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Old 11th May 2019, 10:20 pm   #1
nutteronthebus
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Default Quad 303 re cap

Hi I am about to recap my 303 power amp but on the DADA site they say use 6800uf for the output caps and 4700uf or 6800uf 100v for the power supply yet on the Fidele audio site they fit 10,000uf 63v for the output and 2 X 4700uf 100v for the power supply giving approx. 10,000uf across the HT . What is the best way to proceed



Thanks Dave L
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Old 11th May 2019, 11:15 pm   #2
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Default Re: Quad 303 re cap

Remember that it has a regulated PSU. More C in the power supply shouldn't have much effect unless something is wrong, but it will increase the stress (peak current) on the rectifiers.

Good parts with low ESR and long life expectancy are better than just anything with more farads.

I think i'm a Dadist on this one.

David
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Old 12th May 2019, 12:39 am   #3
Craig Sawyers
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Default Re: Quad 303 re cap

Increasing the value of the output capacitors is no bad thing. Doug Self provides ample evidence that once an electrolytic capacitor has more than 80mV across it, distortion rises rapidly. The fitted 2000uF capacitors have a 3dB point of 10Hz at 8 ohms load. However the harmonic distortion from that capacitor at close to full power will generate distortion rising rapidly from 1kHz downwards.

Increasing the value by a factor of 5 to 10,000uF will mitigate that distortion mechanism.

I don't think that there is much need to mess with the value of the power supply reservoir because the supply is regulated. Increasing that will just increase the stress on the bridge rectifier and transformer.

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Old 12th May 2019, 5:07 am   #4
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Default Re: Quad 303 re cap

Depends on what you want the original sound warts and all or the new improved sound!
To be fair the capacitors available when the 303 was produced can be said to be of a limited choice range I would not go over the top re the PSU caps as already stated rectifiers etc may be overly stressed As to the output cap I would say what spk can produce 10 hz and if it could I bet the distortion would be in the region of 20% plus
I used to retail these in the day Idiosyncratic but at the time a symbol of quality
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Old 12th May 2019, 7:19 am   #5
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Default Re: Quad 303 re cap

Well granted. But even given high distortion in (particularly) woofers at high displacement (see http://www.linkwitzlab.com/frontiers.htm ), there is little justification to adding an extra 0.2% or more distortion though the output capacitor alone.

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Old 12th May 2019, 7:46 am   #6
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Default Re: Quad 303 re cap

Does the increased charge-up transient of the bigger output capacitor cause any problems?

On switch-off, where does the charge go? This sort of thing is rarely planned and transistors are remarkably fragile when reverse bias happens to B-E junctions.

A small increase is one thing, but a 5-fold increase ought to be backed up with some design work to check it doesn't compromise reliability.

David
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Old 12th May 2019, 8:03 am   #7
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Default Re: Quad 303 re cap

That is a good point David. There seems to be a charge bleed resistor across the output terminals of 2k2, and a time constant of about 4.5 seconds. If that is the relevant parameter, it should be reduced if the output capacitor is increased. So to 470 ohms at 10,000uF, 2W or 3W ROX should do it with power dissipation to spare.

IF that is the relevant parameter of course...

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Old 12th May 2019, 8:50 am   #8
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Default Re: Quad 303 re cap

I do not think many speaker systems will have much output at 10Hz. Large horn loaded ones maybe ( Big Klipsch say ). Then one would need a very large room and very deep pockets.

My experience with good quality systems in rooms with longest side in the 18 - 25 feet range is that a series of peaks and troughs in the perceived bass occur at various positions in the room. The optimum ( subjective ) bass almost never coincides with preferred or practicable seating positions.

Personally I wouldn't worry about bass below say 30 - Hz.
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Old 12th May 2019, 8:58 am   #9
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Default Re: Quad 303 re cap

Quote:
Originally Posted by Craig Sawyers View Post
That is a good point David. There seems to be a charge bleed resistor across the output terminals of 2k2, and a time constant of about 4.5 seconds. If that is the relevant parameter, it should be reduced if the output capacitor is increased. So to 470 ohms at 10,000uF, 2W or 3W ROX should do it with power dissipation to spare.
I think that it is only there to 'charge' the cap in case there is no speaker connected when switched on. That saves a large pop when you do connect a speaker up with power on. No need to alter it.

