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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 pre-
amp 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 some-
where 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 volt-
ages. 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 at-
tached 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 in-
tegrity. 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. Heat and patience are the solution.
The old kit companies like Heathkit and Dynaco had a statement in their manuals that 99% of prob-
lems 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.
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