MudBunny - Violet Rams Head Problem

[bram2000]

Hi!

Firstly hello everyone, I'm very new here. I've just completed my first build of a Violet Rams Head MudBunny, and am having some problems. I've built the basic test rig as per the instructions on this site (it's great!) and have tested the board using the audio probe. It's good up until one side of the R8 resistor; the other side of R8 is dead. I've replaced R8 but the problem remains. Have checked both resistors using a multimeter and they both check out fine at ~8.2kΩ. I'm a bit lost as to what the problem could be (the electronics I learned in school is beyond rusty).

Can anyone suggest some further debugging steps?

Thanks,
Bram

[jkokura]

The next thing to do is to check everything that happens after R8, specifically the parts right after. Anything that can be connected to that back side of R8 could be suspect, and you could have a solder bridge or a bad part or a backwards part.

D1, D2, C5, R10, Q2, and R9 all connect there. So start with those parts.

Jacob

[bram2000]

Hi, Thanks for the response.

Sorry for the further newb question, but presumably I have to test those components using a multimeter rather than the audio probe? I don't get signal from any of those components with the probe.

Thanks again,
Bram

[jkokura]

Hey Bram.

So, an Audio Probe won't be helpful here I don't think - if you don't hear anything after R8, you're not going to hear anything later.

The next step is to check these things:
- make sure the values of those parts are correct
- make sure the orientation of the transistor is correct
- make sure there are no solder bridges (parts connecting that shouldn't) and cold joints (parts not connected that should be).

To do the last step, you can use a multi-meter on continuity check. If things are touching that shouldn't be, that needs to be fixed. Use the schematic and layout image as a guide.

You may need to just reflow some solder joints. That might fix it. If that doesn't, there's potential that you've got a faulty part, and it shouldn't be too hard to figure out which one.

Finally, you can measure the voltages on that transistor - make sure you plug the power into the board before measuring the DC on each leg using a multi-meter.

Jacob

[bram2000]

Hi Jacob, thanks for the help!

I think I may have found a problem. I've been doing a continuity check and both sides of R9 seem to be connected to ground as do the collector and base legs of Q2. Trouble is I can't see any touching components or bridged solder. The only thing I'm suspicious of is that I had to replace R8 and in the process sligthly burned the pad nearest the edge. I've lifted this leg out and removed almost all of the solder from the hole, but the problem remains. Do you reckon this could cause my apparent ground issue?

Thanks,
Bram

[jkokura]

That's the issue. I'm not very sure what the fix would be because it's hard to visualize what's going on there, but basically, if that side of R8 connects to ground, you need to fix that problem before it will work.

Are you working on a manufactured or an etched board? If it's etched, you may be able to do more than a manufactured board. It's probably going to look a little less pretty, but either way you should be able to make it work.

Jacob

[bram2000]

Hi Jacob,

It's a board I bought from madbean so manufactured. I'm considering trying to file any remaining solder from the hole with something, and then if I can actually split it from ground again I can just solder the leg of R8 to another component directly (so avoiding this damaged hole). I don't have a file small enough but can try to find one. Does this sound reasonable? Are there any standard tricks for this kind of thing?

Thanks,
Jon

[bram2000]

Hi again.

I'm stumped with my build now. Have completely removed the burned part of board around the R8 leg but the ground loop remains. One strange thing I've noticed is that the two legs of C5 show to be connected using the continuity mode on my mutlimeter.

Any ideas? I'm about ready to move on... 

[jkokura]

Not sure. Are you positive your multi-meter is set right and not malfunctioning?

If you have continuity between the two parts, it again indicates a bridge of some kind. Those two points should not be connected, nor should you have the back end of R8 connected to ground.

I can't say more without seeing it in my hands. It's one of those things where I can tell you what the problem/symptoms indicate, but solving it is in your fingers.

Jacob

[bram2000]

I'm pretty sure the multimeter is working; I can test other parts of the board which are not connected and it will not react, but across the pads of C5 it beeps. R10 is the same, as are the B and E on Q2.

I'm assuming there must therefore be a bridge somewhere between those two sections, but strangely also to ground somewhere. I can't see any evidence of a bridge on either side of the board. Am tempted to start desoldering components from that section, but not really sure what I'm hoping to achieve with that to be honest.

Thanks for all of your help

[bram2000]

I think I've found the problem... I reckon the PCB is faulty.

Take a look at the photo attachment. I've highlighted the C5 Capacity pads. One of the pads is incomplete and appears to connect to ground, where according to the block diagram neither of the pads of C5 should be connected to ground.

Can someone who knows a bit more about this than me take a look at the picture and confirm my theory either way?

Thanks!

[hoodoo]

Hi mate, i don't know if it's just the photo or not , but it looks like you've forgotten to solder one side of R9 and D2 looks pretty iffy as well, good luck, Matt.
Just had a look at a mudbunny i've got and your one is definitely different.

[bram2000]

Hi, Yes I've desperately been trying to debug this board for the last week or so. As such I've pretty much trashed it; desoldered a load of components and even cut a chunk out of the board... to no avail.

Can you confirm that the left hand pad of C5 is a printing error?

[marauder]

Definately a manufacturing defect there, you can check using the build document.

Contact Brian by pm and let him know.

[jkokura]

Manufacturing defects show up on PCBs once in a while. Sucks that you got a bum one!

Jacob