Crack w/ Speedball blowing fuses after working [resolved]

[Steve468]

Unfortunately, I'm at my wits end with this problem and I have to ask someone to please kindly help me out.

Here's the overly long story: I built the Crack stock first, and used it with no problems for two weeks (was really surprised and impressed with how it sounded!). I then installed the Speedball, which worked fine for a few days aside from a hum in the right channel only when RCAs were plugged in. Decided to take a look into the hum issue, see that the sleeve of the black wire from 3U to the headphone jack was frayed a little, so I replaced it with a new length of wire. Powered up fine, but the hum was still there, only a bit quieter. Went back again and resoldered some connections that looked iffy. Found the UF4007 going from 18L to 21L was loose, so I resoldered that. Powered on, and nothing. No voltage readings whatsoever. See the fuse is blown (the little wire inside was broken, I've never worked with these fuses before). Change the fuse, notice the leads of the rectifier I resoldered were touching the other UF4007 going from 18L to 20L, so I bent them back apart. Power on, fuse blown instantly.

After checking some resistances, I've noticed that I'm getting very high resistance at B3 and B6, as opposed to 2.9k ohm, and terminal 3 now has very low resistance. I don't know if that's from the Speedball or something I did. Most every other terminal, including the Speedball boards, read as they should.

I should add I'm very new to this. The Crack is the first thing I've done...ever. I've never soldered, never worked with electrical bits, never even sanded and stained wood before. I had no idea how to use a multimeter, and frankly still don't. So it's been a learning experience to say the least. And unfortunately, this problem is beyond my extremely limited capabilities. I've attached a picture, as well as some closeups of 3U. Can anyone please offer some advice? Thank you!

[Paul Birkeland]

I see a lot of long leads still and a bunch of leads that need to be trimmed.  What kind of fuse did you replace the blown one with?

The resistance at 3 should be 0.

The resistances at 7 and 9 will not be 3K because you took out the 3K resistors and put the Speedball in.

A fuse that blows immediately when you turn the amp on is either not rated for sufficient current, you have things touching that shouldn't be, or a component is in backwards. 

[Steve468]

Thanks for the advice!

I'm using Bussman "fast-acting" GMA 1a fuse, rated up to 250v. I thought those were the right ones, but maybe I misread something.

I take your point about the untrimmed leads. That's something I wasn't even aware had to be done, and it did cause some issues during the initial build. I have been slowly going around clipping things back. After your post I had another look and trimmed back a bunch more. Moved some things around that looked a little too close for comfort, found another loose solder on the power switch that I SWEAR was fine before, checked again that resistances are good (3 is 0), and had another go. Boom, fuse gone. I don't think it's components in backwards, because I didn't change anything around from when it was working. I really just don't understand what I've done to make this go so far south so quickly. Thanks again for the help.

[Deke609]

Try a "slo-blo" 1A fuse. They give you a bit of slack on start up before fixing the max current draw. If the slo-blo fuse blows, then there's definitely a installation error somewhere.

cheers, Derek

[Paul Birkeland]

If you slip the red wire out of terminal 13, then pull it up straight up in the air, will a fuse hold?  (Be aware that you'll need to wait ~2 minutes for the power supply to bleed down if the fuse holds)

[diynewbie]

Are the heat sinks mounted correctly?

[Deluk]

Is B+ on the large board soldered? You could use the meter's buzzer to see if you can find any shorts to earth maybe?

[Steve468]

Thanks everyone for the suggestions, I really do appreciate it. So, stupid confession time: this morning I went through the manual step by step, and much to my dismay found that the black wire on terminal 16 of the transformer (that goes to terminal 13 of the transformer) was NOT soldered. So I soldered that, went through the rest of the manual just to be sure, plugged in my last fuse, and...nothing. Blown again.

I then (after getting more fuses) tried pulling out the red wire from terminal 13, holding it straight up, and it still blew it's fuse. I also checked that the heat sinks were correct, and checked that B+ was soldered. Both look good to my eyes. I'm not entirely sure how to use the multimeter to check for shorts. I've been reading about this, and will keep trying.

But is there a chance I did damage to the power transformer because terminal 16 was not soldered properly?

[Deke609]

But is there a chance I did damage to the power transformer because terminal 16 was not soldered properly?

I think that's very unlikely. The fact that the fuses blow instantly means that current/voltage is cut almost immediately after turn on. People have accidentally shorted half of the rectifier diodes before without reporting any damage to the transformer. Plus, I can't see a loose connection on a PT terminal causing the fuse to blow - if anything, I think it would prevent current/voltage - unless it was also touching something it wasn't supposed to be and creating a short.

Since you're now out of fuses, I would suggest buying a few of the slow blowing (slow acting) 1A kind and trying again. If even that blows, then there is a likely a dead short somewhere - something wired or touching ground that shouldn't be. Or even a stray clipped lead that go lodged somewhere unfortunate (it happens). The usual fix the for the latter is to hold the amp firmly in the air with the wiring facing down and give it a good shake at different angles. 

cheers, Derek

[Paul Birkeland]

What's your AC powerline voltage?

The next test I would do is to remove the red wire from power transformer terminal 11, then see if a fuse will hold.

[Steve468]

It held! I even found that the red wire going from power terminal 11 was a bit loose, but was wedged in so tight with the other things at terminal 19 I never would have found it had I not detached it. I got so excited about finding that issue I re-attached the red wire and had a go, and another fuse bit the dust. But that means that whatever the issue is, it's on that terminal board (17-22), right?

[Paul Birkeland]

I would go ahead and replace all your UF4007 diodes.  It's possible that one has been damaged by the incidental contact earlier that was causing problems.  The fuse holding means your power transformer is OK.  That information combined with the fuse not holding when the wire leaving 13 is removed means there's a short in the high voltage supply somewhere.  You may find that terminal 13 shows 0, 270, or 540 ohms to ground?

[Steve468]

Thanks again for that info. I will replace the uf4007s. I checked terminal 13, and it shows very high, moving resistance, consistent with what I understand the '*' is meant to signify in the manual's resistance checks. 

[Paul Birkeland]

You can move along the chain of 270 ohm resistors to see if any end of any of those resistors measures low.  You can also use the continuity beep function on your meter and place it across each of the UF4007 diodes to see if one makes the meter beep.

[Steve468]

So, I've replaced all the UF4007 diodes, and unfortunately, that wasn't it. I measured all the 270 ohm resistors, and all of them measure just about 270 ohm.  I'm really not sure what to check next. Can I ask yet again for some more assistance?