Manley REFERENCE PREAMPLIFIER - Manual (Page 12)

TROUBLESHOOTING
GENERAL TROUBLESHOOTING
We don't presume you are expert with a sodering iron or that this is your first audio system. This section is for all
and is simply meant to help with a variety of audio problems and some not specific to your preamp.
It is easier to find a problem in a stereo system than a mono system even though there is twice as much electronics
and wires. When you have two channels, there is some built in redundancy that can make fast work of finding a
problem. This is how we teach newcomers to repair 64 channel recording consoles - you have 63 good channels,
use one of them to compare to the bad channel to locate exactly where the problem lives. A little swapping and
bingo -it must be this. A little detective work is advised before you jump on the phone. For example if the left
channel is distorted, you might swap the left and right outputs of the preamp. If the distortion is still in the left
channel, then it must be after the preamp. That means it has to be the amp or the speakers (this logic is easy). The
next step would be to return the preamp connections back to normal, to keep things straight and then swap left and
right power amp outputs (feeding the speakers). If the distortion changed sides or followed the swap then the
distortion is before that point which pretty much indicates the amp is the bad boy. If the distortion is still on the left
side then you can bet the speaker needs work. The idea here is that you can start somewhere in the middle of the
"chain" and follow the problem up or down pretty quickly and with a good deal of "proof". The only rule is that if
the problem follows the swap, then it is before that point and if it stays in the same side then it is after that point.
Educated guesses can help give a starting point and this swapping method gives some certainty. This works
whenever one side is strange. Tube gear is often very easy to "fix" by simply repacing a tube. Remember when
even your parents could fix the TV set by taking a bag of tubes to the corner store tube tester? You are allowed to
swap tubes from one channel to the other (as long as it is the same number) You might want to look at the diagram
on page 9 to see tube locations and the easiest method to pull them. Again educated guesses might help. Would you
like a few examples of educated guesses?
NOISE or HISS. The smaller the signal, the more gain is needed and the more likely it will cause hiss. In the case
of a preamp, the first phono stage deals with the smallest signal. That would be one of the 6072's. It may not be the
6072 but that would be the first guess. You can always switch to, for example AUX, to verify that the hiss is only
when you are switched to PHONO. If it wasn't the 6072, there is a 12AX7 right after it that could be swapped with
the 12AX7 from the other side. Let's imagine that, the hiss has been traced to the preamp but that its on the right
side on every input (CD, phono, AUX, etc). That eliminates the phono tubes and sort of suggests that it might be
the first preamp tube (smallest signal) which would be the right side's 12AU7. The 12AT7 in front of it is used to
drive the RECORD OUT. The last guess would be the final tube, the 6414.
DISTORTION. Here the educated guess would start off with the speaker - only because statistically, speakers are
most often the culprit, then the power amps would be the second bet. Probably the preamp would be the least
likely. As a matter of fact, it is more likely to be a shorted interconnect. If it does seem to be the preamp, once
again a few tube swaps will narrow it down to one tube usually.
HUM. We pointed out a few common sources of hum on the previous page - you might want to try those first.
Here is an interesting technique. Use the preamp to help locate the hum. First (no music) we would turn down the
VOLUME, if the hum also drops then it is almost certainly before the preamp. With the volume up again, switch
the INPUT SELECT. It should show if one source is the cause. At the other end, you could disconnect the inputs to
the power amps and see (hear) if the hum goes away. If it doesn't then we would tell you it has to be the power amp
especially when only one channel hums. The "real" way to do this is to use a shorting connector instead of having a
jack open with no connection. We use old cheap RCA interconnects cut to about 2 inches long with the shield and
center conductor connected together. This eliminates any hum caused by unconnected hi impedance connectors.
The shorting connector simulates the perfect source - low impedance and very quiet. If the hum seems everywhere
- the best method might be to start from scratch. disconnect all the sources and plug them in one at a time until you
get hum. This should point at some piece of gear. It might actually be pointing at two if there is a ground loop.
Check for 3 pin AC plugs. You should only have one AC plug ground pin in use. The others get "lifted" (if that is
legal in your country).
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