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DSJR
03-08-2010, 13:27
I made a silly heart purchase last month and bought a Quad 303 from a house-clearance lot which I thought would need doing up.. In the event, someone (the previous owner?) has beaten me to it.

I'm hoping barry or someone knowledgeable about tweaking these can shed some light on what and why these things were done - it sounds great by the way...

Firstly, the supply and speaker caps..

http://i132.photobucket.com/albums/q8/DSJR_photos/DSCF1448a.jpg

The caps are Rifa 4700uF at 100V and are now screw type, which should make replacing them easier. The trio of bypass caps on each Rifa is the interesting bit, with 100uF + 1uF + something smaller being an interesting take on things...

The driver boards have had new resistors fitted (1% or better), the caps have been replaced with Ansar, Black gates and also a large electrolytic across the back which doesn't feature in the Quad design as far as I can see.

All the presets have also been replaced by multi-turn ones too.

http://i132.photobucket.com/albums/q8/DSJR_photos/DSCF1445.jpg

Have any engineers out there got any opinions? All I feel I can do to put my stamp on it is to consider getting the Net-Audio rectifier board. I'm not sure I'd go as far as to change the output trannies and re-configure for fully-complimentary operation, let alone remove the limiting (I'm not going to drive Apogee, Maggies or 'Briks any time soon with "anything," and the Spendors are a doddle of a load for the Quad in any case.. They certainly sound very charming together, even though the bridged Crowns are the "better" choice, due to better damping and ultimately lower distortion and noise on paper.

P.S. These old amps may well be museum pieces and more than a bit archaic, but the effort that went into designing and making them (no robots in the 33/303 construction) gives them a sort of organic or "human" quality as does the gear Glenn makes by hand.
I dunno, I'd just like to be a careful custodian for these amps as I reckon they'll easily outlive me ;)

Reid Malenfant
03-08-2010, 17:15
Wow, someone went a tad OTT with the bypassing on the main electrolytics :lol: Nice capacitors the Rifas' are, in all honesty all they'd have needed was something like a decent MKT cap of about 1uf or 470nf to bypass them. In reality the long connection wires to the bypass caps isn't too handy. Better off connecting straight accross between the screw terminals imo.

Like yourself i'd ditch the standard rectifier & either replace it with a standard square 35A unit or seperate schottky barrier devices.

No idea about the extra cap that you mention, but nice to hear of low impedance stuff being used to make a difference on the driver board ;)

Barry
03-08-2010, 20:10
Hi Dave,

I can't add much more to what Mark has already said.

The RIFAs are very good no-nonsense capacitors. If it were me, I would fit them up-side down in their capacitor clamps, so the terminals are underside. That way the supply wiring will be shorter and so present less inductance.

0.33uF is the maximum value for high frequency decoupling capacitor. 100uF + is too much. Also, as Mark has mentioned fit the decoupling capacitor directly across the capacitor screw terminals; again to minimise lead inductance at high frequencies.

The 0.03uF 'X' capacitor across the transformer secondary looks like it has been replaced. What has it been replaced with?

I like the idea of replacing the bridge rectifier with something a bit more modern, but if you are going to use a bridge of four Schottky diodes, fit 0.1uF 'snubbing' capacitors across each diode, to supress noise spikes caused by rapid current in-rush due to the fast switching speed of the Schottkys.

Again, I don't know about the extra capacitor on the driver boards. Quad went through a few versions. If you can tell me the board number (M12038 (?)) and the serial number of your 303, I will be able to tell you of the changes that were made.

Regards

DSJR
03-08-2010, 21:49
S/N is 74*** and the boards are issue "10"

The diodes I readily have are UF4007's and I have some 473J snubbers I can solder across them. I know no more and just copied what's now in the X10-D and fitted them in the Quad 33 with no apparent ill effects over the N4001's that were fitted from the Dada kit. The Net-Audio part is a nice little sub assembly that screws into the space left by the original diode assembly.

This is the Net-Audio Shottky board -

http://i132.photobucket.com/albums/q8/DSJR_photos/303rectifier.jpg

Barry
06-08-2010, 19:18
Hello Dave,

Apologies for my slow response. Essentially, the only major change to the circuitry of the Quad 303 occurred from s/n. 11500.

After this s/n. a change was made to the biasing arrangements, where Tr107 and its associated components replace MR103/104, enabling RV101, the quiescent current control, to vary the voltage between the bases of Tr103 and Tr104 without altering the collector current of Tr102. R202 was changed to 8.2kΩ from its previous value of 6.8kΩ.

On most versions the rectifier bridge MR1-4 is an encapsulated assembly: AEI PM7A2L, though early versions used separate diodes.

The BC154 used for Tr100 was replaced with a BC214C.

Early versions of the 303 have the 2000μF electrolytic capacitors (later 2200μF) mounted with the tags facing downwards. This arrangement was altered so that the tags are now uppermost.

The only other change, occurring after s/n 80,500 was the replacement of the 3-pin Bulgin mains chassis plug was changed to the CEE22 (Europlug) connector.

