Quad 303 DIY illustrated guidelines
These are the illustrated step-by-step guidelines for upgrading your Quad 303 with the Dada Electronics upgrade-kit.
There are two versions of the Quad 303. The first version (up to serial no. 11500) has different driver-boards. Make sure you use the schematic that corresponds with the version you have.
We will replace all electrolytic capacitors, the trimmer-potentiometers and some of the cabling. We will also recalibrate the power-supply voltage and the quiescent current.
Electrolytic capacitors dry out after more than 20 years and the capacitance as well as the internal resistance changes. As the 303 uses output-capacitors this will cause a reduction in the sound quality, the amplifier will produce a “behind the curtain” sound and less dynamics, especially in low frequencies. Replacing those caps with high-end Nichicon components will give a dramatic improvement in sound quality.
As the 303 is a push-pull amplifier the correct calibration of the quiescent-current is very important for reducing crossover distortion. We will replace the old trimmers and re-calibrate the output circuit for optimal performance.
We will do the upgrade step-by-step. This is important because some steps are more difficult after other steps are taken. For every step these guidelines will tell you what to do (in Underline) and give you some tips, tricks and advice (in Italics).
When there are any problems, send an e-mail to info@dadaelectronics.eu with a good description of the problem. Some pictures may help. We will do our best to answer within 24 hours 7/7.
When the project is a success you will be listening to one of the best high-end amplifiers ever made with a better-than-original Quad-sound.
Stefaan
Step 1 – The tools & the Components
The tools you need:
- A good quality soldering iron with a fine point (max 30) Watt or a soldering-station.
- A desoldering-pump
- A micro cutting nipper, a wire-stripper and a miniature plier
- A small 2,5x50 flat screwdriver and a 5x125 flat screwdriver
- Tin/lead solder wire
- A digital multimeter
If you don’t have these tools you can order them in the Dada Electronics webshop.
The components in the 303 upgrade-kit:
- 1 4700MF 100V capacitor for the power-supply
- 2 4700MF 100V capacitors for the outputs
- 1 LED red, square with a 22K 1/2W resistance
- ½ M red .75mm flexible cable
- ½ M blue .75mm flexible cable
- ½ M yellow .75mm flexible cable
Components for the power-supply board:
- R200, R201, R204: Resistance 10K ½W 1% (Brown-Black-Black-Red)
- R202: Resistance 8K2 ½ W 1% (Grey-Red-Black-Brown)
- R203: Resistance 2K2 ½ W 1% (Red-Red-Black-Brown)
- R205: Resistance 4K7 ½ W 1% (Yellow-Violet-Black-Brown)
- R206: Resistance 10R ½ W 1% (Brown-Black-Black-Gold)
- R207: Resistance 68R ½ W 1% (Blue-Grey-Black-Gold)
- MR200: 1N4004 diode
- MR201: 12V 1,3W Zener diode
- RV200: 4K7 Trimmer potentiometer
Components for the driver-boards (x2):
- C100: Capacitor 680NF MKT (Yellow)
- C101: Capacitor 470MF >=25V
- C104: Capacitor 22MF >=25V
- C106: Capacitor 47MF >=25V
- RV100: 4K7 Trimmer potentiometer
- RV101: 2K2 Trimmer potentiometer
Options (available in the Dada Electronics webshop.):
- 35mm capacitor mounting-rings
- Red + Black RCA connectors
- 30V 3,75A Toroidal transformer
- 4BA screw-set
- 303 high-end replacement boards
- 2N3055 & MJ802 replacement transistors
Step 2 – Installing the PSU & output capacitors
Remove the non-wired side of the printed circuit-boards from the plastic mounting-clips using a small screwdriver.
In earlier versions of the 303 the PSU & output capacitors were mounted with the connections on the downside, in later versions the connections were on the upside.
In all versions the connection of the capacitors to the cable-beam is as follows:
- Green/Red wire on the left output-capacitor: “+”
- Yellow/Red wire on the left output capacitor: “-“
- Green/White wire on the right output-capacitor: “+”
- Yellow/White wire on the right output-capacitor: “-“
- Blue wires on the PSU-capacitor: “-“
- Red wires on the PSU-capacitor: “+”
We will replace the original 2000MF output-capacitors with 4700MF and the 2 2000MF PSU capacitors (in parallel) with 1x 4700MF.
Modern capacitors are a lot smaller (and better) than their 6-ties ancestors. In some cases you may need to wind some self-adhesive tape around the capacitor to fit it into the mounting-rings (see the picture above).
We will re-wire the PSU-connections with new cable.
- Solder Blue cable between the diode-bridge and the “-“ of the PSU-capacitor.
- Solder Red cable between the diode-bridge and the “+” of the PSU-capacitor
- Solder Yellow cable between the AC-connections of the diode-bridge and the secondary windings of the transformer
The most practical way to solder several cables to the same point is to twist them together before soldering.
Step 3 – Replacing components on the PSU-board
- Replace R200, R201, R204 with 10K
- Replace R202 with 8K2
- Replace R203 with 2K2
- Replace R205 with 4K7
- Replace R206 with 10R
- Replace R207 with 68R
- Replace MR200 with 1N4004
- Replace MR201 with a Zener diode 12V 1,3W
- Replace RV200 with a 4K7 trimmer
The trimmer should be in the middle-position for the time being.
Watch the polarity of the diode’s. The cathode (white or black stripe) should be to the middle of the board.
Step 4 – Replacing components on the driver-boards
- Replace C100 with 680NF MKT
- Replace C101 with 470MF
- Replace C104 with 22MF
- Replace C106 with 47MF
- Replace RV100 with a 4K7 trimmer
- Replace RV101 with a 2K2 trimmer
Watch the polarity of the electrolytic capacitors. The “+” of C104 and C101 is towards the inside of the board. The “+” of C106 is towards the back-end of the board.
It is good practice to upgrade the boards one by one. This way you can compare the boards for the polarity of the capacitors.
The connections of the driver-boards to the cable-beam are as follows:
- Red: + power-supply
- Red: Collector of TR1
- Violet/White: Base of TR1
- Black/White: Emitter of TR1
- Green/White: Output (to output capacitor)
- Blue/White: Collector of TR2
- Orange/White: Base of TR2
- Brown/White: Emitter of TR2
- Brown/White: - power-supply
These are for the left-channel.
In the right-channel White should read Red.
The connections to the PSU-board are:
- Red: + (from the PSU capacitor)
- Brown: Negative voltage & Collector of TR3
- Orange: Base of TR3
- Blue: - (from the PSU capacitor)
Step 5 – Calibrating the voltages and the quiescent-current
- Connect a multimeter between 1 and 9 on one of the driver boards. Adapt RV200 on the PSU-board until you measure exactly 67 Volt.
- Connect a multimeter between 1 and 5 on each driver-board. Adapt RV100 until you measure exactly 33,50 Volt. Repeat this step for both driver-boards.
- Connect a multimeter between 4 and 6 on each driver-board. Set the multimeter to mV-range. Adapt RV101 until you measure 20 mV.
The values will vary around the ideal value when the transistors are cold.
Connect the amplifier & let it play for some hours at high volume. Then repeat the calibration-steps.
And… That’s it ;-)
Stefaan
Price $94.00
Upgrade kit for 303 with all of the hard to find parts.
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Price $229.00
Let us install the Quad 303 upgrade kit for you.Parts and labor included
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Price $429.00
DIY installation including exclusive Dada designed high end audiophile circuit boards. There is NO finer sounding Quad 303 available
This includes the power supply board and booth output driver boards
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