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Old 27th Jun 2019, 5:56 pm   #21
Bazz4CQJ
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Default Re: Quad 303 re cap

A friend of mine has just told me that he has 303 complete with the pre-amp, neither of which has been switched on for...~30 years. I think he has owned it from new and will certainly have stored it well. I've not seen it yet as he lives some distance away.

I'm not really familiar with any of the Quad gear and I simply told him that they are highly collectable and under no circumstance should they be powered up without being "prepared".

Of course, that raises the question of what prepared means. I'd guess that as an absolute minimum, it means connecting some speakers via some external caps and then applying power via a Variac, raising the volts to 240 over a period of some hours, looking at it with my thermal imager and sniffing it frequently?

However, I'd be happy to receive any 'more-considered' inputs as to how best to bring it back to life.

Hope this does not stray too far off topic.

Thanks

B
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Old 27th Jun 2019, 7:58 pm   #22
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Default Re: Quad 303 re cap

The min I would do is to change the main caps on the 303 4700uf 100v RS 771 4747 ( only need 3 as the power supply is 2 x2000uf and is replaced by 4700uf 100v )along with the 4 caps on each board 470uf 47uf 22uf all 25v and C100 3.3uf 25v tant . The 33 will need the 3 1000uf caps on the power supply as a min
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Old 27th Jun 2019, 8:34 pm   #23
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Default Re: Quad 303 re cap

So you are saying that you would re-cap without even investigating response at slow power up?

Not being familiar with the components in question, do people go for simple replacement or is this 'stuffing' territory?

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Old 27th Jun 2019, 10:44 pm   #24
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Default Re: Quad 303 re cap

The first thing to do is remove the covers and make an examination of the power amplifier.I strongly suspect the main power supply and output caps will be leaking. If this is the case they should be replaced The power should then be applied via a current limiting lamps without any signal drive mid rail voltage should then be measured
When this is done the unit can then be set up and tested but to be honest their are well known problems with the electrolytic s so I would replace these as a matter of course .Well worth the effort nice amplifier and worth spending a few pounds on
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Old 28th Jun 2019, 1:57 am   #25
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Default Re: Quad 303 re cap

Also worth mentioning that there were problems with the earlier 303s, in which the can electrolytics were mounted with the "tag" ends of the caps downwards - when they leaked they leaked onto the components/boards beneath them. They introduced a modification (and dealer re-wire kit) in later versions in which the cans were fitted with the tag ends upwards.

Sorry if someone else has covered this point, but if they did I haven't noticed it.

Mike
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Old 28th Jun 2019, 9:58 am   #26
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Default Re: Quad 303 re cap

I can second that! My early 303 suffered in this way, caps leaked and dripped onto boards below. Can't remember what exactly I had to replace (other than big caps) but it was pretty straightforward to get up and running again.
My best amp!

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Old 28th Jun 2019, 11:25 am   #27
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Default Re: Quad 303 re cap

If you look at the DADA site they have a download section ( you need a login but free to join ) and they have a full upgrade path for both the 33 and 303 I use this as a reference to update the preamp/amp


Dave


The other problem is with the transistors ( do they age ? ) as some had gains HFE as low as 7 and others 67 ( normal replacements in the 100's ) used BC550 for NPN and BC 556/560 for PNP type
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Old 29th Jun 2019, 10:16 am   #28
Craig Sawyers
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Default Re: Quad 303 re cap

They are all silicon, so they shouldn't age. But an HFE of 7 is certainly suspicious in a TO18/TO92 package.

It is worth checking the power supply regulator. There is a carbon track adjustment to set the voltage to 67V. If that is intermittent the supply voltage could be going high, which might compromise some of the transistors.

One of the standard fixes is to replace all of them (regulator board and two each on each amplifier) with new parts.

I'd have to look out the reference, but Peter Baxandall (who was involved with the design of the 303 under contract to Quad) reckoned that the only reason the regulator was used is that no one at Quad was confident of the robustness of semiconductors at the time the 303 was designed (it was introduced in 1967). It was Quad's first foray into semiconductor power amps.

But they are pretty bulletproof when fettled. In the mid 70's we used them to power the students union Saturday disco. They had a rough life - certainly beer ingestion, but also smoke from various substances. And mechanical abuse - they were dented and battered on a regular basis. And they just kept working.