Regards

Alex_UK
06-08-2010, 19:56
Are you sure you won't have "The Oracle" as your title, Barry! :respect:

DSJR
06-08-2010, 22:11
The big caps were turned round apparently, because older failing caps used to leak all their gunk over the boards underneath when old and pointing downwards. Apparently, they "breathe" and the air would get trapped in the back of the caps as they expanded. On contraction, the trapped air would push the liquids out of the tiny vent hole.

I'm still playing the 303 and it's delightful just as it is. I love it and am glad I bought it. The Croft is a perfect preamp for it too..

Alex_UK
06-08-2010, 22:25
Great that you're enjoying the 303 Dave. I have to say, I've started to love playing around with all the old kit I seem to keep acquiring - and often I'm finding there is no such thing as better or worse, just different. Getting pleasure from whatever it is you are fiddling with or playing is the real utopia, I think.

Rare Bird
06-08-2010, 23:36
Dave:
Stop messing about gut the 303 & stuff it with With NET Audio clobber.This is what i'm upto anyway

Ali Tait
07-08-2010, 07:55
Great that you're enjoying the 303 Dave. I have to say, I've started to love playing around with all the old kit I seem to keep acquiring - and often I'm finding there is no such thing as better or worse, just different. Getting pleasure from whatever it is you are fiddling with or playing is the real utopia, I think.

Aye,I enjoy fiddling around in my own inept way too.It's very satisfying making an improvement to your system yourself.

DSJR
07-08-2010, 10:04
Dave:
Stop messing about gut the 303 & stuff it with With NET Audio clobber.This is what i'm upto anyway

This one's been rebuilt almost from the ground up but using the original boards, which have good gauge track on 'em. I was hoping it would have been well fugged at the price I paid, but it quite the opposite.

I also like the Dada upgrades.

Alex, I agree about playing with older gear. I think it was the preamps on some of these things that strangled the sound back then, together with less good source setup and bell-wire for speaker cables. I love the 33/303 once the component shackles are removed and the 33 has been a revelation, it's so much better now :)

Rare Bird
07-08-2010, 10:13
The original boards want more than just rebuilding Dave..

DSJR
07-08-2010, 10:16
:)

I think we're agreed that it's a fine old vintage amp though, into the right speakers.

Martyn Miles
25-09-2014, 08:02
:)

I think we're agreed that it's a fine old vintage amp though, into the right speakers.

I own a 303 which is used to power my little Harbeth P3ESRs along with a Quad 34 Preamp. The 303 has been repaired and checked, as was the 34 which required some component replacement. At my age I'm not going to rush into trying different amplifiers...

worrasf
03-10-2014, 15:35
Dave:
Stop messing about gut the 303 & stuff it with With NET Audio clobber.

+1

My 303 and 33 have had the full NetAudio treatment - the 303 is in 303SE spec and the 33 has new PSU, amplifier, tape and disc boards.
I refitted the disc adapter board from scratch

zanash
27-10-2014, 16:39
ok so no one appears to answered your original question ..why the bypass caps ....................this was a very late 80's thing ...the hypothesis was that it made the caps react quicker [deliver there charge] for high freq signals and transients...as small cap react quicker than large ones ...theres a paper some where on the interweb by colin wonfor published in common ground or audio conversions magazine that explains it all

Colin Wonfor
06-01-2015, 09:30
ok so no one appears to answered your original question ..why the bypass caps ....................this was a very late 80's thing ...the hypothesis was that it made the caps react quicker [deliver there charge] for high freq signals and transients...as small cap react quicker than large ones ...theres a paper some where on the interweb by colin wonfor published in common ground or audio conversions magazine that explains it all

Hi, there is but I cant find it sorry,also Martin Column,s and I in the early 1980,s did some work on split foil capacitors which proved to be crap at high frequency as the impedances went up.
Some were used by DNM and other and audio improvement were reported but under extreme examination the caps were very bad for power supplies.
So the choice of caps will and does depend on the load, be it dynamic or static loading and the speed and reaction of the dynamic loading will determine type, ESR, ESL,and current rating all these affect the sound. So be careful what your choice is ! it will normally be a compromise on COST, CURRENT, ESR, VALUE to give you the desired sound you like. NOTE your likes, me I like deep bass, rich warm sound but with speed and dynamic, so the caps I select would have very low esr, huge value high ripple current and cost a small fortune.
Poo, and diodes this is another nightmare area, again depending on the PSU and for another time.

OH Happy New Year all and to Steve who thought I had past on to A Class Hell sorry mate I am still here just he he :lol:

The Barbarian
16-01-2015, 04:33
+1

My 303 and 33 have had the full NetAudio treatment - the 303 is in 303SE spec and the 33 has new PSU, amplifier, tape and disc boards.
I refitted the disc adapter board from scratch

Steve:
I know this thread is a bit old now but looking at that Q33 Board you stuffed. I Recognise the Takman carbon films cos i rebuilt my Ferrograph amp with them..What make are the Polystyrene caps you used? i managed to get hold of some NOS Suflex jobbies..