 to 5, T3 is 0v, which is expected, the other 4 are just 1v to 3v... and the fuse is blown in a minute. In addition, I can see some smoke coming out. And the 2 leds are not lighting

Attached some photos, please give me some clue how to troubleshooting it. I have double-checked all the wires and all the wire looks correct.

 

[kGlerup]

You need to go back and redo a lot of the build:
* Trim all the long leads. Do I see one that actully touches the base-plate? (at headphone jack)
* Fully resolder most of the amp. A lot of joints are not heated properly

Those long excess leads need to go. Have a look at some pictures from other builders to see how it can be done.

 

[Deluk]

All wires that go through a hole should be bent round to give a proper mechanical joint. You then shouldn't be able to pull them off without straightening the wire again. I trim the wire at that point and then solder. With some components it's best to offer them up to the mounting points and trim the excess down a bit to make the leads easier to bend around. Don't cut off too much!
As said there are lots of long ends here. I don't think I've seen as many or as long, on any build. Yes, not enough heat or iron time on the joints too.

 

[yuanjingxue]

Try trimmed the long leads, and re-soldering all the joints, still does not help. Is there any clue that I can check which part is wrong?

 

[Paul Birkeland]

Did you replace the fuse?  Is it still blowing? Can you post some updated photos?

 

[yuanjingxue]

I tried to replace the fuse a couple of times, all blown instantly. The updated photos are attached.

One thing I notice is for UF4007, I didn't follow the instruction on the length of the leads. Not sure that can be the reason why it blown.

Actually, for the first time, things are better, the fuse didn't blow, just the 2AU7 tube does not light up, but after a couple of minutes, the fuse is blown, then whenever I get a new fuse, it blew right away.

Thanks

 

[yuanjingxue]

Some more photos

 

[Paul Birkeland]

Your UF4007 diode leads are still not trimmed.  They are shorting out the HV winding of the power transformer, and I can see that the label we put on the transformer to mark the terminals has gotten so hot that it has turned brown.  I've attached the picture from the manual that shows what this should look like.  The leads that would poke out are trimmed at the terminal strip.  Yours are sticking out and this is absolutely not OK.

Before doing much else, I would pop the red wire out of power transformer terminal 11 and let it poke up into the air.  If a fuse blows when you try to power the amp up like that, you'll need to replace the power transformer (and fix the issues that caused it to burn up).

 

[yuanjingxue]

Unfortunately, just tried a new fuse, it still blows.

Quick question, so the UF4007s' poked-out leads are not touching any other wire, it will still short out the power transformer?

Thanks
Yuanjing

 

[Paul Birkeland]

Quick question, so the UF4007s' poked-out leads are not touching any other wire, it will still short out the power transformer?

I think you're missing the point. 

 

[Paul Birkeland]

I will also qualify why it's those diode leads and not something else. 

In order to burn up the power transformer, you have to demand a ton of current from it and get it super duper hot by doing so.  In the Crack, you have a high and low voltage winding that can be overtaxed to obtain that result.  In the build, we have you do the glow test in the manual in order to be sure that the low voltage winding is wired properly and not causing problems, so that leaves the high voltage winding. 

If you pull a lot of current through the whole power supply, the 270 ohm 5W resistors will give up long before the power transformer will, and we have seen plenty of miswires and shorts where one of the 270 ohm 5W resistors overheats and falls apart.  This has the added benefit that the resistor will fail open and the short will no longer be allowed to draw excessive current.  In your amplifier, your 270 ohm 5W resistors are in perfect condition, so that tells me that the short drawing down your amplifier is most likely upstream from those components.  When examining the area, I saw multiple potential shorts with the diode leads, and this has the potential to blow out your power transformer without anything else in the amplifier appearing to be amiss.

 

[Doc B]

The untrimmed leads create a hazard and must be trimmed before you plug the amp in again. Once they are trimmed you really need to remove the red wire at 11 and do the voltage test that PB is recommending. Once these two things are done we can help you to determine the next step.

 

[yuanjingxue]

OK, just trimmed the leads that PB mentioned, but I don't have good fuse any more, let me purchase some and try again will update here later.

 

[kGlerup]

I would strongly advice that you do not continue by just popping in new fuses.
You may already have a failed transformer in your build, which first needs to be troubleshooted. In my opinion you would need to step back and sort the basics first, starting with the simple glow test. Then a full review of the entire build.

 

[yuanjingxue]

So I have re-wired the UF4007s, and try with some new fuses, also detached terminal 11. So the fuse still blows.

Some photos are attached to show my current status. So do I need to get a new transformer and try again?

Thanks

 

[Paul Birkeland]

Yes, that power transformer is well cooked.