Hi
Ive been given a 303 for repair as its been sitting on the shelf for abt 2 yrs, its a 80k+ serial number (i think that means its an 80s model but not sure), so about 30yrs old give or take but this has had a hard life seeing over 12h a day action over most of its life.
It was in use connected to a set of Rogers in a freinds home studio till one day it let the magic smoke out. Freind looked and saw a resistor on the power board which burnt completley (it is the one directly below the and closest to the heatsink), my wiring diagram reading skills are not briliant so cant work out which one it is. It is one between pins 12+13 and also one leg of a transistor. I think its R204.
So from that point it was just left standing before comming to me after 2 yrs.
Ive looked, it needs new PSU and Speaker caps as they are all buldged on the top and also some have started to leak.
So is this all fixable and could it have taken out anything else? All other boards look ok.
I was thinking getting the Net Audio, full replacement kit of caps, psu board, rectifier bridge and sound board caps plus replace all the open frame bias parts with newer versions.
We are also looking to stop using the Quad 33 Pre so i was thinking of either replacing the front plate or just adding additional RCA connections to the existing plate along with a pot to adjust line in, this would then allow us to plug the unit directly into the soundcard output.
Any help would be much apreacitated to restore this unit back to working.
Ive been given a 303 for repair as its been sitting on the shelf for abt 2 yrs, its a 80k+ serial number (i think that means its an 80s model but not sure), so about 30yrs old give or take but this has had a hard life seeing over 12h a day action over most of its life.
It was in use connected to a set of Rogers in a freinds home studio till one day it let the magic smoke out. Freind looked and saw a resistor on the power board which burnt completley (it is the one directly below the and closest to the heatsink), my wiring diagram reading skills are not briliant so cant work out which one it is. It is one between pins 12+13 and also one leg of a transistor. I think its R204.
So from that point it was just left standing before comming to me after 2 yrs.
Ive looked, it needs new PSU and Speaker caps as they are all buldged on the top and also some have started to leak.
So is this all fixable and could it have taken out anything else? All other boards look ok.
I was thinking getting the Net Audio, full replacement kit of caps, psu board, rectifier bridge and sound board caps plus replace all the open frame bias parts with newer versions.
We are also looking to stop using the Quad 33 Pre so i was thinking of either replacing the front plate or just adding additional RCA connections to the existing plate along with a pot to adjust line in, this would then allow us to plug the unit directly into the soundcard output.
Any help would be much apreacitated to restore this unit back to working.
The 303 is a fairly straighforward design and an easy DIY prospect but you need to know how to read a circuit and how to make at least make basic tests.
R204 on my circuit is a 10K
The series pass regulator transistor and its driver would be suspect together with the output stage of the amp and associated low value resistors but without doing simple tests its all guesswork. First steps would be replacement of all the electroylitics as a matter of course and then checking all the passive components (resistors) to make sure they are withing tolerance. Whether rebuilding or just fixing the amp I would look for and fix the amp first before just replacing components.
R204 on my circuit is a 10K
The series pass regulator transistor and its driver would be suspect together with the output stage of the amp and associated low value resistors but without doing simple tests its all guesswork. First steps would be replacement of all the electroylitics as a matter of course and then checking all the passive components (resistors) to make sure they are withing tolerance. Whether rebuilding or just fixing the amp I would look for and fix the amp first before just replacing components.
I have a 33 + 303 set too that was given to me when the power amp blew up. Well worth fixing, beautiful designs and they sound really nice. So far I've only recapped the preamp but it needs a new volume pot - finding one with a switch is a problem... the power amp is next.
I second that but would also recommend replacing the trimpots on all boards. This will mean setting them up again from scratch, but you'll probably have to do this anyway if replacing transistors.
You might find the following sites useful:
Quad 33 and Quad 303
http://www.net-audio.co.uk/quad303caps.html
NET Audio
Quad Spot: Quadrevision Live: Peter's 303 Explosion
I also have a pdf copy of the 303 service manual if want it - just send me a PM.
I second that but would also recommend replacing the trimpots on all boards. This will mean setting them up again from scratch, but you'll probably have to do this anyway if replacing transistors.
You might find the following sites useful:
Quad 33 and Quad 303
http://www.net-audio.co.uk/quad303caps.html
NET Audio
Quad Spot: Quadrevision Live: Peter's 303 Explosion
I also have a pdf copy of the 303 service manual if want it - just send me a PM.
