Restoring an
original amp
As I wrote in the Quad 303 upgrades
article, I stop
short of circuit modifications for several reasons and
instead concentrate on restoring the original circuits,
spending the budget instead on good components which
perform better than the original parts.
When I strip down an amp to restore, I usually separate
it into three parts - connection panel, heatsink
assembly, and main chassis - as this enables each part
to be disassembled easily.
For example, once the four main capacitors have been
removed, the mains and audio connections can be cut from
the input panel and from the transistors on the heatsink
assembly, and then each component can be easily
disassembled into component parts to be cleaned,
repainted where necessary, and generally restored before
being reassembled.
Component brands
I've been asked many times which brand of components I
use, I simply use premium components from established
manufacturers which are suited to the application I am
using them in. I stay away from niche 'audio quality'
brands, purely because many of the claims made about the
advantages of these types of components are dubious at
best and on several occasions in the past I've had
equipment with components such as expensive Black Gate
capacitors fitted which have leaked and degraded
en-masse.
Capacitors
I change all of the electrolytics
for modern low ESR (Equivalent Series Resistance), high
ripple current capable types.
All of the electrolytic capacitors in the 303 are
working as either power reservoirs or in the case of the
loudspeaker outputs as AC coupling (blocking DC bias
voltages but letting AC signals pass), for both of these
situations you want a component which will present as
low an impedance as possible at high frequencies and
which can store and deliver large current peaks on
demand.
I also bypass all electrolytic capacitors with a 100:1 ratio
down to a few hundred nanoFarads (nF). 'Bypasses' are
essentially just a smaller capacitor connected in
parallel with a larger one and the reason for doing this
is that larger a capacitor, no matter how good quality it is,
the worse it performs at higher frequencies; this is due
to factors such as inductance of the leads and internal
structure of the component. Smaller capacitors are inherently more
perfect in this respect and when you connect them in
parallel they effectively supplement the operation of the
larger capacitor at higher frequencies and hence make it
perform much better.
Finally,
I also change the lone tantalum in each signal line with
something better (polypropylene or similar). Tantalum
are actually good capacitors - they are similar to
electrolytics in that they are polarised, but perform
better at higher frequencies (for this reason they often
get used in power supplies, to filter transient noise).
Still, in the interests of audio quality I still prefer
to change them for something which is at the least equal
and most probably better in the audio spectrum.
Connectivity
I usually add a set of RCA phono sockets in addition to
the original DIN socket, I prefer phono sockets over DIN
plugs (they look/feel nicer to use and are more common)
although I doubt that they make a huge difference to the
signal at the relatively high voltage levels seen here;
DIN connectors don't look great, but they do work
surprisingly well. That said, having RCA sockets means you
can easily connect anything to the 303 and are also able
to use better quality interconnects as well (DIN
connectors struggle with any screened cable pair above
about 3mm outer diameter), and you're also assured of a
good connection with low noise levels.
The original loudspeaker sockets are a tough one, they
are plastic bodied and I do prefer to change them to
something better as occasionally in the past I have
had connection problems even with good quality banana plugs.
The problem is that the sockets are packed quite tightly
together on the panel and good quality binding post type
sockets are just too bulky to fit safely. When I do change the sockets,
I use high quality metal bodied flush fitting sockets,
which are better than the originals but which
dimensionally are similar except that they have a round
instead of square flange on them.
Repainting
If an amp is to be repainted, this requires all painted
panels to be removed. The main outer case is steel, but
the end panels are aluminium and require slightly
different treatment. The only way to refinish the panels
properly is to strip them back to bare metal and repaint
them; this is done using an airbrush.
For stripping, we use a combination of acid dip,
sandblasting and wire wheel, depending on the condition
and type of surface. After being mostly stripped, the
parts are finished by hand and then degreased ready for
priming - either etching in the case of aluminium - or
zinc in the case of steel. At this stage any dents,
chips and other damage are rectified, this may take
several stages.
The base coat is custom matched to the original Quad
colours and is applied in several coats using a
combination of an automotive airbrush and smart repair
gun.
Not a Quad 303, but this is a Quad 33 which has just
been restored & refinished. The knobs are a nightmare to
restore; they require painting and then the engraved
lettering picked out with a scalpel while looking
through a magnifier and filled in with white paint,
before the whole thing is then lacquered. The result
isn't quite factory perfect, but is good enough; often
the aluminium is corroded under the paint after 40 years
and to be honest, the original quality of Quad's metal
parts wasn't that great in the first place.
And, a close-up of the paint finish. The lacquer is
matted manually to give the desired sheen.
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Last updated April 2014
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