(I think) blown capacitor on new crack build [resolved]

[rat3hat]

Hi

I have just "completed" my 1st crack build. Not there yet it seems! It initially passed all the tests on the "resistance tests" page 47 of the current manual.

When doing the voltage check, there was a small pop sound/ electrical crackle and a touch of smoke, I believe from one of the 100uF, 160V caps on the side of the chassis closet to the headphone jack. This happened somewhere testing the tabs on the bar near the A socket. There are tiny flecks of brownish oil on the chassis, but I can't see a fault in the capacitor or anywhere else. I turned off/ removed the power, tubes, etc.

I have re-checked every step in the manual, checked all the wiring, etc. I have repeated the resistance tests and found that now, at B6 I get no resistance reading. It doesn't make a ton of sense to me (surprise, beginner) and I can't see anything clearly wrong.

I have attached several pictures of the build to see what I have done wrong!

Tx
Chris

[rat3hat]

Also, assuming I need to replace those 2 caps at least, and I am in the UK, are these OK to use as replacements?

"100uF 160V Audio Note Standard Electrolytic Capacitor" (these are not expensive and they are available locally). Not sure why I can't post external links, but the name should do.

Tx
Chris

[Deke609]

You'll want the designer, PB, to help troubleshoot this. But in the interim:

There are tiny flecks of brownish oil on the chassis

That's normal and harmless. It's flux from the solder. If you use a high flux solder like Cardas Quad Eutectic, you'll get a lot of those specks. They're actually hard - you can pop them off with a fingernail (but you don't need to - all of my amps have those everywhere).

but I can't see a fault in the capacitor

It hasn't popped, so it might still be fine.

now, at B6 I get no resistance reading.

I don;t think the cap should affect the resistance reading at B6 - so I suspect a wiring/soldering problem. Do you still get a reistance reading of 2.9K at terminal 9? If so, the bad resistance reading at B6 points to a bad connection between B6 and terminal 9. Try reheating the connection of the red wire at 9 and add a tiny bit more solder.

cheers, Derek

[rat3hat]

Tx v much Derek.

"Cardas Quad Eutectic" - nailed it. Tx.

A bit more solder on B6 and that point it is reading fine now. All resistance and voltage tests pass now.

I think the B3 point made a bit of an "zzzt" noise when the tube powered on. That said, it passed all the tests and didn't do that again.

I guess I need to power this thing on with headphones... finding sacrificial headphones to try; not keen to plug in my Verite right off the bat!

[Deke609]

Cool.

But just to be extra sure, I'd also reheat and add a touch of solder to the terminal 9 joint with the red wire. It's possible that the bad connection is in fact there and wasn't at B6, and that the slight movement of the red wire when resoldering B6 temporarily brought the red wire back into contact with terminal 9.

cheers, Derek

[rat3hat]

Tx very much, again. I have done just that, and touched up a few more points on the other capacitor and B3.

[Paul Birkeland]

What resistance do you get at terminal 9?

There isn't anything going on here that would lead me to believe you have a blown capacitor, definitely don't replace it. 

[rat3hat]

Thanks for coming back. It is reading 2.9K as expected. LEDs on, tubes glow, etc.

The small-pop/small smoke during voltage testing threw me. I've been over the connections and made a few touches to ensure 6,7,9,10, B3, B6 are all good.

I think it is OK. All tests at least are good and there is no more snap/crackle/pop. I'll get my inexpensive DAC from the office this week & use sacrificial (cheap) headphones to give a go ASAP. Even though the tests pass, I am afraid to plug it into expensive headphones/DAC, until I hear it working for a bit.

[Paul Birkeland]

You didn't use any additional flux right? (Just the solder on its own)

[rat3hat]

Just the solder, no extra flux.

[rat3hat]

I have gotten back to the Crack project. All tests pass, but there was still a very audible hum/buzz. After searching the forum, I went back and reflowed all the solder joints in the amp. It still crackled, so I swapped tubes. It still crackles. I am using inexpensive low impedence headphones to see of "it works at all", so I don't plug in expensive headphones and hurt them. I did plug in a pair of high impedence headphones and the noice is VERY loud. Is the advice to reflow the joints again? Not sure what to do next. Pictures of the circuit are above, and happy to post pics of the reflowed joints if that helps.

[Doc B.]

Typically background noise would get more quiet rather than louder with higher impedance headphones. The fact that it changed when you plugged in a different set of cans indicates that it could be a bad joint where the black wires connect to the headphone jack. This is a pretty common spot for a cold solder joint that causes noise.

[rat3hat]

Thank you. I will check there.

FWIW - the high impedance headphones made a reliably louder hum (swapping back/forth) i.e.; hum w/ low impedance, louder hum with high impedance, Back to low, back to high, same pattern.

[Paul Birkeland]

Can you post some updated photos of your amp?  Looking over the ones you first posted, I see a lot of joints that never got adequate heat.  You need to heat your solder joints until the solder flows into them, and from what I see there are a lot of joints where the solder is just sitting on top of things. 

[rat3hat]

Thanks again for reviewing my cack-handed work.  Perhaps when reflowing yesterday, I added too much solder at some points... not really sure. My soldering prior to this has been mostly DIY headphone cables and simple tube adapters. Whilst I practiced on simple tab strips also, perhaps I really just mucked up the soldering?? 

Here are some more pics.