R204 seems to be listed in the parts list on the service manual as a 6.8K, is this a print error as R204 is listed as 10k in the circuit diagram. What is the spec of the 10k that i need.
What could cause the resistor to blow up?
The resistor feeds a zener diode (so its value isn't critical as such) although I would try and replace with the same. Can you make out any markings ?
Either 10K or 6K8 will work and the resistor needs to be at least 1watt rating.
Nothing could cause it to blow up. If the zener went short (or problem with the regulator transistors) then it could see almost the full unregulated supply across it which would cook it slowly.
That said it could just fail through age and use as high value resistors with high voltage across them were a common failure point in lots of commercial gear. Depends on the quality of the resistor really. Old carbon composition types were really poor.
You need to check the main series pass transistor and driver is OK and that the output transistors of the amp are OK too.
Either 10K or 6K8 will work and the resistor needs to be at least 1watt rating.
Nothing could cause it to blow up. If the zener went short (or problem with the regulator transistors) then it could see almost the full unregulated supply across it which would cook it slowly.
That said it could just fail through age and use as high value resistors with high voltage across them were a common failure point in lots of commercial gear. Depends on the quality of the resistor really. Old carbon composition types were really poor.
You need to check the main series pass transistor and driver is OK and that the output transistors of the amp are OK too.
repair quad 33 and 303
hello I'm about to start an overhaul of mij quad amps though I'm tempted to buy an upgrade kit from Dada I'd rather like to select my own parts. Is there someone who can send me a complete parts list for the quad 33 and quad 303? It would greatly speedup the start of my repair,
djacco
hello I'm about to start an overhaul of mij quad amps though I'm tempted to buy an upgrade kit from Dada I'd rather like to select my own parts. Is there someone who can send me a complete parts list for the quad 33 and quad 303? It would greatly speedup the start of my repair,
djacco
The Quad service manual has all that info. If you can't find it anywhere else, join the Dada site, and download it along with replacement transistor information, and their latest upgrade info.
The Quad 303 is a single rail power supply item, which regulates the -ve / earth rather than the +ve rail.
For this reason attaching it to a computer with a shared earth is NOT recommended. To solve this use the coaxial / optical audio out of the computer to drive a DAC. This is not only safer but will be much more HiFI even with a cheap 30 Euro DAC from china ebay sites.
Replace ALL the capacitors in Quad 303's and do not hesitate to do this leaving any old capacitors will just cause you to end up replacing them a couple of weeks months later, at least this is my experience. Quad 303 has in my opinion a lovely sound in the mid range and above but is a little high output resistance for modern bass reflex speakers.
Don't forget one reason people like Quad so much is their service department, they consider them selves a company that services musicians and not a HiFi company and consequently considers repairs a service and not an excuse to exploit. If your not so confident it might be cheaper and easier to just buy new boards from Quad. Also excellent clone boards exist on ebay for 303's so you could consider building new boards to replace the old ones. These Chinese PCB's seem better made than the Quad original PCB's but I had already rebuilt the amps using the original PCB's so my Chinese ebay Quad 303 clone PCB's are just sitting in the spares box at this moment.
For this reason attaching it to a computer with a shared earth is NOT recommended. To solve this use the coaxial / optical audio out of the computer to drive a DAC. This is not only safer but will be much more HiFI even with a cheap 30 Euro DAC from china ebay sites.
Replace ALL the capacitors in Quad 303's and do not hesitate to do this leaving any old capacitors will just cause you to end up replacing them a couple of weeks months later, at least this is my experience. Quad 303 has in my opinion a lovely sound in the mid range and above but is a little high output resistance for modern bass reflex speakers.
Don't forget one reason people like Quad so much is their service department, they consider them selves a company that services musicians and not a HiFi company and consequently considers repairs a service and not an excuse to exploit. If your not so confident it might be cheaper and easier to just buy new boards from Quad. Also excellent clone boards exist on ebay for 303's so you could consider building new boards to replace the old ones. These Chinese PCB's seem better made than the Quad original PCB's but I had already rebuilt the amps using the original PCB's so my Chinese ebay Quad 303 clone PCB's are just sitting in the spares box at this moment.