4 x 4700uF 100 volts caps are a good compromise all round. I think that is what Dada used to recommend in the past...
Alan
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Old 18th May 2019, 8:03 pm   #10
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Default Re: Quad 303 re cap

Having opened the 303 some body has been playing . The 2000uf power caps have been replaced by 2 X 1500uf ones ( now have a single 4700uf ) and the output caps were well out ( high esr and reading 3600uf now a pair of 4700uf ) following the DADA upgrade path I have replaced the elec caps ( need 22uf 25v as out of stock ) but a lot of the resistors have been replaced on one driver board with what looks like a lower wattage ones and the bias pot RV100 replaced by a fixed resistor . Its amazing how good the 303 sounds even with all the bodges looking forward to completing the upgrade

Dave L
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Old 19th May 2019, 11:53 am   #11
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Default Re: Quad 303 re cap

I recently modded my 303 that I bought new in 1970 with the Dada components and it has transformed it! Brilliant!
When setting up voltages I had it all laid out on the bench to 'soak' before I finally trimmed the pots but found almost no drift from the cold setting.
I can only think that a gradual deterioration over the years had gone un noticed by my finely tuned ear?

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Old 19th May 2019, 3:34 pm   #12
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Default Re: Quad 303 re cap

Quote:
Originally Posted by alanworland View Post
I can only think that a gradual deterioration over the years had gone un noticed by my finely tuned ear?
Alan
Yes, that's very common. It's a great help to sales people who like to demonstrate a nice new modern widget against your old one.
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Old 24th May 2019, 9:28 am   #13
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Default Re: Quad 303 re cap

I have just won an other 303 from that site and this one has not been played with but been stamped all over the insides with Jack boots I will post photos later the driver board looks like it has been repaired with a poker and gas ring and I will need a new one ( they sell the bare bones on ebay for £10.00 for a set power 2 x driver )
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Old 24th May 2019, 9:48 am   #14
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Default Re: Quad 303 re cap

Yes purchased a set of boards built them up easy - peasy provided you can get the devices
Always loved the 303 just connected with it at the correct time in my life I suppose
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Old 24th May 2019, 11:34 am   #15
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Default Re: Quad 303 re cap

Quote:
Originally Posted by nutteronthebus View Post
I have just won an other 303 from that site and this one has not been played with but been stamped all over the insides with Jack boots I will post photos later the driver board looks like it has been repaired with a poker and gas ring
I once extracted a PCB from a Sommerkamp (Yaesu) transciever that illegal CBers had tried to convert to naughty frequencies. It had then been bought by an amateur in our club. I held it up to the gathered bunch and announced "It was the Marx Brothers, in the kitchen, with the blowlamp"

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Old 30th May 2019, 5:27 pm   #16
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Default Re: Quad 303 re cap

I have now recapped both of the 303's and time to set the bias etc they are v10 boards and I have replaced both rv100/101 and the one on the power board . 67 V set on the power OK 33.5v on both driver boards OK . DADA recommend between 6mv and 15mv across pins 4 and 6 on each of the 303's one sets OK at 10mv the other side will not set at all but the amps work! . Has anyone had this on there Quad 303 ( I think the fault was on from the start


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Old 30th May 2019, 8:45 pm   #17
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Default Re: Quad 303 re cap

First suspicion would be a solder short or a copper whisker on the board that may have been there some time...

David
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Old 30th May 2019, 9:17 pm   #18
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Default Re: Quad 303 re cap

I have found this old post https://www.vintage-radio.net/forum/...ighlight=bc461 this is the same problem that I have
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Old 21st Jun 2019, 9:22 am   #19
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Default Re: Quad 303 re cap

I have been look into the faults on the 2 303's re unable to set the bias on 1 it was the cracked hammer syndrome ( faulty tools ) the adjuster was turning within the shaft not the trimpot . The other is work in progress as TR103 had a HFE of 7!!! waiting for spares
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Old 26th Jun 2019, 5:49 pm   #20
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Default Re: Quad 303 re cap

As promised photos of the bodge job on the 303 with the caps that are leaking and the new PCB in place sounds nice even with a MK9 on one side and a MK10 on the other
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