Basic Troubleshooting - A Rationale
by Doc B.
One of the most tense and frustrating moments in the adventures of a Do-It-Yourselfer is when the project we have finished fails to "start right up the first time".
Fear not, this happens to everyone, even veterans with years of construction experience.
There are a few general, logical techniques to troubleshooting that can make the process a lot less painful and actually quite a rewarding lesson. Remember, we don't learn from doing something right, we learn from our mistakes. So forget the panic, take a deep breath, and get analytical for a moment...
While a really hot shot tech might be able to diagnose a few problems based entirely on his past experience with a particular circuit, one really needs a minimal collection of test gear to sort out a problem. Most important (and most likely to show the location of the typical problem ) is the Volt-Ohm meter, whether an analog meter or a digital voltmeter (a.k.a. DVM or DMM). Along with this a few clip leads can be a tremendous help in hooking up meters, substitute parts, etc. For real "tough dogs" and new designs an oscilloscope is almost mandatory, but we will deal here with the more straight forward kinds of problems that are usually encountered when one completes an electronics kit.
Let's create a scenario to help us understand the troubleshooting process. You've been playing your preamp for a couple of hours, and suddenly you started hearing a noise in one channel of your system. It's making you crazy, and you're wondering how you ever talked yourself into the idea that you could build your own gear.
Rule number ONE
Break the system into its component parts
I cannot emphasize this enough. The first step is to determine where in the system and/or where in the component the problem lies. This is done by the process of elimination.
So let's slow down and get logical. Start by eliminating the parts that we know aren't broken. First we know the problem is only in one channel. So let's not bother with the other channel, in fact if we can turn the amp off on the channel that's OK, let's do so.
Now let's disconnect the preamp from the amp on the noisy side. Shut the amp and preamp off and disconnect the interconnect between them. What we want to do now is to short the input of the amp. You can do this with a shorting plug, which is just an RCA plug that has the center pin connected to the outer shell, or you can just use a clip lead to short the center conductor of the RCA jack in the amp to the ground tab on the RCA jack. Turn on the amp and see if you hear the noise.
For grins let's say you don't hear the noise. OK, now you know that the problem is not in the amp. So it must be in the preamp or the source component ahead of the preamp ( i.e. your CD player, DAC, or phono setup). Now shut everything down again, hook the preamp back up to the amp, and disconnect the source component from the preamp input. Once again use your shorting jack or trusty clip lead, this time on the preamp input. Fire the system up and listen.
Let's assume that we hear the noise. OK, now we know that it's not the amp, not the source, so it must be a problem in the preamp.
Rule number TWO
Analyze the component parts of the equipment in question
Now we need to figure out exactly what is wrong inside our preamp.
First off, let's think about how the circuit is developed. For sake of argument let's assume we are looking at a Foreplay preamp. There's a power supply, and following that there's two separate preamp circuits, one for each channel. Each of those preamp circuits is divided into a gain stage and a cathode follower stage.
OK, what can we assume so far? Well, we know that the noise is only in one channel. And we know that the same power supply supplies both channels. So we may deduce that the problem lies somewhere beyond the power supply in this case, or it would affect both channels.
Rule number THREE
Start with the easy stuff
OK, so the problem must be in the preamp circuit of the channel that's making the noise. So, what's the most obvious, easy thing to try first? Of course it's swapping the tubes from side to side. If we swap tubes between the left and right channels, and the noise follows one of the tubes, you are done. Time to get a new tube.
But let's not just end our story here. We'll assume the noise stayed in the same channel when we swapped tubes.
Rule number FOUR
Measure your voltages
At this point it's time to turn our preamp over and examine it's guts. You need to go back through the check out voltage measurements for the channel in question. Better yet, re-measure all your voltages. Log this info, so that if you need to call us for advice, we have some info to work with.
Let's say you've checked to make sure that your component wiring matches the drawing or photo provided in your kit, but you find a funny voltage in there somewhere. Look at the components attached to the terminal you are measuring. In particular, make sure they are in the right place, and that the solder joints look good. Shut the preamp off, and measure the resistance at the same points as you measured the voltages. A funny reading may lead you to the problem.
Rule number FIVE
Resolder your joints
In our case of a funny noise being generated, one of my hunches would be that we have a broken ground somewhere. The most likely culprit would be a bad solder joint. Once again I must get on my high horse. Just looking at a solder joint tells you absolutely nothing about it's electrical integrity. The only way to be sure a solder joint in question is good is to carefully re-solder it, adding a little solder in the process. And then re-measuring the resistance and voltage measurement at the terminal in question. In particular, there can be problems using our magnet wire, because it can be difficult to see if the varnish coating has completely boiled off under the solder. Heat and patience are the solution.
The old kit companies like Heathkit and Dynaco had a statement in their manuals that 99% of problems with a kit were due to cold solder joints. Still holds true today.
In our case, we found a cold solder joint at the ground buss, resoldered it, and now we have clean uninterrupted music.
Different sounds can indicate different problems.
Deep hum without any buzz - this is 120Hz hum, and is usually encountered in DHT amps. Usually a touch up of the hum balance pot will eliminate it
Buzzy hum, like a bee - This usually means a lifted ground connection. Usually fixed with a careful examination of solder joints and an arbitrary re-solder job
Scratchy sound, hissing, crackling, popping - usually a bad tube, but if just one of these four types of noises is heard, it can also be caused by a bad capacitor or resistor.
Popping on start-up - usually a grid short in the output tube causes this. It will often clear itself and the amp will play OK.
Microphony, a tendency to ring when tapped or a loud musical transient plays - again this is most often due to a bad tube, but on rare occasion a capacitor can create this symptom too.
No sound - something isn't getting voltage or it isn't getting signal. Before you dive in to tear things apart, check the obvious - are your speaker cables hooked up properly? Interconnects? Is everything switched on? We see some slightly embarrassing moments at VALVE meetings, where the equipment is often swapped in and out of the system at a frantic pace. About 95% of the time a "dead" piece of gear just isn't hooked up right.
Muffled sound - this usually means something is not getting all the voltage it is supposed to get, and that you need to get inside the equipment and start taking voltage measurements.
Pay attention to the LEDs on C4S boards
If a board doesn't light up, be sure to check out your component numbers, orientation on the PC board, and solder joints. To test the transistors, put your meter leads across each of the three possible combinations of transistor leads and test for resistance. If you get a very low resistance reading at any pair, the transistor is blown. Reversed LEDs are another fairly common problem.
Last tip -
If you see smoke, SHUT IT OFF!!!