Replacing Capacitors on the Quad 303
As the Quad 303 is quite long in the tooth as the original design is
over 30 years old, it is nevertheless an extremely good amplifier. The
one problem with amps of this vintage is the vulnerability of components
whose performance reduces with age, namely electrolytic capacitors.
There are a number of these within the 303, a few on each amplifier
circuit board, and most importantly the main PSU and output capacitors.
These output and PSU capacitors are rated at 2000uF 100V (2200uF in
later models). Over time the internal electrolyte begins to evaporate
and the capacitors ability to function correctly decreases, as would
your car battery if it was not topped up with distilled water and it
dried out. Capacitors cannot be topped up as they are sealed at
manufacture, but they can be replaced with newer, higher quality types.
Sonic evaluation tests have resulted in the choosing of BC components
capacitors for the power supply and speaker coupling as they are hard to
beat on performance and price.
The PSU capacitors tend to be stressed the most as they are nearest
the transformer which tends to keep them warm, increasing the likelihood
of electrolyte evaporation, and they are also running at the highest
voltage, around 85V. The output capacitors have around 33V across them,
and only have current flowing through them when playing music.
Subsequently a lower voltage, higher capacity items can be replace
the original output devices. A 1000uF 63V capacitor would not only
improve the bass response, but would also improve the overall sound
quality as capacitor technology has improved markedly over the last few
decades. An audio grade BC Components 10000uF 63V capacitor has been selected for its sonic
qualities as a superior replacement for the originals. These are supplied
with high grade 0.1uF bypass capacitors which reduce the high frequency
impedance and distortion.
For the power supply, 100V capacitors must be used, but
improvements can still be had. 4700uF 100V capacitors can be fitted,
lowering the PSU impedance across the audio spectrum and improving sound
quality. This gives nearly 10000uF on the supply instead of the original
4000uF.
The amplifier circuit board itself is not immune, it also has
electrolytic capacitors. These age just the same, even faster in some
cases. Not because of heat, but because some of the older style of
capacitors had a bakelite type enclosure which perishes and cracks over
time, letting the electrolyte evaporate. More modern types have
aluminium enclosures with rubber seals which have a longer, more
predictable life span. There are four main electrolytic capacitors on
the amplifier boards, the input coupling cap, the input stage decoupling
cap, the feedback capacitor and the bootstrap capacitor. The input
capacitor is normally a tantalum bead which is a solid type and has a
long life. The other 3 should be replaced along with the PSU and output
capacitors. This provides the opportunity to use audio grade devices of
better sonic quality than the original items. Elna audio grade
capacitors are a very good choice, providing greater depth, detail and
transparency as well as reliability. The input capacitor (C100
on the 303) has had different types fitted to the 303 over the years but
a very good choice for this part is a 1uF CMK capacitor from Evox-Rifa.
It has excellent sonic properties and although the leads have to be bent
slightly to fit in the original C100 holes, it does fit.
BC Components Audio Grade 4700uF 100V PSU capacitors and 10000uF 63V
Speaker Coupling Capacitors.
Panasonic TSUP Capacitors are now
supplied instead of the BC Parts.
Optional capacitor mounting clips for the above capacitors.
Elna Audio Grade Capacitors for the Amplifier PCB.
Speaker Coupling Bypass Capacitors
 These
fit across the 10000uF Capacitors
Input Coupling Capacitors to replace C100 (now white SMR types)
RED 100nF Wima Capacitors
Fitting the Capacitors
To fit the new capacitors, remove the bottom cover and the top wrap
around lid.
Under the amp there will be three circuit boards on clips. Unclip the
circuit boards on the right hand side (looking from the front connector
end) and lift them up to reveal the bottom connections of the amplifier
as in the picture below.
Capacitor Wiring.
 
The Main PSU capacitor polarity is shown in the drawing to the right.
The white bypass capacitors are now RED Wimas.
Note the polarity of the wire connections to the capacitors, the
yellow wires on the speaker coupling capacitors (10000uF 63V) are
connected to the capacitor terminal with the black dot (-ve) and the
green wire is connected to the capacitor positive terminal. On the PSU
capacitors (4700uF 100V) the blue wires are connected to the capacitor
terminal with the black dot (-ve) and the red to the positive terminal.
