Quad 33 desoldering PSU

[John_73]

Hi folks.

Just about to remove the psu and replace the caps, but not sure how to proceed with removing the wires from the psu pins. Does one just heat up the centre of the pins and gently pull the associated wire away? Thought I'd best check with anyone who's done it rather than end up making a lash-up of it. Later units had a direct connection I believe, but this one has the pins. Need to think anout where to desolder that thin earth wire too - the socket end or psu end.

Cheers,

John.

 

[John_73]

Have upped C502 from 400uF to 1000uF, and C501/502 from 640uF to 1500uF. Have just noticed after a quick Google image search, that some go as high as x2 4700uF here!

How much is too much? Will hold fire installing these psu caps if it'd be best to up the values more...

 

[Arkless Electronics]

Have upped C502 from 400uF to 1000uF, and C501/502 from 640uF to 1500uF. Have just noticed after a quick Google image search, that some go as high as x2 4700uF here!

How much is too much? Will hold fire installing these psu caps if it'd be best to up the values more...

There's no upper limit. The values you have fitted will be fine.

 

[John_73]

There's no upper limit. The values you have fitted will be fine.

Thanks Jez. I've just found a couple of 2200uFs here too, but if 1500uF is more than enough I'll stick with those then. Don't want to stray too far from the original design.

 

[LPSpinner]

Yup I found a couple of 2200uf/50v caps for C500 and C501 as well as an axial lead 1000uf/50V for C502 that fit in my 33 so they should last a life time since the new caps are running well below their maximum rating an don’t forget that newer electrolytes tend to be much better and allot more stable over time as well. Like Jez said more “uf” is better, I like to leave a good safety margin on the maximum voltage rating as well. These new caps will probably last me the rest of my life.

As for the pins, just heat the pins and the wire should just slide out, get your self a good “solder-sucker” to clean up the pins and the reworked leads should be easy to re-insert once you have finished doing up the PSU board.

Lots of luck ….

LPSpinner

 

[John_73]

Thanks LPS! The ones I have are Panasonic FC 2200uF 35V, which is still 10V over the original rating so should be fine. Just have to decide whether to use a third 2200uF FC for C502, or stick with an axial type there to keep the appearance of the original board - I have a 1200uF 25V Vishay/BC cap I can use there.

Thanks for the advice about the pins. As you said, they slid out very easily. Will clean the pins up & the pcb whilst it's out of the unit.

Any thoughts on Quad 303 chassis caps? Several recommend x2 4700uF 100V for the psu caps, and for the outputs to. Others caution sticking to x2 2200uF for the psu caps so as not to stress the original rectifier diodes. As BHC/Kemet ALT22A caps are quite pricey I'm going to do the PSU first, for safety. Will do the output caps in another couple weeks.

Cheers,

John.

 

[Arkless Electronics]

Thanks LPS! The ones I have are Panasonic FC 2200uF 35V, which is still 10V over the original rating so should be fine. Just have to decide whether to use a third 2200uF FC for C502, or stick with an axial type there to keep the appearance of the original board - I have a 1200uF 25V Vishay/BC cap I can use there.

Thanks for the advice about the pins. As you said, they slid out very easily. Will clean the pins up & the pcb whilst it's out of the unit.

Any thoughts on Quad 303 chassis caps? Several recommend x2 4700uF 100V for the psu caps, and for the outputs to. Others caution sticking to x2 2200uF for the psu caps so as not to stress the original rectifier diodes. As BHC/Kemet ALT22A caps are quite pricey I'm going to do the PSU first, for safety. Will do the output caps in another couple weeks.

Cheers,

John.

The rectifiers should be fine with the bigger caps.

 

[John_73]

Thanks Jez, good to know.

I'm decided to 'live dangerously' and power it up tonight and see how it sounds with the stock 2000uF caps. They look to be in perfect condition, plus the unit has had very light use, and the owner had it up and running to test it still worked only a couple weeks back.

Since then I've replaced all driver board electrolytics, all trim pots, and two resistors that had gone high value (which just happened to be the only two carbon comps in the amp). I'll see if I can set all voltages easily and keep a close eye - and ear! - on things.

I know the source isn't meant to be connected during this phase. Are speakers also meant to be disconnected to?

Thanks.

 

[Arkless Electronics]

Thanks Jez, good to know.

I'm decided to 'live dangerously' and power it up tonight and see how it sounds with the stock 2000uF caps. They look to be in perfect condition, plus the unit has had very light use, and the owner had it up and running to test it still worked only a couple weeks back.

Since then I've replaced all driver board electrolytics, all trim pots, and two resistors that had gone high value (which just happened to be the only two carbon comps in the amp). I'll see if I can set all voltages easily and keep a close eye - and ear! - on things.

I know the source isn't meant to be connected during this phase. Are speakers also meant to be disconnected to?

Thanks.

It's best to leave them disconnected although as it's capacitor coupled it's less important than normal. Make sure the bias presets are set to minimum on switch on!

 

[John_73]

It's best to leave them disconnected although as it's capacitor coupled it's less important than normal. Make sure the bias presets are set to minimum on switch on!

Thanks for all that Jez.