Some components from Farnell:
1
1198552 PANASONIC
ECOS1HA103EA CAPACITOR, 10000UF, 50V Yes 842 £5.04
£40.32
Line Note:
Alternative Products Available
2
9451390 MULTICOMP
MCGPR50V226M5X11 CAPACITOR, 22UF, 50V Yes 15851 £0.05
£0.50
Line Note:
Alternative Products Available
3
9451366 MULTICOMP
MCGPR50V225M5X11 CAPACITOR, 2.2UF, 50V Yes 9810 £0.037
£0.37
Line Note:
4
9342648 MULTICOMP
MF12 150K RESISTOR, 0.125W 1% 150K Yes 16508 £0.025
£1.25
Line Note:
Alternative Products Available \u2022 Accessories Available
5
9342800 MULTICOMP
MF12 200K RESISTOR, 0.125W 1% 200K Yes 14153 £0.025
£1.25
Line Note:
Accessories Available
6
9343059 MULTICOMP
MF12 33K RESISTOR, 0.125W 1% 33K Yes 8206 £0.025
£1.25
Line Note:
Accessories Available
7
9343474 MULTICOMP
MF12 68K RESISTOR, 0.125W 1% 68K Yes 11903 £0.025
£1.25
Line Note:
Accessories Available
8
9752625 EPCOS
B32561J3474K CAPACITOR, 0.47UF, 250V Yes 888 £0.23
£0.92
Line Note:
9
9342427 MULTICOMP
MF12 100K RESISTOR, 0.125W 1% 100K Yes 131634 £0.025
£1.25
Line Note:
Accessories Available
1
1198552 PANASONIC
ECOS1HA103EA CAPACITOR, 10000UF, 50V Yes 842 £5.04
£40.32
Line Note:
Alternative Products Available
2
9451390 MULTICOMP
MCGPR50V226M5X11 CAPACITOR, 22UF, 50V Yes 15851 £0.05
£0.50
Line Note:
Alternative Products Available
3
9451366 MULTICOMP
MCGPR50V225M5X11 CAPACITOR, 2.2UF, 50V Yes 9810 £0.037
£0.37
Line Note:
4
9342648 MULTICOMP
MF12 150K RESISTOR, 0.125W 1% 150K Yes 16508 £0.025
£1.25
Line Note:
Alternative Products Available \u2022 Accessories Available
5
9342800 MULTICOMP
MF12 200K RESISTOR, 0.125W 1% 200K Yes 14153 £0.025
£1.25
Line Note:
Accessories Available
6
9343059 MULTICOMP
MF12 33K RESISTOR, 0.125W 1% 33K Yes 8206 £0.025
£1.25
Line Note:
Accessories Available
7
9343474 MULTICOMP
MF12 68K RESISTOR, 0.125W 1% 68K Yes 11903 £0.025
£1.25
Line Note:
Accessories Available
8
9752625 EPCOS
B32561J3474K CAPACITOR, 0.47UF, 250V Yes 888 £0.23
£0.92
Line Note:
9
9342427 MULTICOMP
MF12 100K RESISTOR, 0.125W 1% 100K Yes 131634 £0.025
£1.25
Line Note:
Accessories Available
1
1165909 MULTICOMP
MJ15003 TRANSISTOR, NPN, TO-3 Y-Ex 767
£2.99
£29.90
Line Note: Accessories Available
2
8820872 VISHAY BC COMPONENTS
222210119222 CAPACITOR, 2200UF, 100V Yes Out of Stock
£15.84
£126.72
Line Note: Accessories Available
3
8821216 VISHAY BC COMPONENTS
222210619332 CAPACITOR, 3300UF, 100V Yes 69
£22.48
£89.92
Line Note: Accessories Available
4
8813345 NICHICON
LLS2A472MELC CAPACITOR, 4700UF, 100V Yes 55
£7.18
£28.72
1165909 MULTICOMP
MJ15003 TRANSISTOR, NPN, TO-3 Y-Ex 767
£2.99
£29.90
Line Note: Accessories Available
2
8820872 VISHAY BC COMPONENTS
222210119222 CAPACITOR, 2200UF, 100V Yes Out of Stock
£15.84
£126.72
Line Note: Accessories Available
3
8821216 VISHAY BC COMPONENTS
222210619332 CAPACITOR, 3300UF, 100V Yes 69
£22.48
£89.92
Line Note: Accessories Available
4
8813345 NICHICON
LLS2A472MELC CAPACITOR, 4700UF, 100V Yes 55
£7.18
£28.72
Hi
Where did you plan to use this cap? The output cap. would be very risky and you certainly can't use a 50V cap. on a ~80VDC raw supply. 😕
I realise it is possible to "stack" electrolytic capacitors with added balancing resistors to get a higher voltage rating but it's not really advisable, IMHO.
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