Quad 303 replacement components

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Good evening,

I own a pair of Quad 303's which I plan to upgrade. My concern has to do with the driver board capacitors and transistors. Electronics is not my field but I can understand a couple of things. The capacitors on these boards are (apart from the electrolytic and tantalum ones) the old tubular ceramic (erie AP, BP, etc). These units where stored on a normal temp environment but I found a couple of moisture signs on them. So I'm thinking that these should be replaced if I want to be on the safe side. I have the parts list from Quad service manual but I'm not sure about the voltage rating I should use. I plan to go with EPCOS or WIMA MKS4 ones for replacement. Since the amplifier rail voltage is 67v, is this a safe point to start from, I mean use caps at least 100v? Or is this a foolish thought?

I also have a replacement list for the transistors but I can't find an exact match on some cases. For example a BC556 is suggested but I can only find BC556A/B/C. I remember that going for higher hfe is generally recommended but can create distortion problems (?). Should I go for the closest min hfe?

Please excuse my ignorance! Thanks for any help!
The semiconductors, unless faulty, should not be replaced. You will cause issues for yourself!

Only replace the electrolytic capacitors, the 'water' marks you see are probably from the washing process used in production of this amplifier.
The main smoothing capacitors are a weak part after all these years, especially if been in storage and unused for a long time.
Film caps, ceramic caps do not usually wear out so no need to change them unless you find a faulty one.

Some electrolytics and tantalums may be tired and need replacing. The Quad will have been built using normal commercial quality components so that is all you need.

As far as possible, replace like with like. Semiconductors should only be replaced if faulty.
Look up DADA electronics (Home) for information and parts for updating the 303 amp.Lots of options including converting to full comp output as well as recommended replacement transistors.New circuit boards are available from DADA as well as Chinese boards with or without parts,resistors etc.Chinese boards for the 303 are very well made and inexpensive.You have the option of leaving the circuitry original or bringing it up to date and improving the sound quality.Some like it old and slow sounding as original some will want it to be less veiled,your choice but replace the caps,power supply and outputs etc, as they are well past their prime.Cheers.
Hi koozoop,
Follow the advice so far given, if it is working leave the transistors alone.
The caps C2 and C3 (on the Quad service manual) need to be 100 volt working types. There is 80+ volts before the regulator that is set to 67 volts.

Dada Electronics (Quad 303 Upgrade and Revision kit) do an 'upgrade kit' for the 303 and have an excellent 'what to do' guide that you can follow. It has the component values you need
(http://www.dadaelectronics.eu/LiteratureRetrieve.aspx?ID=31879). This will download a .pdf file for you.


beaten by rlr274!
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If you go to youtube and search for Quad303 an excellent video is available by the author" The old git" where he shows the options from Dada and others as well as the Chinese boards available from e-bay.He shows his errors and how to correct for them and links to various other update sources.Cheers.
This may be useful to those who like to play at being an engineer;
TR1: 38494 - 2N3055 MJ15003
TR2: 38494 - 2N3055 MJ15003 (or Push-pull alternative* MJ15004)
TR3: 40411 - MJ802
TR100: BC154 - BC560 BC214C
TR101/TR107: BC109 - BC109 BC237C BC184K
TR102/TR103: U17219 - BC546B ZTX304
TR104: U17229 - BC556B ZTX504
TR105: 38496 - BC461 2N5322
TR106: 38495 - BC441 2N5320
TR200: U17229 - BC556
TR210: 38495 - BC441
MR 100/101/105/106 can be replaced by 1N4148
The main smoothing capacitors can be replaced with;
Aluminium Capacitors | RS Components
Wow! Thank you all for your help! I will leave the ceramic capacitors alone and change all electrolytics and tantalums. The main reason I wanted to change the transistors is that there are different transistors on each channel on one of the amplifiers. They do work but I like the "symmetry".

I've been through Dada's documents. Amazing what you can still do with these amplifiers, I mean keep it as is or give it a fresh touch. I'll stick with the "veiled" sound for the moment because they drive speakers of the same age (KEF CS1 kit). I'm thinking of vertical bi - amping. Later on I might upgrade them with all new components and try a minidsp crossover, etc. One thing I might do though is rewiring with .75mm tinned copper cables and try to re-route in a way that there will not be any interference.
One of the problems with the 303 is the way all the leads going to and from the circuit boards and the output transistors as well as the power supply are tightly bound together.This leads(no pun intended) to loss of ch separation and noise problems.Separating the wires for each ch and routing them so they are as far from each other as possible helps,the power supply leads seem to be the noisiest.I have gone so far as to remove the circuit boards and installing them in an old Dyna 120 case keeping the leads to the heat sink,an old Pioneer heat sink,from the circuit board as direct as possible.The s/n ratio improved by 11 DB's and with changing the transistors doubled the bandwidth.I am not suggesting you do this but it does point out a problem with the 303 layout.Cheers and good luck.
Its a classic example of how not to layout amp wiring in fact...

Seriously, I know Quad would have done that when they built them if it was an improvement.
If it ain't broke, don't try to fix it.
I don't believe that. The layout of the whole amp is clunky to make it fit the posh boxes. It was likely a compromise to help production, not the intention of the engineers designing the circuitry. It was common to use off-board output devices then, and its never a great idea because the leads to the output devices generate highly non-linear interference - at the very least each device's leads should be twisted together and routed separately, and kept well away from input signal wires.

The inputs to the driver boards are shielded cables because they have to be in that situation - but there was no excuse for lacing them in the same harness as the high current stuff!
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It's really a very tight space one has to work with. It would be costly for Quad to properly route the cables in this enclosure. But for us, who have the time and patience to diy, might worth it. Question is "how well away" should each channel be from the power supply leads? Will something like that: Heat Shrink Tubing Kit, EMI Shield, 0.5: Amazon.com: Gateway do the job?

Maybe a multi-clone 18awg shielded cable but it will be hard to work with.
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Unlacing the existing bundles, separate out the parts, twist them into new bundles (or relace, or tightly fitting spirawrap). Be interesting to go in with an EMI probe set and see whats radiating most.

BTW spirawrap is great, if you've never used it get some, it can be wrapped on, or twisted to feed itself onto when access is restricted, its tidy and allows wires to emerge anywhere along a bundle, and can be reused. I hope the inventor got royalties!
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