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Old 29th Jun 2019, 10:20 am   #29
Craig Sawyers
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Default Re: Quad 303 re cap

Having said that I have come across various plastic package transistors that have gone strange, or even open circuit. I suspect (and it is only suspect) that it might be down to the early formulations of epoxy putting stress on the silicon or bond wires.

BTW - stupid question - it was wired into your transistor tester right? If collector and emitter are swapped it will be working in inverse active mode and have an apparent very low beta.

Craig
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Old 29th Jun 2019, 12:27 pm   #30
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Default Re: Quad 303 re cap

Quote:
Originally Posted by Craig Sawyers View Post
... It is worth checking the power supply regulator. There is a carbon track adjustment to set the voltage to 67V. If that is intermittent the supply voltage could be going high, which might compromise some of the transistors.
...
I'd have to look out the reference, but Peter Baxandall (who was involved with the design of the 303 under contract to Quad) reckoned that the only reason the regulator was used is that no one at Quad was confident of the robustness of semiconductors at the time the 303 was designed (it was introduced in 1967). It was Quad's first foray into semiconductor power amps.
My understanding of the 67 volt setting is slightly different. Or there may be two reasons?
The 303 was a logical partner to Quad's electrostatic ELS57 speakers and they can only take 33 volts (peak to peak). Hence Quad setting the 303 to prevent possible damage.

Alan

Added 303 service data just in case it is needed.
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Old 29th Jun 2019, 12:29 pm   #31
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Default Re: Quad 303 re cap

HI I use a peak DAC55 tester with auto lead id the transistor now tests as a diode but other ones test as HFE 95 against the replacement of HFE 514. Others test ok but to be safe I changed all the small signal transistors with BC550 for NPN and BC556/560 for PNP . It maybe by old ears but the recapped amp with new transistors sounds brighter than the just recapped amp

Dave
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Old 29th Jun 2019, 2:46 pm   #32
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Default Re: Quad 303 re cap

That looks like a culprit of early epoxy. If it tests like a diode either the emitter or collector is O/C.

I have a DAC55, and find it exceptionally useful. The more expensive DCA pro is much more comprehensive, particularly with JFETs where it give Vp and Idss.

And Snowman - It is a classic series voltage regulator with components to start it on power up.

Also the Quad 405 has a jumper position to limit its output to suit the ESL57.

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Old 3rd Jul 2019, 8:21 am   #33
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Default Re: Quad 303 re cap

I now have 5 Quad 303 that I have updated with 10 turn pre-sets for the bias and mid point voltage ( trying to set the bias with a normal pre-set is a pain) on 1 I was unable to set the bias and found that the PNP transistors had all gone low gain ( as low as HFE = 7 and others all below 100 ) the NPN ones were all ok but I changed the lot as RS only sold them in the 100 pack . The 303 now sounds brighter than the others that I have done and I may have to revisit the other 4 and do the same ( 3 1/2 as 1 has a new board in it )

Dave
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Old 13th Jul 2019, 7:23 am   #34
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Default Re: Quad 303 re cap

Can anyone suggest which makes of capacitor would be a good trade-off between cost and quality to re-cap the power supply and output on a 303 and where could these be bought?

Thanks
B
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Old 13th Jul 2019, 8:33 am   #35
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Default Re: Quad 303 re cap

I use RS for the caps as they just need a bit of tape to make them fit the clips see https://uk.rs-online.com/web/p/alumi...itors/7714747/ ( I now get most of my parts from them /CPC ) you need 3, 1 for the power supply ( was 2 X 2000uf and 2 for the output upgrade 2000uf to 4700uf )

Dave
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Old 17th Jul 2019, 12:09 am   #36
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Default Re: Quad 303 re cap

Thanks for that. I've now got this amp at home and have got as far as taking off the base and confirming that there's been no leakage from the caps on this one.

The owner basically just wants to sell this unit and I'm just wondering about the trade-off between spending time on it and maximising it's desirability.

I'm wondering about connecting a DC psu to the big electrolytics and slowly applying a few volts and seeing if they look like they are polarising well. Then if so, powering the amp up slowly on the Variac. If at that stage it basically works, pass it on as "working" and let someone else choose exactly how they want to restore it?

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