De solder the wires on the four
capacitors, turn the amplifier over and unscrew the pinch bolts so that
the old capacitors can be removed. If you are using the optional clips,
unscrew the eight mounting bolts and remove the clips and old
capacitors.
Capacitor Mounting Clips in Place.
These are now metal clips and are almost identical in construction to
the original Quad 303 items except that they are a direct fit for the
replacement capacitors.
Customer photograph showing the capacitors installed into his
amplifier.
Solder the bypass capacitors across the 10000uF capacitors terminals and then
place the new capacitors in their clips. Mount the 10000uF 63V
capacitors at the front
of the amplifier, and the 4700uF 100V at the rear nearest the rectifier. Hold
them in place with the eight mounting screws, but do not tighten. Rotate
the capacitors in their clips so that the correct wire goes to the
correct terminal and then solder the wires onto their prospective
terminals. The rectifier wires are slightly different to the standard
wiring in the picture as this gives better performance. Once the wires
are soldered correctly, fit the pinch bolts and tighten to hold the
capacitors firmly, but not so that they get deformed. Tighten the eight
clip mounting screws. Recheck your work to make sure all the connections
are well soldered and that there are no short or open circuits. Trace
each wire to make sure that the output capacitor negative terminal goes
to the red speaker terminal and the the PSU capacitors positive terminal
goes to the rectifier positive terminal.
Fitting the Capacitors to the Amplifier Boards
To replace the capacitors on the amplifier board, note on the
polarity of the capacitor and its value and its position on the board.
The tracks on the 303 boards are very delicate and can be lifted easily.
The use of solder wick or careful use of a solder sucker is recommended.
Remove the three old capacitors on each amplifier board, and replace
them with their new equivalents. Solder them in and check for short
circuits or solder splashes.
New Capacitors fitted to earlier and later versions of the 303 PCB.
The capacitors are as follows:-
Top Center 100uF 35V
Large cap (bottom center) nearest the input wire 470uF 16V
Small capacitor to the left of the 470uF 16V is a 22uF 35V or to the right
of the 470uF on the right hand side photograph is a 22uF 35V. C100 is
replaced with the light grey 680nF capacitor. C100 is the tube shaped blue
coloured capacitor near the input wires.
For earlier 303 MK1 amps - below serial number 11500 -
the amp boards are slightly different. The Starget capacitors are still
fitted in the same place as the photograph. The blue tubular input
capacitor on the earlier boards is next to the input wires.
Please note the polarity, the top capacitor has the negative stripe
facing upwards in the photograph, whilst the two capacitors at the
bottom have their negative stripes nearest the bottom edge of the board
in the photograph. Also the left hand board has had new pots fitted.
The amplifier should now be ready to power up. Carefully check all
the connections and orientation of the capacitors and make sure there
are no short circuits. Replace the lids and
screws, power up on the bench and if all is OK unplug and reconnect to your Hi
Fi after a few minutes to let the speaker capacitors discharge.
Quad 303 Amplifier Full Replacement Capacitor Set
303 Replacement Capacitors for both channels of a Quad 303 Include:-
- Two BC Components 10000uF 63V Speaker Coupling Capacitors
- Two Bypass Capacitors Capacitors
- Two BC Components 4700uF 100V PSU Capacitors
- Six Elna Audio Grade Capacitors (3 per channel)
- Two SMR Input Capacitors
- Four capacitor clips (optional, please add separately if you
require them)
Total Price excluding P+P = £46
£5.00 for four optional capacitor mounting clips
Quad 303 Amplifier Main Capacitor Set
303 Main Capacitors for both channels of a Quad 303 Include:-
- Two BC Components 10000uF 63V Speaker Coupling Capacitors
- Two Bypass Capacitors Capacitors
- Two Beyshlag 4700uF 100V PSU Capacitors
- Four capacitor clips (optional, please add separately if you
require them)
Total Price excluding P+P = £34.00.
£5.00 for four optional capacitor mounting clips
Quad 303 Amplifier Board Replacement Capacitors
303 Amplifier Board Replacement Capacitors are available separately
if you do not require the 4 large PSU and Speaker Coupling capacitors.
This set will replace the electrolytic capacitors on both channels of a
Quad 303:-
- Two Elna Audio Grade 22uF Capacitors
- Two Elna Audio Grade 100uF Capacitors
- Two Elna Audio Grade 470uF Capacitors
- Two SMR Input Capacitors
Total Price excluding P+P = £12.00.
|