Well setting the voltages & quiescent current went without incident, and I have music! Very pleased indeed! Going to keep it running for an hour or so and then recheck the settings after its been warmed up. Surprising how sensitive the (new) adjustment pots are. Running it directly from a DAC with variable output at the moment as the 33 is still awaiting a few cap changes.

If it still sounds this good after a few hours playing and there's no signs of any bulging in those four chassis mount caps I think I'll replace the cover & base and enjoy it. I'll still feel more comfortable with it once I have a new set of psu and output caps fitted though.

Anyway - Progress!

John

 

[Tony L]

Even the fancy blue multi-turn pots on the Dada boards are sensitive! Their advice is to get a provisional setting, play some music at a normal level for a few hours, then fine tune it at what will be normal operating temperature.

PS I decided to stick with Quad's second spec of 2x2200uf for the PSU as I'd read stories of things being damaged with inrush current when people tweak them too far. The RCA transistors are rare and quite expensive, not worth taking any risks IMO. I've used Dada's recommendations for 4700uf for the output caps though and that does give a bit more grip once they've bedded in. I'm very happy with mine now.

 

[Arkless Electronics]

I rebuilt a pair for a mate many years ago in which I replaced the regulator board with a design of my own and lots of other mods.... originality was not part of the remit. I put 10,000uf caps in these two and they are still in daily use with no problems....

 

[John_73]

I seem to recall it was the same with that 303 I did the article on, way back. It frustrates my OCD side, but I have it as near as I'll ever get it I think. Will reset once more after this evenings session.

To be honest, I'm rather taken back by how good it sounds. It's been many years since I had that last 303, and I don't recall it being quite this good. But my sources weren't so good back then either. Taken back in that - subjectively - it appears to have utterly trounced my (serviced) Quad 606! The midrange on vocals in particular is just... spot in! So natural. Warm yes, but naturally so - not 'pipe & slippers' warm And detail aplenty, and dynamics, but again - all so very natural. Encourages you to turn up the volume & disappear into the recording! No artificially pumped up treble masquerading as detail, or 'in your face' (ears?!) sonics. Plucked guitar strings are just leaping out of the speakers, with the ability to startle. Amazingly, it's even doing perfectly good, and punchy, bass. From the original 2000uF caps into speakers that dip to 6-ohms...!

It sounds so good I'm suddenly loath to change the PSU or output caps now. Wonder how much more life they have left...?

Playing the rather sublime first volume of 'A Monstrous Psychedelic Bubble' right now and, on the well recorded tracks, it's a superbly attention grabbing and addictive 'sound'.

Apologies for all the superlatives, but I'm honestly over the moon with it!

Peter Walker - genius!!!

 

[Tony L]

I keep on saying it, but there is something very special about the 303 mid-band assuming it's working properly and driving speakers it likes. So much so I'd be very reluctant indeed to tweak/hack it into something else. There are countless more powerful and better specified amps that don't get close to what a decent 303 gets right IMO.

 

[John_73]

In complete agreement. It's very rarely I've heard amps make a decent stab at slide guitar (for example) - that fullness and 'weight' is so realistic. Has the feel & 'presence' of the real thing. And the attack - it's genuinely surprised me after some of the 'good for its time' comments you see around netsville. Whatever it gets wrong in my opinion is utterly benign. Not using particularly special/fancy speakers either - just running into a pair of Mission M72s!

 

[Arkless Electronics]

I always found them a bit too rolled off at the frequency extremes and lacking resolution myself... not in a huge way but enough to be always aware of it... if that makes much sense...

 

[John_73]

I always found them a bit too rolled off at the frequency extremes and lacking resolution myself... not in a huge way but enough to be always aware of it... if that makes much sense...

To be honest that was what I was expecting. The stereotyped Quad II sound with just a bit more heft in the bass, which is why I'm quite surprised. Unless my speakers have a slight treble peak and the 303 is just a perfect match for them.

Whatever the technical reason, I'm honestly over the moon with it

 

[John_73]

 

[Tony L]

I always found them a bit too rolled off at the frequency extremes and lacking resolution myself... not in a huge way but enough to be always aware of it... if that makes much sense...

I can understand that perspective, but for me what's missing is that edge, glare or sheen of amps designed to be "impressive" for five minutes in a quick dealer A B dem. I really hate 'bright'! The 303 just sounds natural to me, though My context is three very easy to drive sets of speakers and I've certainly heard them sound flat, tubby and shut-in when asked for more than they want to deliver into awkward loads.

PS John, those boards really do look amazingly clean! Did you get the caps and pots in without lifting any tracks?! I'll be interested to hear how you feel it compares to your IIs.

 

[John_73]

If it wasn't for the fact its partnering 33 had slight signs of use, it'd be easy to believe this 303 was, in effect, NOS! The boards are completely clean, I got lucky with this set.

As for desoldering, all tracks were fine, but I've developed a 'technique' which seems to work well. I use the old classic XS-25 25W Antec soldering iron which seems to be gentle on old pcbs. I also use Chemtronics desoldering braid. The secret is to press the braid with the tip of the iron just hard enough to touch & melt the solder, so you're barely applying any pressure to the board itself. I also - where possible, depending on component - heat the solder pad and gently pull through the component from the other side. This is safe as its pulling 'into' the board, away from the pad trace side. I then wick off any remaining solder with the braid.