Fault Finding a Build

I thought that some sort of debugging guide would be a useful addition to the blog.  I get a lot of requests about non-working builds and always have basically the same answers, so this is a first port of call for anyone having problems.

The build I'm using in this guide for simplicity is an NPN silicon Fuzz Face I built a while ago:

and yes I know this one wouldn't work without transistors! :o)


1) Transistor Orientation

With my earlier layouts (hopefully not so many recently) I used the transistor symbol purely as a graphical representation of a transistor.  I always included the required pinouts but often people have taken the orientation as correct (understandably unless you've known what I've written about it in the past) and so if you're having problems with a transistor build (especially from an older layout) then first thing to do is check the datasheet for your own transistors to make sure the orientation matches.  I always socket transistors to allow me to experiment with different types and gains and would always suggest everyone else do the same.  You can use PCB header sockets (found on eBay) which cost me £5.55 for 20 strips of 40, which is enough to socket 266 transistors.  Not a lot of money to allow you to experiment easily with every build.  As a bonus it's also easy to turn a transistor round if the orientation is wrong.

2) Faulty components

This is really something to check before a build.  Once the components are in circuit you can't guarantee they will measure accurately and as you can get a cheap multimeter that can measure both resistors and caps for not a lot of cash, then everyone should be doing this first.  I've done a number of builds where I was tearing my hair out and as I destroyed the board in anger, measured components afterwards, usually to find a faulty electrolytic cap.  If you have the patience then do it because it can save you a lot of heartache.  If you don't have the patience, then develop some! :o)


3) Cold solder joints

If you're getting noise (or no sound at all) from your build, then a good thing to check is for cold solder joints. 

You want all soldering to be shiny and so look out for dull looking points or pitted solder, and reflow if necessary to make sure you've got a strong connection.


4) Unwanted Bridges

When you make a track cut in vero you always run the risk of leaving a burr that is making an unwanted bridge across tracks which will almost certainly stop the build from working properly.  Similarly solder can stray sometimes to cross the groove between strips and cause the same problem.

If you've got a multimeter the best way to test this is to use the audible diode test to check for continuity between consecutive tracks including points in the row which have been isolated by track cuts.  So in the Fuzz Face example this is where you would want to look for continuity.

If an unwanted bridge is found then use a sharp knife to cut between the tracks and break the bridge.  It may be worth using a magnifying glass to make sure nothing is left which could cause additional problems in the future.

If you don't have a multimeter then just score between all the tracks with a sharp knife, or better still a small hacksaw, to make sure there is complete separation.

5) Checking placement and cuts

I often come across problems with builds where a part was soldered to the wrong hole, or a cut misplaced and again in most cases this would stop the effect working.  With smaller layouts it's easy enough to go over everything and double check but I use a visual method to help me check for placement errors.  Anyone who uses a graphics editing program like Photoshop or Paint Shop Pro should be able to do this easily enough, but not everyone wants to mess around with graphics programs to fault find a build, and if that is the case then just check your placements critically against the layout, counting holes to make sure everything is where it is intended to be.  You can ignore the rest of this section and skip to number 6.

I take a front and back pic of the board and then use the layout pic to create a semi transparent layer over the top.  For the top of the board just select right round the board layout and copy and paste it as a new layer over the top of the front photo. Then you can make it semi transparent by setting the layer opacity.  Then use a deformation tool to line up the layout with the photo (it's easiest to try to lineup the vero holes as points of reference).  The results make it very clear where everything should go and highlight errors

Do the same with the reverse side (don't forget to mirror the layout before pasting over the picture so it's the correct way round.

Using a filter on the track side can make it more obvious where the photo and layout cuts are.  This was using a simple "Darken" blend mode

6)  Measurements

If the above all looks good then we need something else to give us a clue to where your problem arises.  The best way to provide this is to measure the DC voltage between all transistors, ICs, regulators etc and ground.  Any voltage that isn't in the right sort of ball park will stand out like a sore thumb, so if you want help make sure you do this first to give us something to work with.

To assist you in giving us the correct information, ICs follow the following numbering convention:

1----8
2----7
3----6
4----5

and if the circuit contains transistors be sure to identify collector, base and emitter when giving voltages. 


7)  If all else fails .....

If all of the above check out ok, then the problem must be an incompetent layout designer! :o)  In this case post a message to the board including any front and back pics you've taken, along with the pin voltages mentioned in (6) and that could really help identify the problem area.

Hopefully you have more successes from this blog than failures to make the frustrations worthwhile.