Thorough checking during the final stages
of rebuilding is critical, especially in the case of
large vintage amplifiers, because of the high currents
available. Large
current supply is essential for good power and solid
bass delivery into your speakers but if something begins
to go wrong there's a lot of power reserve there to
create heat and some impressive fireworks at the expense
of quite a few hard-to-find components.
It goes without saying that you need some good equipment
if you're serious about repairing audio gear, not least
a well lit place to work, a good selection of hand tools
and ideally a good selection of test equipment (at the
very least a multimeter or three, preferably a signal
generator and oscilloscope).
Another handy tool is a Variac.
The Variac
A Variac is basically a (usually large) transformer
where you put mains voltage in and get an infinitely
variable voltage out (usually anywhere between 0v and
270v on good quality ones used on a 230v supply),
somewhat like a volume control.
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Testing a Rotel amp on
a Variac |
The output voltage is controlled by turning a knob which
is connected to a rotor and carbon brush which contacts
a bared portion of the main winding, thus varying the
output voltage. There is a rub here however; due to
their design, they are termed an 'autotransformer'
(Variac was merely a brand name which these were once
sold under) which means they only have a single winding,
they are essentially just a high power AC version of a potential divider
- compared to a regular transformer which has two
electrically isolated windings and which means the
output supply is completely isolated from the mains.
So, even at low voltages, you are always connected
directly to the neutral line of the mains power supply.
A Variac is inherently dangerous, a health and safety
nightmare; if you ever use one, use it with extreme
care. Running the Variac from an isolating transformer
increases the safety factor a great deal but working
with high voltages is still dangerous if you aren't
careful.
Variacs are useful when repairing and testing audio
equipment, their main use here is to bring voltage on
equipment such as a power amplifier up slowly, the idea
being that if there was a fault, you'd be able to detect
it before you had too much voltage going through the amp
which could cause damage - in comparison with plugging
straight into the 240v mains supply and having full
power instantly there to do whatever damage it pleases
if anything is wrong.
Variacs also make a great variable power supply, again
you should really use them in line with an isolating
transformer to make them at all safe; in all my years
repairing audio, I've connected myself to the mains on
two occasions and I can tell you that it's not a nice
experience..
About rebuilding
Usually, if numerous faults are suspected, or
otherwise if the objective is a full restoration, the amp is stripped to board level and worked on
from there. This means disassembling as much as the
chassis as possible, removing any mechanical parts as I
go. Some brands of audio gear are more modular and use
plugs and connectors, which makes it easier to remove
boards one by one to work on - some (for example many
Pioneers) are mostly hardwired which creates a nightmare
when you need to strip them down.
The most important thing is to clean the PCBs - this not
only makes soldering easier but also makes everything
look better once it's put back together. Over the years
I've tried various cleaners but nowadays I use a
combination of 2 part specialist cleaner and water
- yes, water. Water won't hurt anything as long
as you make sure everything is dried thoroughly
afterwards and that you don't get it into unsealed
components, like line transformers for example.
The 2 part stuff works out expensive but works better
than anything else I've ever used. The first part is a
foaming cleaner which removes huge amounts of dirt,
after which the boards are rinsed with clean water. Next
the boards are flushed with the second part of the
cleaner, a neutraliser which removes any traces of
cleaner. Then the boards are left to dry in a warm place
with plenty of air circulation.
Once the boards are cleaned, switches are cleaned
(I do this after cleaning because the switch cleaner
leaves a protective coating on the contacts which
rinsing the boards would probably remove), Any suspect
components are examined and tested and any new
components (capacitors etc) fitted. Boards are checked for
dry joints and resoldered where necessary (or preferably
resoldered entirely, depending on the amount of time to
spare and the age/condition of the amplifier).
Some higher end amplifiers also have a very complex
construction, making it difficult to comfortably work on
certain areas without a lot of disassembly, perhaps even
cutting and removing lots of wiring and removing large
parts of the chassis. Pioneers are among the worst offenders
here, whereas many brands (Quad, Musical Fidelity etc)
tend to design in a more 'modular'
form of construction, in the golden days Pioneer virtually hard wired most
things, soldered wires or 'tie wrap' pins which aren't
designed to be pulled apart and which generate a huge
mess of cut wires which need to be carefully labelled if
you do.
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Disassembled Pioneer
SA-9500 II |
In this case I'll resort to making written notes, take
some photos to help identify wiring layouts etc later
on. Usually I'll cut any wiring which is attached to
'tie wraps', leaving a short stub attached to the pin
to help identify the colour of the wire to make
reassembly easier - this stub can then be unwound
from the pin, the pin cleaned and the wire soldered back as everything is
reassembled.
Really, the assembly and disassembly part is a skill in
itself, over the years you get a feel for which order
you need to do things in, even on a model which you
haven't touched before. Of course, a solid knowledge of
electronics is a must - once you can understand which
function each component and each wiring run performs, it
makes the whole thing appear much less complicated than
it would otherwise.
In the image above is a Pioneer SA-9500 II, these
are a good example of a difficult amplifier to rebuild.
Far more complex than the SA-9500 Mk 1 which it replaced, it had
dual mono power supplies and was inevitably difficult to
restore. The amplifier in the photo was being stripped
to component level to be restored, power transformers
were both removed at this point.
Cooling
In the old days, the main power transistors in
an amplifier were often set in thermal grease
with plastic mica pads so that they could conduct heat
to the heatsink but were insulated electrically. This was
fine back then but over time the grease dries up and the
thermal contact with the heatsink is gradually lost and
the transistors run hotter, possibly failing eventually.
This doesn't always happen, but at times I have seen
amplifiers where the grease had completely dried and
gone to powder, it's better just to take the time to
renew it however it looks.
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Outputs
on a NAD 7020e with thermal pads |
Remove the grease and the mica pads will likely
disintegrate as they are removed, luckily new technology
means that the grease/mica are now often replaced by
thermal pads, which, are apparently much more efficient
at conducting heat than the grease/mica combo yet will
still insulate it electrically from the metal heatsink.
You need to be careful when using these pads however, as
they're often a soft woven type material you need to
take care that the edges of the power devices don't cut
through the pad and contact the metal of the heatsink
(hence, creating a short circuit and fireworks when the
amplifier is switched on).
When using these, the screws
need to be tightened snug but not over-tightened,
usually hand tight then another ¼ to ½ a turn is enough,
you need to learn the knack of feeling when they are
tight enough. Insulation checks afterwards are a must.
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Technics
SU-V8 outputs with thermal pads |
Sometimes,
output transistors on more powerful amps are an unusual case size and new pads aren't
available so need to be cut from a sheet of thermal pad
material. Think of this as cheap insurance, the main
output transistors are the major components which send
the music through your speakers, they handle a lot of
power and are sometimes difficult/expensive to replace
if they do go wrong. Making sure everything is kept cool
helps to minimize the chance of something going bad.
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