Crack w/ Speedball blowing fuses after working [resolved]

[Paul Birkeland]

Can you fold the 220uF caps down and post some photos of the power supply build around the power transformer?

[Steve468]

Here's some pictures. Thanks again for taking a look.

[Paul Birkeland]

I would put a 3A fuse in there and see if you can get something to smoke and sizzle a little bit. 

[Steve468]

Really? Um...okay... what would I be looking to sizzle, and what would that tell me?

[Paul Birkeland]

Yeah, it seems a little odd, but it may be a matter of something on the amp that we can't see that's conducting, or a marginal part that needs a little more motivation to make itself known.

[kgoss]

Well something in your amp is causing a short which is blowing the fuse. PB will help you resolve the problem so don’t get frustrated. Your issue is definitely unusual.
One thing I would do is go over the amp with a bright light and a magnifying glass. Look for a clipped lead that is touching something and the plate. Or maybe bridging two terminals. That lead could have been in the amp since you built it. Then moved when you worked on the amp to cause the problem. Look closely at the edges of the power transformer too. It won’t be easy to find or you would have seen it already.

Good luck!

[Steve468]

I was definitely thinking it was a stray clipped lead somewhere at first, but doesn't the fuse not blowing when transformer terminal 11 is disconnected mean it's more likely something miswired in the transformer area? At least, that's my understanding. It is hard not to get frustrated (and I'm certainly driving my wife nuts!) but everyone here, particularly Paul Birkeland has been a great help so far. I've POURED over this amp, and I just can't seem to find this short! Anyway, I'll try Mr. Birkeland's next piece of advice soon and see where that takes me.

[Paul Birkeland]

I was definitely thinking it was a stray clipped lead somewhere at first, but doesn't the fuse not blowing when transformer terminal 11 is disconnected mean it's more likely something miswired in the transformer area?

That is the output of the transformer to the high voltage supply.  Something in the high voltage supply is drawing excessive current, which is blowing the fuse.

[Steve468]

Well, I put in a 3A fuse in and it...just worked. Everything lit up fine, nothing sizzled. I let it warm up for about 30mins, and though it seemed like everything got a bit warmer than I remember it getting after installing the Speedball, I wouldn't say anything got excessively hot. I would say the top plate got as warm as it did before I installed the Speedball, and the heatsinks were radiating a tiny bit of heat. This is all based on my foggy memory, as I don't have actual temperature readings and it's been a few weeks since I let it warm up in any way.

I was afraid to poke around too much, because I'm not certain about the safety implications of running it with the 3A fuse. Is it safe to do voltage readings? What else can/should I look for while it's on?

[Paul Birkeland]

Yes, check the voltages.

[Steve468]

Here's the readings I got:

1: 69.1
2: 216.7
3: 0
4: 217.3
5: 69.9
6: 0
7: 117.2
8: 0
9: 116
10: 0

Speedball large board:

OA: 115.5
OB: 117.2
G: 0
B+: 217.8

Terminals 2, 4 and B+ are all a bit higher than they were when I initially measured them. Other than that, everything else is basically the same.

[Paul Birkeland]

That looks like a working amp to me. 

[Steve468]

Right, so, I can just use it with the 3A fuse? Is that safe? Thanks again for all your quick replies, even on a Sunday!

[Doc B.]

It would give you a better margin for error to use a 1A fuse. There is no harm in going back and trying a 1A fuse now that you have gotten the amp to run. If it holds, stick with the 1A.

[Steve468]

I tried already, it still blew. I do have some 1A slow blow fuses on the way (they don't seem to stock them anywhere in my backwards 3rd world country - Canada), so maybe I'll have better luck with one of those?