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What goes wrong?
Noisy pots and switches:
The bane of my life! It's so embarrassing to have a half-decent hi-fi that
you can't adjust the volume! The failure mode is deposits on the track, particularly
at the lower end of the scale. Due to the "design" of the preamp,
the problem is compounded: as the wiper hits dirt and goes high-resistance,
the volume is effectively max'd for a brief time. If the configuration was
conventional, the sound would do the opposite, making the effect much less
objectionable...
Also, the switches are affected for the same reason - the input op-amp stage
puts dc currents through the switches...
As noted above, replacing the pot and switches is a short-term solution.
If you retain the original preamp, the new components will go the same way.
Faulty electrolytic capacitors:
This is the second big problem with these amplifiers. Heat is a major enemy
to electrolytic capacitors, so it will be no surprise to hear that they are
a particular problem with this amplifier! The original components were fairly
low quality to begin with, which clearly doesn't help.
Look in an electronics component catalogue, and you'll see that electrolytic
capacitors have a finite life expectancy, typically around 2000 hours for
an average-quality device. This is quoted at an operating temperature of 85°C,
and as a rule of thumb, reducing the operating temperature by 10 degrees will
double the life expectancy. As the surface of the heat sink is around 60-65°C,
the internal temperature will be even higher. Remember, 2000 hours might sound
like a lot, but that's only 2 years if you use the amplifier for 3 hours a
day!
The symptoms of faulty capacitors are hard to predict. Basically, it only
takes one degraded capacitor to affect the performance of the circuit, and
as it's hard to detect a gradual change in sound quality, you can be forgiven
for not noticing faults developing. When they get particularly bad, it's possible
that other components can fail as a result of the changes operating conditions
within the amplifier. The message is simple - if you have a fault with the
amplifier, don't start trying to apply logic until you've replaced all the
capacitors. In 95% of the cases I've come across, this will clear the fault.
Better quality capacitors will be rated at 105°C, and will have a longer
life span as well. Follow the logic of doubling the life span with every 10°C
fall in temperature, and you'll soon see why fitting anything less is a false
economy.
Failed output stages:
The output devices expire with the heat, possibly prematurely if the poor
application of heat sink compound that I discovered is anything to go by...
Luckily, this amplifier is relatively "fail safe", and not prone
to the catastrophic failures that affect many audio power amplifiers - failure
of an output device normally leads to cascaded faults further back in the
circuitry, which is one reason why service centres are often reluctant to
take on such jobs. But having said that, you should carefully check every
active and passive component in the power amplifier stage before re-applying
power. And make sure your speakers are not connected!
As there is no dc protection circuitry, a faulty output stage will probably
damage loudspeakers if there is significant dc offset present... Maplin sell
(sold?) a Vellmen kit that protects the speakers from the amp under these
circumstances, and is definitely worth adding, if it is still available...
Så er man da advaret
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