Runt seems to be shorting

[sjaustin]

Hi all, I'm a regular over at BYOC, but this is my first post here. I built the Smoothie with success but am having trouble with the Runt. I think the only part substitution I made is a a 2.2n cap instead of 3.3 at C8.

Bypass works as expected. LED works when effect is engaged, but it passes no sound, and the circuit board gets quite hot after 10 seconds or so. The heat seems concentrated at the top by the 3 electros. The same problem occurs both using a breakout box connected with alligator clips and with the pedal quasi-assembled in the enclosure (the PCB hanging loose with extra wires).

What I've done so far:

Double, triple, quintuple checked all wiring and parts orientation
Reflowed all solder joints
Tested wire-grounding both I/O jacks instead of only the input jack
Swapped the transistor orientation (just in case the PCB was wrong), then socketed and replaced the transistor, which is why it's just stuck in the socket without the leads clipped.
Checked continuity among all ground points on the PCB, the switch, and the sleeves of all the jacks

I feel like this has to be a power short to ground, but I can't seem to find it! Photos:

[midwayfair]

Check that D3 isn't blown or shorted.

[Axe_34]

You might want to check the 9V board connection too. I see you melted some insulation along with the wire.

Also, last time I had a board heat up on me, I had the transistors in the wrong way.

And I can't see for sure, but it looks like you may have some bridges on the back side of the gain connections at the board. It looks like some strands are touching all three pads.

[sjaustin]

Thanks for your replies.

I replaced D3. Problem remains. (I also lifted the square pad, so I had to jumper across from it to the components on either side.)

I do have continuity from the DC jack to 9V+ on the board. I think that rules it out as a problem, but if not, it wouldn't take much effort to clean it up.

[Axe_34]

I just added some photos to my post for clarity.

[sjaustin]

Thanks again. I have cleaned up the wiring on the volume pot and the 9V+ solder pad. No improvement. (Also, re: tranny orientation, I've swapped it both ways, no change.)

[sjaustin]

Also, I took voltages on the IC. Never done that before, but I think I did it right. (Pedal power connected, DMM set to 20V, black probe on enclosure, red probe on IC pins.)

1. 6.9V
2. 6.0V
3. 6.7V
4. 6.8V
5. 0.5V
6. 4.7V
7. 6.5V
8. 6.3V

These assume this orientation:

[jkokura]

You're definitely not getting good voltages. That's a single op amp, and even with it being different than your average single op amp, it should have close to 9V on pin 7, and pin 4 should be 0.

So basically, you've obviously got a short from +9V to GND. That's why things are getting hot.

As to how to find it, that's something you'll have to do. I'd remove the wiring and then see if you get continuity between your +9V and GND pads. If you do, that means the short is on the board. If you don't, perhaps it was in your wiring. If it's in your wiring, rewiring is a good step.

Don't power it up again until you've found the short.

Jacob

[sjaustin]

As to how to find it, that's something you'll have to do. I'd remove the wiring and then see if you get continuity between your +9V and GND pads. If you do, that means the short is on the board. If you don't, perhaps it was in your wiring. If it's in your wiring, rewiring is a good step.

Thanks, Jacob. When you say "remove the wiring," should I include the pots in that? Or just the +9V, I/O, and two G wires?

[jkokura]

I would remove it all, only because once you start it's easier to rewire everything rather than just some things. Besides, some of your pot connections look fishy on the board.

Jacob

[sjaustin]

OK, I removed all the wiring. There is no continuity between +9V and either Ground pad. If I'm reading you right, this means the short was in my wiring. So I will rewire it with extreme care. Then is it okay to power it up again, or is there a better way to check for the presence of the short?

[sjaustin]

Bah. Hooked it up to the breakout box last night—same behavior. I moved on to another project. 

Could this be caused by a bad IC? That's the thing I've been dreading removing since it's not socketed, and all through the troubleshooting I've been afraid of frying it for the same reason.

Any other advice?

[pryde]

Definetely could be the IC.  Since you did not socket it the direct soldering may have killed it. I would def use a socket, especially for the more expensive ICs.

are your IC voltages still the same after re-wiring and all?

[sjaustin]

Definetely could be the IC.  Since you did not socket it the direct soldering may have killed it. I would def use a socket, especially for the more expensive ICs.

are your IC voltages still the same after re-wiring and all?

Yeah, the Baby Board Guide recommends against a socket, so I took it real slow on the soldering, but all this heat from the short and removing the transistor may have fried it even if the initial installation didn't. I think I'm going to test it with an audio probe next and see where that leaves me.