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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 Panasonic 10000uF 63V Speaker Coupling Capacitors
  • Two Bypass Capacitors Capacitors
  • Two Panasonic 4700uF 100V PSU Capacitors
  • Six Panasonic Audio Grade Capacitors (3 per channel)
  • Two Input Capacitors
  • Four capacitor clips (optional, please add separately if you require them)
Total Price excluding P+P = £46

£8.00 for four optional capacitor mounting clips

Quad 303 Amplifier Main Capacitor Set

303 Main Capacitors for both channels of a Quad 303 Include:-

  • Two Panasonic 10000uF 63V Speaker Coupling Capacitors
  • Two Bypass Capacitors Capacitors
  • Two Panasonic 4700uF 100V PSU Capacitors
  • Four capacitor clips (optional, please add separately if you require them)
Total Price excluding P+P = £34.00.

£8.00 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 Panasonic Audio Grade 22uF Capacitors
  • Two Panasonic Audio Grade 100uF Capacitors
  • Two Panasonic Audio Grade 470uF Capacitors
  • Two Input Capacitors
Total Price excluding P+P = £12.00.