DIY Factory

DATA - The Burnley Files
Tips and tricks from Mark Burnley

Updated 23 December 2004

Getting your DIY projects working -

Signal Tracing

People often build a DIY project only to find that upon power-up, there is little or no audio signal passing. Checks on the power supply show that all is well here, and you realise the fault must lie somewhere on the PCB . One of the best ways to fault-find in this situation is to trace the path of the audio signal from the input connector to the output connector: where it stops will usually be the trouble zone ( there are a few caveats to this rule ... more of that later ... )

Anyway, all you need for simple fault finding is some form of audio amplifier which allows you to "listen in" to the circuit at the different stages along the way. There is a wide choice here, but I find either an old HiFi amp connected to a speaker, or a small guitar practice amplifier is best suited. Headphones are not recommended, because very LOUD sounds and NOISES can be encountered on the way, as well as widely differing signal levels.

So, you've got a "squawk box" for listening in. What else? Well ... some sort of probe which allows you to hunt around without danger of shorting anything out, or upsetting the DC voltages found around the average audio circuit.

Here is one type of probe that I use a lot for general fault finding. It's cheap, quick to make, and simple to use. I've even made them up in-situ from bits in my toolbox when I've been "caught short"....

Things you'll need:

Shopping list :

* 1m length of single core screened audio cable
* 1x 1/4" Jack plug or RCA connector (to suit amp input)
* 1x 100k to 1M Log (audio taper) Pot (value not that important)
* 1x crocodile clip with 30cm length of wire
* 1x 0.01uF (10nF) to 0.1uF (100nF) capacitor. Doesn't matter what make/style/value- as long as it is rated for at least 1.5x the maximum power supply voltage used in the equipment under test!!!. I don't recommend the use of this "lash-up" version for valve (tube!) equipment due to the "open" nature of the build- I've built a version in a small diecast box for this purpose with a >1000V DC rated blocking capacitor!!

You can save time/effort here by using an old guitar lead- you've got a jack plug on one end already, and you can use one of those crocodile-clip test leads for the Ground wire- just chop one crocodile clip off,

Okay, first thing, cut the lead into two pieces- a shorter piece with the jack/RCA plug on, and a longer lead with nothing connected either end. Leave the piece with the connector to one side for now. Take the longer piece and prepare the end like this:

Then, cut the screen down and solder on the length of wire with the crocodile clip on the end:

The end of the central cable will become the "tip" of the probe, so make sure you only leave a small amount of the wire showing. Tin the end too to make it stiff and probe-like. Fold the ground wire back and insulate the solder connection. You now have a Probe and a Ground Flying Lead:

All cool so far ?

Right. The next part needed is the DC-blocking capacitor and the Variable Attenuator.

When using a signal tracing probe, you connect the Ground Clip to the 0V rail of the equipment under test. You then apply an Input Signal to the equipment via its input XLR/jack etc. The signal can be a sine wave (mind blowing after 2 hours of work ) or a music signal (less mind blowing! ) Sometimes for tracing slight distortions a sine wave is better, but for level-dependent problems a music signal is more appropriate. You then have a look at the schematic and see how the audio signal flows through the circuit, and you can place the tip of the probe on these points on the PCB. Because it is only a small stub of wire, it can touch the pads and tracks below the PCB, or actually on the resistor/capacitor/transistor/IC leads on the topside of the board.

The signal is "listened to" with-respect-to-ground which is fine because nearly all audio circuits have the 0V rail as their Ground Reference. The problem with just getting and amplifier and connecting a lead to its input socket is that you often encounter signals in a circuit which vary greatly in their level. For example, a mic pre-amp will have signals from mic level to line level on the same PCB. So if you used a guitar amp input as your tracer, even with the Volume control turned down, the input stage of the amp would distort massively when you hit the mic pre line-out driver stage!

So an attenuator pot is placed after the probe before the input to the amp, so you can adjust the level before it hits the pre section of your monitor amp. Because this pot is effectively being placed across the point in your circuit you're probing and ground, it needs to be de-coupled using a capacitor. This cap allows the AC audio signal to pass unscathed, but blocks any DC voltage present at that point in the circuit. This means that DC bias voltages intended for setting the operating points of transistors etc. are not affected by probing.

So we connect the cap (0.01uF to 0.1uF polyester, ceramic, polyprop etc) in the probe lead in the "Input" connection to the attenuator pot:

The ends of the Probe and Connector cables are prepped ready for connecting to the pot. The shields of the cables are twisted together and soldered. These are the ground connection for the "bottom" end of the attenuator pot wiper.

Notice the cap connected to the Upper end of the pot track.

The centre wire of the Probe cable connects to the DC blocking capacitor, which connects to the 'Upper' end of the attenuator track. The centre wire of the Output cable connects to the wiper of the pot:

And that should look like this:

To test, connect the jack/RCA connector into the monitor amp and start with the volume turned down. Set the attenuator pot to about 1/3 up. Clamp the crocodile clip to a secure point on the 0V rail. Often a good point is the "Star Ground" tag in the chassis. Power up the equipment (and remember to be careful- the full mains voltage may be accessible with the lid off!) and connect a signal source. Set the controls of the equipment to operating-levels (i.e. how you'd expect it to function normally) and then take the probe and touch it to a point in the circuit where you'd expect a signal. A good test point/starting point is the terminals where the input signal enters the board from e.g. the input XLR socket. Then, adjust the attenuator pot and/or monitor amp volume to set a good listening level. You will have to juggle these two controls to stop distortion in the monitor amps pre-amp, and to stop deafening yourself with higher-level signals!

One thing to watch out for is that if you touch a power supply rail other than 0V, the capacitor charges momentarily and causes a >POP< in the speaker ... so don't use headphones or precious studio monitors

More on common signal-tracing problems like hitting a virtual-earth point with op-amps etc later.
Anyway, hope this helps someone ... and happy signal tracing!

 

Many thanks to Mark Burnley of Liverpool in the UK

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